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Design of Stormwater Tanks - Recommendations and Layout

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The Grundfos handbook “Design of Stormwater Tanks – Recommendations and Layout” covers every aspect of stormwater tank design, sizing, installation and positioning, control strategy and equipment, and …

The Grundfos handbook “Design of Stormwater Tanks – Recommendations and Layout” covers every aspect of stormwater tank design, sizing, installation and positioning, control strategy and equipment, and shows how such measures ultimately offer savings on investments.

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  • 1. GRUNDFOS waStewater StORmwateR taNkSDeSign of Stormwater tankSRecommendations and layout
  • 2. 3 The concept of storm water detention is to temporarily store excess storm water runoff. This is to avoid hydraulic overload of the sewer system, which could result in the flooding of roads andDeSign of Stormwater tankS buildings with untreated wastewater or its release directly into the environment, causing pol- lution. When space is available in the sewer system, the detained water is released at a rate notRecommendations and layout exceeding the capacities of the sewer system, and the tank should be cleaned ready for the next flush. the challenge aheaD the ground as normal, and instead water is guidedthe challenge aheaD ...........................................................................................................................................................3 into the existing sewer system. This creates a sub- Climate change is increasingly the cause of ex- stantial hydraulic load in the pipes network and treme weather phenomena around the world, for at the wastewater treatment plant. Furthermore,Stormwater tankS in DUtY-moDe of oPeration................................................................................................ 7 example extreme rainfall. This challenges the exist- paved and impermeable areas also collect more air- ing sewer system, not least because an increase in borne pollutants which are washed off along with FILLING ..................................................................................................................................................................................... 7 rain intensity of 40 to 60 % will reduce the return rainwater. FIRST FLUSH............................................................................................................................................................................8 period of severe flooding events. Stormwater tanks RETENTION TIME..................................................................................................................................................................8 can store much of this excess rainwater, thereby re- Storm water runoff is the term normally used for ducing the hydraulic load on the existing sewers. this kind of runoff water. Storm water is of concern RISK FOR OVERFLOW ..........................................................................................................................................................8 Properly sited, such tanks can reduce the need for for two main reasons: one is related to the volume CLEANING AND MIXING ...................................................................................................................................................9 expensive renovation of the existing sewer system. and timing of the runoff water; and the other is EMPTYING .............................................................................................................................................................................13 related to potential contaminants that the water Retention of these extreme peak flows of rainfall may be carrying. As we have seen above, both canPhYSical DeSign of Stormwater tankS .................................................................................................................. 17 is also a major challenge because of the extent be expected to increase. of paved or impermeable areas in urban areas. In these situations, water cannot naturally seep into FORM AND SHAPE ............................................................................................................................................................. 17 BENCHES .............................................................................................................................................................................. 19 DIMENSIONS .......................................................................................................................................................................21 STORAGE VOLUME .............................................................................................................................................................22 Figure 1: Dry weather COMBINED SEWER SYSTEM Wet weather INLET .......................................................................................................................................................................................23 OUTLET .................................................................................................................................................................................. 24 Combined (top) and OPEN/CLOSED TANK CONFIGURATION ................................................................................................................... 29 separate (bottom) Spout Spout sewer systems OBSTACLES ........................................................................................................................................................................... 30 during dry and wet SERVICE ACCESS ................................................................................................................................................................ 30 weather flow. Storm StormrainJetS .......................................................................................................................................................................................32 drain drain CONSIDERATIONS FOR SIZING .....................................................................................................................................32recommenDation for inStallation anD PoSitioning .................................................................................35 Sewage from Combined domestic Combined sewer commercial and sewage and overflow RECTANGULAR TANKS......................................................................................................................................................35 industrial sources storm water CIRCULAR TANKS WITH SUMP PLACED AT THE WALL.........................................................................................37 To WWTP To WWTP CIRCULAR TANKS WITH CENTRE SUMP ................................................................................................................... 38 PIPING AND MANIFOLD .................................................................................................................................................40 Dry weather SEPERATE SEWER SYSTEM Wet weather PIPE INSTALLATION ...........................................................................................................................................................40 TUBE AND CHANNEL TANKS ........................................................................................................................................ 43 Spout SpoutrecommenDation for controlS anD Set PointS ............................................................................................ 44 CONTROL STRATEGY ........................................................................................................................................................ 44 Storm Storm drain draineQUiPment for Stormwater tankS .......................................................................................................................... 46 Sewage from Sewage from domestic domesticBiBliograPhY .......................................................................................................................................................................... 49 commercial and industrial sources commercial and industrial sources To WWTP To wet To WWTP To wet retention pond retention pond
  • 3. 4 Stormwater tankS Stormwater tankS 5 retaining Storm water Untreated wastewater can also back up in the sew- Figure 3: er system and result in backflow into buildings and Off-line detention Storm water is collected in the sewer network. A basements, for example through toilets and drains In-line and off-line sewer network can either be a combined system or and result in property damage. The concern here is Inlet storage a separate stormwater system (Figure 1,). The lat- possible economical losses for private homes, com- ter is of no interest for this guideline, as the storm panies and public buildings due to the destruction water from this system is often guided directly to of property, of contents and the cost of cleaning. Outlet the recipient or into wet retention ponds. However, storm water from the combined sewer system is a Stormwater tankS an effective SolUtion more complex issue and can cause environmental, aesthetic and hygiene problems for the recipients, Stormwater tanks are an effective way of reducing in the form of combined sewer overflows of un- peak flow and equalising flow rates from storm wa- treated wastewater, because the systems are not ter runoffs in the sewer system. Placed strategically, In-line detention constructed to cope with these large water flows. stormwater tanks mean better utilisation of the ex- isting sewer system, allow for intelligent manage- In many parts of the world, there is now an ac- ment of storm water flows, and ultimately save on Overflow knowledgement of and increased focus on these is- infrastructure investments. sues and requirements through local legislation for Inlet discharge from combined sewers has become com- Stormwater tanks are a cost-effective solution, be- mon. At a general level, this greater awareness can cause sewer lines are already constructed and gen- Outlet clearly be seen in efforts to prevent and reduce pol- erally have a substantially remaining lifetime, and Overflow lution, promote sustainable water use, protect the replacing existing pipes in an urban environment is aquatic environment, improve the status of aquatic – in addition to being very expensive – troublesome. ecosystems, and mitigate the effects of floods. Stormwater tanks can be relatively easily adapted to in-line anD off-line retention fraction of the suspended solids will settle during health, SafetY the sewer system, and during heavy rain the sewer retention in the tank whereas the rest will remain anD the environmental iSSUeS system is relieved by guiding excess storm water to Stormwater tanks are classified according to how in suspension (Fig. 4). the stormwater tank for temporary storage (Fig. 2). they are connected to the sewer system. For storm- Of major concern to regulatory bodies are the health All this illustrates the advantages of self-cleaning, water tanks connected in series with the convey- and safety issues for populations living close to wa- pump-managed stormwater tanks. ance system, the term in-line storage or detention Figure 4: ters with bacteriological and chemical pollution. is used. Storage facilities connected in parallel to TDS The pollution resulting from these discharges, even the sewer system are termed off-line storage or de- Diagram showing though only brief and periodical, is also in focus be- tention (Fig. 3). the fraction of the cause it can cause severe ecological consequences total solids (TS) and impact more heavily on the environment than With in-line detention, both dry and wet weather that will settle (TSS TS the steady load from a wastewater treatment plant. flow passes through the tank. The outlet for in-line settleable) in the Minimising the overflows from combined sewers detention tanks has less capacity than the inlet, and TSS stormwater tank can thereby improve water quality and ecological consequently flow passes through the tank unde- TSSsettleable during retention. stability in many ways. tained until the inflow rate exceeds the outlet ca- pacity. The excess inflow is then stored within the tank until the inflow rate decreases, where the de- tained water then empties through the outlet. Off-line storage is connected in parallel to the sew- er pipe, and as such dry weather flow bypasses the Figure 2: storage tank, leaving the stormwater tank empty FACTS: Dry weather COMBINED SEWER SYSTEM Wet weather between storms. Off-line storage is first achieved Detaining water for a period of time can also be utilised in applications Combined sewer when a predetermined flow rate is exceeded and other than the temporary storage of storm water in the sewer system: systems with the flow from the conveyance system is diverted Spout Spout • Wastewater treatment plants: Ensuring efficient treatment of waste- an integrated into the stormwater tank by means of pumping or stormwater tank gravity. The detained water is stored in the tank un- water a constant hydraulic load must be provided throughout the during dry and til sufficient conveyance or treatment capacity be- cleaning processes securing final effluent quality. Inflows of water wet weather comes available downstream and the water can be exceeding the capacity can then be temporary stored in a detention flow. Storm Storm pumped back. tank. drain drain • Large blocks of flats and buildings: Discharges to the sewer system Inflow from a storm water runoff event entering are often subject to limits due to the capacity of the public sewer net- the stormwater tank carries organic and inorganic work. If the hydraulic load is exceeded, detention tanks can be used matter. This can include macro pollutants such as for temporary storage. fine particles, vegetation and litter or micro pollut- • Discharges from factories: Batch volumes of process water with a cer- ants such as nutrients, bacteria, heavy metals and tain chemical or physical property must sometimes be stabilised or Sewage from Combined domestic Stormwater sewage and Stormwater chemicals. The usual definition of all these mate- equalised before discharge to the sewer system. This is to ensure that commercial and tank storm water tank industrial sources rials together is total solids (TS), which consists of the processes taking place at the receiving wastewater treatment To WWTP To WWTP a suspended (TSS) and a dissolved (TDS) fraction. plant are not disturbed. For storm water detention, this incoming material is left to settle during the detention period. Only a
  • 4. 6 Stormwater tankS Stormwater tankS 7 enSUring effective cleaning Stormwater tankS Mechanical scrapers keep a clearance from the bot- If the detained water is conveyed directly back tom of the tank, leaving material on the tank floor in duty-mode of operation into the sewer system, the majority of the settled which will give rise to the odour problems described matter will stay in the tank, where it will build up. above. This system will therefore not be suitable for The variation in stormwater tank design that might – provide an insight into how the hydraulics act in Once established, sediment deposits in the tank stormwater tanks that are emptied between storm arise due to differences in construction means that the tank. are difficult to remove, even with pressure cleaning water events. Compared to the single flush system, the operation of each stormwater tank must be equipment, and these deposits will take up storage the continuous flush system has the advantage that considered as a unique case. However, regardless of The colours of the arrows and contours show the capacity. If the settled matter is not removed from it can continue the flushing cycle until all detained whether the tank is renovated or newly construct- magnitude of the chosen variable according to the the tank, the tank will also appear dirty and poorly matter has been removed from the tank. ed, the determining factor for the effective use of scale in the picture. Furthermore, the arrows on the maintained. Furthermore, anaerobic decomposition the structure for efficient cleaning is the hydraulics vector plots show how the direction of flow of wa- of the detained organic matter due to biological Relatively cheap, flexible and automatic cleaning of the system, as it is the flow of water that cleans ter is in the tank at the specific conditions. activity creates unpleasant odours and toxic gases with a continuous flush system is obtained by in- the tank. from the stormwater tank. stalling RainJets. This type of equipment is categor- filling ised as a continuous flush system that can easily be The hydraulics of the system require the consider- The accumulated matter contains various toxic com- adjusted to fit most stormwater tanks. Grundfos ation of filling capacity, mixing, cleaning and emp- When the tank is being filled, two conflicting pa- pounds adsorbed to the settled organic particles as RainJets are available in two different versions: the tying as well as the retention time of the storm rameters are important to provide relief for the well as a variety of pathogenic bacteria. Manual Water/Water ejector (RainJet WW), and the Water/ water runoff. Equipment installed in the tank can sewer system and let detained material settle. In- cleaning in connection with tank operation is there- Air ejector (RainJet WA). The RainJet does not need directly influence the hydraulics, and the selection flow to the stormwater tank should take place at a fore problematic and should be eliminated. Instead, an external source of fresh water supplied for clean- of equipment must address these issues. pace that ensures the sewer system delivering wa- an effective, automatic and controlled cleaning of ing, as it uses the detained water already in the tank ter to the tank does not become overloaded, with the stormwater tank should be implemented. for flushing. This completely circumvents the pos- Grundfos has a strong tradition in using state-of- the consequences that follow. On the other hand, sible problems of backflow into the potable water the-art simulation tools to improve our products and the inflow should not arrive at a pace or in a man- Cleaning efficiency is however dependent on system. has more than 15 years of experience making simu- ner that makes turbulence in the tank an issue for stormwater tank design. Due to the flexibility when lations using computational fluid dynamics (CFD) the settling of detained material, should an over- building with concrete, which often is used for the To ensure that the effective and automatic cleaning to describe flow patterns in for example pumps and flow from the tank occur. construction of stormwater tanks, these storage of the stormwater tank runs smoothly, it is impor- pumping stations. Grundfos has utilised CFD tools facilities can assume many forms and be fitted in tant that the tank is properly designed, both in re- for describing, optimising and visualising the hy- The extent to which pumps should be used in storm- where space is available. Describing them all is not gards to equipment, physical design and controls. draulic processes that takes place during stormwa- water tank operation depends on site topography, feasible and this guideline only considers rectangu- To aid the process of creating a durable solution, ter tank operation with the finite volume method vertical fall in the sewer system and the amount of lar and circular tanks, but the principles described this handbook aims to help provide an overview of (Figure 5). Simulations in this guideline have been control that is desired for the flow in the system. In Figure 5: can be transferred to other shapes and forms, with some of the important points regarding equipment made using the general purpose CFD software suite some cases, where site topography allows it, gravity modifications. used for automatic control, stormwater tank design, ANSYS CFX that combines an advanced solver with can be used to cope with the flow either in or out of CFD simulations placement of equipment and automatic operation. powerful pre- and post-processing capabilities. the tank. However this strategy does not provide as can be used to Stormwater tanks are most often flat-bottomed, tight a control with the runoff as is possible when describe the hy- which are not self-cleaning and therefore require The best practice for CFD modelling has been ap- applying pumps for the task. draulic processes special equipment. For automatic cleaning of these plied to generate the simulations shown here. This taking place in the tanks different types of equipment are available, involves generating appropriate computation grids The pumps chosen can be either centrifugal or pro- stormwater tank. such as mechanical bottom scrapes, and single and selecting the relevant physical models. peller pumps, depending on the flow and head re- At the top can be flush or continuous flush water systems. The CFD models give the possibility for studying quired. The free passage of the impeller in the pump seen an example of details in the hydrodynamics of the RainJet, which should however be considered to avoid possible a velocity vector plot is not possible with traditional experimental meth- clogging issues. Some sewer systems and stormwa- and at the bottom ods. Figure 5 shows examples of flow patterns ter tanks have incorporated screens to trap major a contour plot which would take weeks to measure manually. The impurities, while other systems let everything pass showing the water resulting simulations can – with the use of vector and will either pass through or block the pumps velocity at the or contour plots in various intersections of the tank operating the system. bottom plate. the grUnDfoS rainJet range RainJet Wa RainJet WW Velocity Velocity With longeR- With gReateR Streamline 1 Contour 1 ReaChing Jet floW foR 0.500 0.500 0.450 0.450 and mixed WideR tankS 0.400 0.400 0.350 0.350 With aiR 0.300 0.250 0.300 0.250 0.200 0.200 0.150 0.150 0.100 0.100 0.050 0.050 0.000 0.000 [m s^-1] [m s^-1]
  • 5. 8 Stormwater tankS Stormwater tankS 9 firSt flUSh Wastewater carried to the stormwater tank from FACTS: the combined sewer system contains a certain load In the first minutes of a storm water runoff event, a of organic matter. The organic matter in the de- batch of water termed the first flush will arrive. The tained water decomposes under the consumption Gas development problems related to anaerobic digestion of materials in storm water tanks: first flush carries high concentrations of organic of oxygen in the water phase due to biological activ- • Hydrogen sulphide (H2S): H2S has a very characteristic odour and smells strongly like rotten eggs and inorganic matter to the stormwater tank. This ity. During extended periods of detention, oxygen is and flatulence. It is a highly toxic and flammable gas that is detectable in low concentrations (ppm). material has accumulated in the sewer system or consequently depleted, which gives rise to the for- H2S deadens the sense of smell in higher concentrations or after prolonged exposure. Respiratory on the drained area during periods without rain. Ab- mation of anaerobic zones in the tank. At elevated paralysis and death may occur quickly at concentrations as low as 0.07% by volume in air. sorbed in this material are different chemicals such water temperatures (15 ºC or above), the microor- • Methane (CH4): CH4 is a colourless and odourless gas at room temperature and atmospheric pres- as heavy metals and xenobiotics. These chemicals ganisms speed up the decomposition of organic sure. CH4 is not toxic but it is highly flammable and may form explosive mixtures with air. originate from the surfaces of roofs, drains, con- matter and thus the depletion of oxygen happens CH4 is a relatively potent greenhouse gas which contributes to global warming because it in the crete areas and roads in the catchment areas. even faster. As oxygen in the water becomes lim- atmosphere is converted in to carbon dioxide and water. ited, anaerobic decomposition of the organic mat- Because of the intake of large amounts of organic ter will be the main mechanism for decomposition. and inorganic matter in the first period of a runoff Anaerobic decomposition creates foul smelling and event, it might be advantageous to divide larger flammable gasses such as hydrogen sulphide and now makes it mandatory in most places to monitor crease the efficiency of the disinfection methods, stormwater tanks into separate sections, which methane that will emit to the surroundings from and report the number of overflows, duration, vol- simply by lowering the bacterial removal rate and are then filled successively and controlled by weirs, the stormwater tank or get trapped inside if the ume, etc., for each individual stormwater tank. furthermore will result in the formation of toxic by- valves or other arrangements (Fig. 6). Filling the tank is covered, creating a hazardous environment. products. sections successively gives two advantages. Firstly, To avoid gross material flowing into the receiving only the section(s) of the tank that have been filled The RainJet used for cleaning can be supplied both waters, screens can be installed on the overflow cleaning anD mixing must be cleaned and not the whole tank. Secondly, with and without an aerated jet stream. Choosing a to cope with this. The screens will trap impurities all major impurities are retained in the first section, RainJet WA will introduce air into the detained wa- depending on the clearance and reduce organic The principles for the automatic cleaning of storm- minimising the risk of having suspended material ter, which might eliminate or postpone the forma- pollution of the receiving water. In vulnerable ar- water tanks using RainJets are described below. in the effluent in the event of an overflow. tion of anaerobic zones and the inconveniences this eas, a further treatment of the overflow might be The cycles that take place in the tank during a storm might bring. required before it is guided to the receiving waters. water event from inflow of wastewater to the com- retention time For this purpose, membranes, filters or disinfection pletely cleaned tank are described below. Automa- riSk for overflow equipment can be installed. tic control of the installed equipment is normally Retention time of the stored water is controlled by done depending on water levels. Stormwater tank available space in the sewer system, and this deter- There is an economic consideration about how big Disinfection of the overflow can eliminate bacterio- operation is basically divided into four periods: mines when water from the tank can be conveyed the stormwater tank should be constructed, in rela- logical contamination and the hazards that bacte- back. This can normally be done immediately after tion to the expected inflow of storm water. Some ria can cause. Disinfection can be either chemical 1) a filling period where the inflow exceeds the the storm water runoff event and the tank can be storm water events will therefore exceed the capac- or physical. The chemical methods comprise dosing outflow; empty within 24-48 hours after the first inflow. Dur- ity of the tank and an emergency overflow must of chlorine or its derivatives as well as ozone, where 2) a settling period; ing extended periods of detention, unintended is- consequently be incorporated. a widely used physical method is exposure of the 3) a depletion period where the tank empties when sues such as odour problems might arise, resulting water to UV light. For disinfection of overflows from the outflow is greater than the inflow rate; from the combination of wastewater quality, tem- Legislation setting up the environmental goals for stormwater tanks, high-rate disinfection is often 4) a cleaning period where the tank is flushed, perature and time. the receiving waters and the degree of safety con- required to ensure adequate removal of pathogens cleaned and completely emptied. cerning health issues for the people living nearby from short-term high volume overflows. (See figures 7-11). Figure 6: Figure 7: Section Inlet Step 1-2: Successive 2 Section Filling of the tank filling of a 1 and settling of de- stormwater tank Section 3 tained material Overflow Spillway The disinfection equipment must be able to be Cleaning controlled based on intermittent high flows with variable temperatures, suspended solids concentra- During the filling period it is desirable to let the or- tions and levels of microorganisms. ganic and inorganic matter settle on the tank floor so it does not run into the recipient if overflow from Due the presence of suspended solids in the de- the tank happens (Figure 7). Non Outflow tained water, it might be necessary to install return valves pump screens or filters in front of the equipment. If these organic impurities are not removed, they will de-
  • 6. 10 Stormwater tankS Stormwater tankS 11 During the subsequent depletion period it is desir- In the depletion period mixing must therefore be Figure 10: able to resuspend the settled material for trans- provided, which can be done using RainJets or a port back to the sewer network and further to the combination of RainJets and mixers (see Figure 8). The dynamic wastewater treatment plant for treatment. cleaning effect where the water jet oscillates from Figure 8: side to side can be visualised using Step 2 – Mixing transient CFD and resuspension of calculations. settled matter. Rectangular tank: Circular tank: Switching on the RainJet ensures that the side Switching on the RainJet will start building the walls and the centre area of the tank floor are inertia of the circular movement which will kept free from deposits due to the mixing force clean the stormwater tank. In the direction of provided by the installed equipment. A properly the jet stream the settled matter will be resus- When depletion of the tank is close to being chosen RainJet resuspends most of the settled pend due to the mixing force provided by the completed the water level in the tank is between matter in the tank, with only the corners as an RainJet. Furthermore the side walls will also be 30 - 60 cm and the RainJet will be partially unsub- exception. scoured as a result of turbulence in the tank. merged and the operational cycle proceeds into the cleaning period (see Figure 11). During the depletion period the water level is re- thereby also decreases. However as the RainJet de- duced by pumping the detained water back to the livers an unchanged thrust force, this allows the sewer system (see Figure 9). The force required for RainJet to resuspend the remaining settled matter mixing the decreasing volume of water in the tank in places further away from the machine. Figure 9: Figure 11: Step 3 – Resuspen- Step 4 – Cleaning sion and clearance proximity of the of places further RainJet away from the RainJet in the depletion period Rectangular tank: In the cleaning period the length of the jet stream decreas- Circular tank: es, reaching approximately the centre of the tank where As the RainJet becomes unsub- Rectangular tank: Circular tank: a standing wave forms. This wave terminates the jet and merged the water jet length de- The excess thrust force during the depletion pe- During the depletion period the circulation spreads out the stream. As the water level decreases in the creases. This implies that the wa- riod is further assisted by an effect of a dynamic speed increases due to the excess power input. tank, the jet length from the RainJet similarly decreases ter delivered from the RainJet will cleaning motion where the water jet oscillates This ensures that all settled material at the tank until the hydraulic jump disappears and the jet stream be concentrated in the middle of from side to side, effectively resuspending set- periphery will be resuspended and dragged with simply protrudes along the tank floor. the tank flushing any remaining tled material in the corner of the tank furthest the water towards the centre of the tank. material at the centre into the away from the RainJet (Figure 10). The back flow of water is now generated from the centre of sump of the tank. the tank, gathering the solids in the proximity of the Rain- Jet and flushing them to the sump.
  • 7. 12 Stormwater tankS Stormwater tankS 13 The sump of the stormwater tank can now be com- For a tank having a water level of 4 meters, a Rain- pletely emptied by the outlet pump. All organic and Jet delivering a water flow of 25 l/s is insufficient to Velocity Velocity inorganic matters are now removed from the tank. create a high enough velocity – and therefore suffi- Streamline 1 Streamline 1 The tank is now thoroughly cleaned and eventual cient mixing – in the tank to provide an acceptable 0.500 0.500 odour problems are totally eliminated. coverage of the tank corners furthest away from the 0.375 0.375 RainJet. mixing 0.250 0.250 Increasing water flow to 75 l/s will result in energy The hydrodynamics of the RainJet makes it possible being wasted at the tank wall and water surface, as 0.125 0.125 to both control the mixing and the cleaning of the there is a high velocity when the contour colours are tank at the same time. In the depletion period the matched to the legend in the figure below. Howev- 0.000 [m s^-1] 0.000 [m s^-1] detained water must be mixed to resuspend the er, a performance of 50 l/s seems to be suitable, as settled matter and obtain an efficient cleaning. the water jet of the RainJet is sufficient to reach all The RainJet is capable of both. When submerged the way to the backwall with an appropriate veloc- the RainJet utilises the flow of water created in ity to sweep the corners, resuspending the settled the ejector to act as a powerful mixer, resuspend- material without wasting energy on the backwall stormwater tank will not concentrate so much that (see Figure 26), in order to design a solution where Figure 13: ing the matter in the tank due to the thrust force and water surface. they exceed the solids handling limit of the outlet the mixing capacity will not be oversized during and high power jet formed. At low water levels the pump. Furthermore, when deciding the water level most operational hours. Simulation of RainJet acts as a powerful flushing device, cleaning Even though velocities in the middle part of the for complete mixing, it is also necessary to take into the difference in the tank floor and walls from organic matter, as ex- tank seem low compared to a performance of 75 account that the RainJet has enough time for resus- emPtYing velocity streamlines, Figure 12: plained in the section above. l/s, this has no effect on the cleaning efficiency. pension of the settled matter in order to perform a indicating mixing This is due to the fact that this area of the tank is perfect cleaning. Emptying the stormwater tank after the storm capabilities, at dif- Velocity contour The equipment installed in the stormwater tank being cleaned during the end of phase 3 and start water runoff has ceased should be done at a pace ferent water levels plots showing what must be sized to deliver a large enough water flow of phase 4, where the water flow of the ejector is The simulation in Fig. 13 shows the difference in ve- that does not overload the sewer system and the when using the happens at different and thus a thrust force to mix the water volume concentrated in this area. Furthermore, at a perfor- locity streamlines and thus mixing capabilities at an downstream-situated wastewater treatment plant same power input RainJet capacities of the stormwater tank. Choosing a RainJet with a mance of 50 l/s the water velocity in the area above equal power input of the RainJet at different water that it initially relieved. This can easily be controlled of the RainJet. all other conditions too large water flow will not enhance cleaning ef- the sump is low which favours sedimentation. This levels in the stormwater tank. Based on the velocity using variable-speed pumping, which is regulated are equal. From the ficiency but only waste energy in for example the means that materials will collect in the sump where streamlines, it can be seen that the RainJet is not based on measurements of the actual capacity of top 25 l/s, 50 l/s and mixing phase of the cleaning cycle, as illustrated in it will be carried away through the outlet pipe. capable of mixing the entire volume of water at a the sewer system to provide a near-constant and 75 l/s shown in a Figure 12. water level of 4 meters, but as the water level drops controlled flow to the sewer system. plane 0.15 m above If instead of mixing the entire volume it is only nec- to 2 meters the mixing capacity of the RainJet will the bottom plate The velocity gradients, as shown on the simulations essary to mix a predefined volume of the stored wa- be more suitable to the stored volume of water. If the flow to the sewer system does not have any (left) and in a plane below, will give rise to a certain shear stress at the ter, a RainJet with a smaller capacity can be used. hydraulic limitation when emptying the stormwa- through the RainJet tank bottom, which at large velocities will be suf- However when choosing this solution it is impor- For stormwater tanks, a given number of RainJets ter tank, emptying should not be done at a pace axis (right). ficient to keep the bottom plate free from deposits. tant to make sure that materials detained in the will always be capable of providing the necessary faster than is required for the cleaning equipment thrust force to mix the complete volume of stored to have time to resuspend material and clean the water. However this may entail that an excess clean- tank. As the cleaning action of the RainJet is not ing efficiency is installed and mixers can instead be based on a single flush but on continuous flushing, used to supplement the thrust together with an ap- an extra flush time/cycle can be used if desired. To Velocity Velocity Contour 1 Contour 1 propriate number of RainJets. do this the outflow pumps must be discontinued 0.500 0.450 0.500 0.450 for a certain time, leaving an appropriate volume of 0.400 0.350 0.400 0.350 Whether the RainJet is used alone or in combination water in the sump for final flushing. When this ex- 0.300 0.250 0.300 0.250 with a mixer will be a consideration made on the tra flush time is completed the outflow pump can 0.200 0.150 0.200 0.150 basis of e.g. length and volume of the tank, physical once more be switched on, emptying the remaining 0.100 0.100 shape, and obstacles. Furthermore considerations volume of the detained water to the sewer system. 0.050 0.050 0.000 0.000 should be given to the typical filling volume of wa- [m s^-1] [m s^-1] ter to be stored according to e.g. a duration curve However the emptying time should also be per- formed at a pace so that the biological breakdown Velocity Velocity of organic matter does not create an odour issue FACTS: Contour 1 Contour 1 0.500 0.500 due to formation of anaerobic zones in the tank. 0.450 0.400 0.450 0.400 A balance between this and the above mentioned 0.350 0.350 factors should be established. 0.300 0.250 0.300 0.250 A mixer can only be operated when it is sub- 0.200 0.200 merged and it has a higher thrust-to-power 0.150 0.150 PUmP DeSign for emPtYing the tank 0.100 0.100 ratio than the RainJet. When the water level 0.050 0.050 0.000 0.000 drops to a certain level above the mixer, it [m s^-1] [m s^-1] Implicit in emptying (or filling) a stormwater tank is might create a vortex which lowers perfor- that huge variations in the hydraulic characteristics mance and creates unbalanced conditions of the system will be experienced. Velocity Contour 1 Velocity Contour 1 around the mixer. 0.500 0.500 0.450 0.450 Controlling the outflow provides some challeng- The RainJet on the other hand can work with es, as the hydraulic characteristics of the system 0.400 0.400 great performance even when it is unsub- 0.350 0.350 change drastically during drainage, from large flow 0.300 0.300 merged, as long as the suction pipe provides 0.250 0.250 0.200 0.200 with low head to low flow with high head. Pumping 0.150 0.150 enough suction pressure to overcome the 0.100 0.100 with a duty point too far either to the left or right of 0.050 0.050 required Net Positive Suction Head (NPSH). 0.000 0.000 the best efficiency point can result in reduced pump [m s^-1] [m s^-1] lifetime in addition to increased energy costs.
  • 8. 14 Stormwater tankS Stormwater tankS 15 Due to this huge change in duty conditions during However, the design of the stormwater system emptying specific demands are set when design- might have certain constraints that must also be ing the outlet system of the stormwater tank and it taken into consideration when draining the tank. h1 Max.water level h1: highest water level ΔHmax: highest differential headloss should be secured that the duty point will not reach For example, this could be desired emptying time or h2: lowest water level ΔHmin: lowest differential headloss these extremes of the pump curve. maximum and minimum flow. Qmax: highest flow A: area of the tank h2 Qmin: lowest flow T: required time for emptying When system conditions result in an operating Qin: inflow to the tank Kv: pipe flow resistance point exceeding the preferable operating range, this FACTS: Qin g: gravitational acceleration Min.water level modifies the pressure acting on one of the sides of Minimum velocity for riser pipe ρ: density the impeller. This will result in unbalanced condi- To make sure that sand and similar materi- tions, resulting in radial thrust which will deflect als can be pumped out of the tank through the pump shaft causing for example excessive load the riser pipe a minimum velocity, for Figure 16: on the bearings, deflection of the mechanical seal example 1-1.5 m/s, must be maintained, and irregular wear on these parts. depending on pipe configuration and 1) Pump hydraulics based on required emptying time and maximum outflow Schematic particle characteristics. This velocity cor- representation of If the duty point is too far to the left on the curve it responds to a minimum flow that must be the system defining can result in labile pump operation and excessive kept in the riser pipe. Assuming the average flow can be calculated as the Qmin + Qmax h – h1 how to calculate wear due to recirculation of abrasive particles in the mean between the maximum and minimum flow: =A 2 2 T pump hydraulics pump because they cannot be removed from the and emptying time. system. If the duty point on the other hand is too Obeying the constraints put on the system provides far to the right on the curve it may cause cavitations some challenges when selecting pumps due to the h2 – h1 due to an increase in NPSH. great changes in hydraulic characteristics. Using The minimum flow can then be calculated as: Qmin = 2A – Qmax T the model and equations below, it is possible to es- Controlling the outflow can be done using vari- timate: able speed controlled pumps or by gravity through motor valves, but in both cases the system curve • Pump hydraulics based on required emptying Figure 17: for the application will vary as shown in Figure 14. time and maximum outflow The two working points of the system (Qmin ; ΔHmin) H Pumps and motor valves are normally controlled via • Pump hydraulics based on required emptying are given by Working points of a flow measurements in the sewer system, so it will time and minimum outflow pump system. Note not be hydraulically overloaded during emptying of • Tank emptying time based on a specific pump (Qmax, ΔHmax) and (Qmin, ΔHmin) (Qmax ; ΔHmax) that with this nota- the tank. curve tion ΔHmax < ΔHmin as illustrated in Figure 17. h2 When gravity cannot do the job, frequency con- These estimations then show the specific required trolled pumps can match smoothly the actual ca- hydraulic performance for the pump, to make sure it h1 pacity of the sewer system during emptying (see stays within the acceptable duty area, even though Q Figure 15). large variations are encountered (see Figure 16). The headloss (ΔH) of these working points can be calculated based on maximum and minimum flow from the following equations: Figure 14: Min.water level ΔHmax = KvQmax² + ρgh1 H The great difference in h1 ΔHmin = KvQmin² + ρgh2 hydraulic charact- Max.water level Max.water level eristics when emptying a h2 EXAMPLE: h2 stormwater tank Given an application with the following conditions: is shown here. Min.water level h1 T: 11 hours = 39,600 sec Q h1: 6 m h2: 0 m Qmax: 0.1 m³/s Qin: 0 m³/s Figure 15: A: 500 m² The great differ- H Min.water level Capacity of Flowmeter sewer system The minimum flow reached during emptying within the given conditions is: ence in hydraulic characteristics h1 Qmin = 2 x 500 x 6 – 0 – 0.1 = 0.05 m3/s Max.water level VFD when emptying a Max.water level 39,600 stormwater tank. To h2 h2 respect the capacity 50 Hz The minimum flow should then at least be 0.05 m3/s. The two working points of the pump system can 40 Hz of the sewer system, 33 Hz then be found when knowing: h1 25 Hz the pump is being Min.water level ρ: 1000 kg/m³ controlled by a Q g: 9.82 m/s² frequency converter.
  • 9. 16 Stormwater tankS Stormwater tankS 17 Kv: 2,455,000 kg/m7 at Qmax phySical deSign Kv: 9,820,000 kg/m7 at Qmin of Stormwater tankS ΔHmax = 2,455,000 x 0.12 + 1000 x 9.82(8 – 6) = 44,190 pa ≈ 4.5 mWC* The physical design of the stormwater tank is of up force on an empty tank can be so powerful that utmost importance for obtaining trouble-free op- it might collapse or float. Coping with this can be ΔHmin = 9,820,000 x 0.05 + 1000 x 9.82(8 – 0) = 103,190 pa ≈ 10.5 mWC* 2 eration and is also important in regards to how ef- handled in different ways: by permanently lowering ficiently it can be cleaned. When thinking about the the water table, increasing the ballast of the tank or physical design of the tank, it is not enough to con- incorporating gate valves into the tank, which lets (0.05 ; 10.5) H sider only the dimensions, total volume of the tank in water if the water table rises. and for existing tanks the remaining lifetime of the concrete structure. Just as important is the physi- Bottom Plate (0.1 ; 4.5) cal layout of the bottom plate and benches as well as including a properly designed sump and drain- Design of the stormwater tank bottom plate is a age channel, among others. An illustration defining critical parameter for an efficient removal of solids 8m technical terms in connection with the physical de- from the tank. For automatic cleaning with a Rain- sign of a stormwater tank is shown in Figure 18. Jet, the bottom plate should have a slope of around 2m 1-4 % towards the tank outlet. In addition to the Below is an elaborated description of what is im- slope, the texture of the bottom plate is also impor- Q portant when designing the physical structures of tant. The plate should be smoothened to minimise the tank. roughness and avoid local depressions in the plate *)1 mWC = 9,806.95 Pa where water preferentially will run and form stream form anD ShaPe channels or impose additional flow resistance. 2) Pump hydraulics based on required emptying time and minimum outflow For the purpose of storm water detention, both new The determining factor for moving solids deposited In contrary to the above, the minimum required flow can be a prerequisite together with the desired time and existing tanks can be used. New tanks can be at the bottom plate to the outlet during step 3 and for emptying. The maximum flow of the pump can then be calculated to keep the required time and, simi- constructed specifically for the purpose whereas 4 in the cleaning phase is the shear stress (τ). The lar to the above, the two working points can be found based on the maximum and minimum flows. existing tanks with a little effort can be rebuilt to shear stress is the force acting between the flow meet the layout requirements. of water and the deposited material which can be h2 – h1 calculated as: Qmax = 2A – Qmin The form and shape of the tank does not limit T the usability for storm water detention, meaning square, rectangular or round tanks and other forms τ=γxRxI and shapes can be used for the purpose. This gives 3) Calculating emptying time based on specific pump curve the flexibility to use existing tanks or fit in new tanks wherever space is available. τ: Shear stress (N/m2) For a given pump or pump system it might be necessary to estimate the emptying time of the tank. This γ: Specific weight of water (N/m3) time can be calculated based on the maximum and minimum flow obtained from the pump operating Rebuilding existing tanks might provide some R: Hydraulic radius (m) points of the system by the following equation: challenges. Besides estimating the remaining life- time and the specific layout of the construction, I: Bottom plate slope (m/m) 2A(h2 – h1) the buoyancy of the tank must also be taken into T= Qmax + Qmin consideration. This should be considered, as the Figure 18: EXAMPLE: Given an application with the following conditions: Inlet pipe Bench Bottom plate Spillway Overflow The terms used in pipe connection with the h1: 6 m physical design of h2: 0 m an open rectangular Qmax: 0.1 m³/s stormwater tank Qmin: 0.05 m³/s Qin: 0 m³/s A: 500 m² Outflow pipe The time for emptying will be: Bottom slope T = 2 x 500 x 0.1 – 0.05 (6 – 0) = 40,000 sec ≈ 11 hours 0.1² – 0.05² Sump with benches
  • 10. 18 Stormwater tankS Stormwater tankS 19 Figure 19: The shear stress needed to ob- The actual slope of the tank will in practise be de- • The fact that the effective jet length of the Rain- tain self cleaning can vary sig- termined by several factors which together ensure Jet is inversely related to the slope of the bot- Bottom displace- nificantly depending on what that the desired displacement force is met. These tom plate, with regards to having sufficient fall 6 ment forces as a kind of material is contained factors are: towards the sump, so water and material will be function of slope in the storm water. This is in transported in this direction. 5 and flow. The simple regard to size, weight and tex- • Ensuring that the tank is dug as deep as is fea- spreadsheet model ture where greasy, and heavy sible 4 used to construct material consisting of large particles requires a higher τ to Displacement • Selection of the RainJet with the lowest possible the graph assumes 3 forces [N/m2] that the water flow- obtain self cleaning. In storm- power consumption to deliver the required jet ing from the back water sewers a τ of 2-4 N/m2 is 2 length and flow wall of the tank and normally considered sufficient back to the sump is for obtaining self cleaning and 0,080 1 equally distributed used as dimensioning criteria. 0,060 This is therefore also seen as a Flow [m3/s] 0 BencheS over the entire cross guide for the bottom plate of a 0,040 2,50% section of the tank. 1,50% 2,00% 0,50% 1,00% Figure 21: The model does not stormwater tank during final Slope [m/m] To avoid accumulation of solids in for example the take into account emptying. corners of the tank, flow patterns with least pos- that parts of the sible hydraulic losses should be established to mini- Velocity profile of cross section is taken mise required power consumption of the cleaning a liquid flowing over up by the water jet For stormwater tanks with relatively low water Furthermore it can also be seen that at a slope of equipment. When water flows over a surface the a surface. generated from the depth, the hydraulic radius can be considered to be 8% and a RainJet performance of 50 l/s the water velocity will decrease the closer the distance to the RainJet, which flows equal to the natural depth of water over the bottom jet partly terminates prior to reaching the back wall surface is due to the friction losses exhibited by the Distance to wall in the opposite plate. This depth can be estimated from the Man- as it hits the bottom of the tank. At the interception texture of the surface (Figure 21). direction. ning equation based on the surface roughness, bot- with the bottom plate the water jet looses energy. tom slope and water flow from the RainJet. This interception reduces the cleaning and mixing In corners or where the tank walls and bottom action of the jet which also is evident when com- meet, two surfaces are in close proximity, and this Visualised by a simple spreadsheet model increas- paring the velocity contours at the simulations. will decrease the velocity significantly due to the Figure 20: ing either the water flow through the RainJet and friction losses from both of these surfaces. The low thereby the hydraulic radius or the bottom slope Increasing RainJet performance to 75 l/s ensures velocity in this area will favour sedimentation of The bottom slope will result in increasing displacement forces, ensur- that the water jet reaches the back wall even at material. Of course, applying excessive power could directly influences ing that the tank will be self-cleaning. a slope of 8%. However the water jet has reduced well maintain a high velocity; however this cannot Velocity the jet length of the power for mixing and cleaning as some of the ener- be justified economically. Slanting benches could RainJet, where an However, increasing these two parameters infinitely gy is lost at the bottom plate which is evident from be made to minimise the effect of friction losses in increasing bottom is not practical or economical in relation to power con- the velocity contour plots when comparing the two these areas (Figure 22), and increasing power input slope decreases the sumption for example and may induce undesirable simulations with different slopes and RainJet per- ought not to be considered. These benches can ei- effective jet length. side effects that minimise cleaning efficiency. formance of 75 l/s. ther be made in the corner or as benches running Left: tanks with along the entire side of the tank. slope of 1%. Right: The CFD simulations in Figure 20 illustrate how the tanks with slope effective jet length of the RainJet is directly influ- of 8%. Top: RainJet enced by the slope of the bottom plate. From the performance of 50 flow velocity profile it can be observed that the jet l/s. Bottom: RainJet length in a tank with a bottom plate slope of 1% has performance 75 l/s. a better reach compared to a slope of 8% at equal All other conditions RainJet performance. are equal. Velocity Contour 1 Velocity Contour 1 Figure 22: 0.500 0.50 Benches in a 0.450 0.45 0.400 0.40 0.350 0.300 0.35 0.30 stormwater tank to 0.250 0.200 0.25 0.20 minimise friction 0.150 0.100 0.15 0.10 losses at corners and 0.050 0.000 0.05 0.00 to mimic natural [m s^-1] [m s^-1] flow patterns. Left: benches run- Velocity Velocity Contour 1 Contour 1 ning along both sides of the tank in 0.500 0.50 0.450 0.45 0.400 0.350 0.40 0.35 the entire longitudi- 0.300 0.250 0.30 0.25 nal direction. 0.200 0.150 0.20 0.15 Right: bench at the 0.100 0.050 0.10 0.05 corner at the wall 0.000 0.00 furthest away from [m s^-1] [m s^-1] the RainJet.
  • 11. 20 Stormwater tankS Stormwater tankS 21 The effect on wall shear stress in the stormwater the corner is eliminated. The remaining part of the DimenSionS tank when using benches has been simulated. Ve- side bench will first give maximum benefit during locity contour plots showing a corner of the storm- step 3 and 4 of the cleaning phase when the water Area water tank are presented in Figure 23 in situations level is low. Basically, the physical area of a tank, whether new Besides the height differences, available space (foot both with and without a bench. or old, does not limit the applicability for storm wa- print) also helps the decision whether to build shal- In this situation a similar effect will be seen as with ter detention and implementation of a subsequent low tanks over large areas or deep tanks that do not As can be seen the corner bench and the side bench the corner, where areas with low shear stress are hygienic cleaning. However it is not only the specific claim much land. However, building deep might be will exhibit more or less the same effect close to the eliminated, due to the slanting benches. tank area that must be taken into account when de- expensive due to a high groundwater level, for ex- back wall of the tank where the low shear stress in ciding which equipment to install in the tank, the ample, so the final design will be a compromise be- length/width ratio is even more important and ba- tween actual conditions on site, minimum required sically determines which type of RainJet to install. slope and maximum available foot print. In Figure 24 a RainJet capable of cleaning 78 m2 has been installed in a tank of equal area in two differ- ent positions. Velocity Contour 1 In the first position the length/width ratio of the 0.500 tank suits the water jet generated from the Rain- 0.450 0.400 Jet through all phases of stormwater tank opera- 0.350 0.300 tion; from mixing to the final cleaning phase. In the 0.250 0.200 first case almost all of the energy in the jet stream 0.150 0.100 will be utilised for mixing and cleaning, and subse- 0.050 0.000 quently the jet will be able to spread all the way to [m s^-1] the side walls during final cleaning. In the second position, where the RainJet is placed on the longest side, most of the power in the water jet is lost on the tank wall which means that less power will be Velocity Contour 1 available for mixing and cleaning. Furthermore, 0.500 when the water level is at the end of the depletion 0.450 0.400 period the water jet will not be able to spread all the 0.350 0.300 Zoom area way to the corners, leaving settled material at the 0.250 0.200 side of the tank. 0.150 0.100 0.050 0.000 [m s^-1] length/ eJeCtoR type Width Ratio RainJet Wa ~3 WateR/aiR Figure 24: RainJet WW ~2 WateR/WateR The tank surface area Wall Shear should be made in a cer- 0.50 Velocity Contour 1 tain length/width ratio 0.45 0.40 0.500 0.450 in order for the RainJet 0.35 The length/width ratio of the RainJet changes de- 0.400 to provide proper mixing 0.350 0.30 0.25 pending on the actual bottom slope. At a slope of 0.300 and cleaning. around 1 % the RainJets have the following specific 0.250 0.20 0.200 length/width ratios of their water jets. 0.150 0.15 0.100 0.10 0.050 0.05 0.000 0.00 When constructing new tanks specifically for the [m s^-1] [pa] purpose of storm water detention, it is therefore advantageous already in the design phase to con- sider what type of RainJet is intended for keeping the tank clean so the tank area can be designed in a Figure 23: specific length/width ratio. Shear stress contour plots to show differences in wall shear stress when using benches in a stormwater tank. Depth Simulations are made at high water level under steady state conditions and an increase in shear stress will be The depth of the tank is chosen based on the height Velocity expected as water level drops during the depletion period. differences in the catchment area and the actual Contour 1 0.500 depth of the sewer system. This height difference 0.450 0.400 Simulations show a zoom on one of the corners furthest away from the RainJet. sets the basis for tank layout as there is a limit for 0.350 0.300 the maximal acceptable back water level in the sys- 0.250 From the left: tank without benches; tank with bench in the corner; tank with bench running along the side. 0.200 tem. This results in either a shallow self-emptying 0.150 tank being used or, as used in most instances, a 0.100 0.050 deeper tank, where emptying must be done by 0.000 [m s^-1] pumps.
  • 12. 22 Stormwater tankS Stormwater tankS 23 Storage volUme When designing a tank, Qin must be chosen based inlet Figure 27: on the stormwater event that will give maximum Dimensioning a stormwater tank consists of es- volume for a predefined overload criterion. This can Guiding storm water into the stormwater tank can, Scumboards An example of a Inlet tablishing the storage volume that will ensure a be based on the overload frequency or repetition depending on the placement of the tank, be han- flow split structure desired equalisation in the runoff based on a given period. Qout on the other hand can be determined dled either by gravity or by using pumps. The inlet Overflow where water at a weir maximum overload criterion. freely but it is desirable that it is as constant as pos- should be constructed with backflow protection to flow exceeding the sible to avoid overload of the sewer system when avoid the stormwater tank putting an unintended outlet flow will be To make this calculation, the system can be simpli- the water is guided back. hydraulic load on the system that the tank is in fact diverted into the fied by only taking the inflow (Qin) and outflow (Qout) designed to relieve. stormwater tank for that corresponds to the storm water runoff flow. For As actual data for stormwater events in an exact temporary storage. a combined sewer system, the dry weather flow is area is seldom available, the stormwater events For off-line detention, the inlet structure can be de- The flow split struc- therefore not a part of the calculation. used for dimensioning the storage volume are signed as a weir overflow that functions only when ture can be designed based on validated constructed data or historical a predetermined flow in the sewer is exceeded (Fig. with or without The continuity equation for a tank during filling or stormwater events. These data contain information 27). The overflow can then be guided from the over- scumboards. emptying is described by: about intensity, volume, duration, frequency, etc., flow structure to the stormwater tank by gravity or To stormwater that can be used to estimate the storage volume. using pumps. tank dV dh Qin – Qout = =Ax Visualising the stormwater events on a duration For in-line detention, throttling in response to dif- Outlet dt dt curve or by similar representation can help to es- ference in pipe dimensions between inlet and out- Where V is the volume, A is the area and h is the wa- timate the necessary or cost-effective storage vol- let pipes will ensure that water will backup in the ter level in the tank. By integration of the continuity ume that will ensure that the predefined number of stormwater tank (Fig. 28). Here, gravity can normally Inlet equation the storage volume can be found as: overloads from the tank is not exceeded. From the be used to convey water both in and out of the tank. curve can be read how large the volume of the tank In-line detention requires that the backwater level hmax t1 should be to be able to store the water from, for ex- does not exceed the lowest points in the system it V = ∫ A x dh = ∫ (Qin – Q out) dt ample, 90% of all stormwater events, or for events relieves, if flooding of upstream areas is to be avoid- hmin 0 having a volume equal to or bigger than a defined ed. value. The duration curve can furthermore be used Where hmin is the minimum water level, hmax is the to estimate whether it will be beneficial to split up When guiding water to the stormwater tank, the maximum water level and t1 is the filling time. the storage volume of the tank into several sub vol- inflow must fulfil what may appear to be conflict- umes. This could be useful to consider if the tank ing tasks. The inflow should be as fast as possible to Inlet The needed storage volume for a certain in- and only becomes partly filled during most stormwater relieve the hydraulic load of the sewer system; and outflow to the tank can be graphically shown as in events as it will only be necessary to clean a section yet the momentum of the inflow should be limited Outlet Figure 25. of the tank and not the whole tank. to avoid that material in the tank is in suspension. Overflow Outlet to recipient During the filling period, sedimentation of organic matter is desired due to the risk of overflow, and Figure 28: Outlet turbulence in the tank should therefore be mini- dry weather mised. The only hydraulic disturbance in the tank An example of throttling in an in-line detention, Figure 25: during filling will be the inflow jet. Dissipating where the inlet pipe has a bigger diameter than flow (Q) the momentum in the inflow jet will decrease the the outlet pipe. When the capacity of the outlet is Storage turbulence, enhancing the settling of material in exceeded water will accumulate in the stormwater volume Inflow hydrograph illustrating how to estimate the necessary storage volume based the tank. Slowing down the inflow jet can be done tank. Qout on inflow and outflow of the stormwater using a weir overflow, a baffle, or other similar ar- Qin tank or upstream a flow split device. rangements. Figure 29: grit anD Stone chamBer Outlet Screen A grit and stone The significant amount of water that arrives at chamber can be time (t) the stormwater tank during rainfalls is powerful Filling period Emptying period integrated in the enough to drag large concrete chunks or other large sewer system before obstacles with it. Having these objects scattered on Waste the stormwater the tank floor will reduce cleaning efficiency as well collection tank in order to trap from screen as damage the installed equipment. In some cases, large contaminants Figure 26: the inflow can then be guided through a separate Volume (V) instead of letting reservoir where grit, stones, litter and other large them enter into the Graph used for estimating for example the objects can be captured, preventing this problem Overflow stormwater tank. volumen needed storage volume of a stormwater (Fig. 29). This reservoir must be constructed in a way Sediment from tank trap tank, the number of compartments needed that ensures organic matter does not settle but is Weir to minimise cleaning, or the number of dragged into the stormwater tank, to avoid odours wall Designed expected overflows based on a certain tank from the chamber. volume size. Occurrence is defined as the number of tank To stormwater of events that give rise to a water volume Independent of chamber design, an access must be tank equal to or bigger than a certain volume, constructed in order to empty the chamber for the divided by the total number of events. trapped material at regular intervals and for service Inlet Occurrence (n) inspections.
  • 13. 24 Stormwater tankS Stormwater tankS 25 oUtlet If the outlet flows uncontrolled from the tank When using spillways or overflow weirs, floatables Figure 31: through an outlet pipe, a flow regulator device is and other suspended pollutants will be washed Outlet to Scumboard recipient From the stormwater tank there must be at least often incorporated to work as a brake for the capac- to the recipient. To minimise this pollution, the Examples of a skim- two outlets; a controlled outlet and an emergency ity. However this will not adjust precisely to the ac- overflow arrangement should be equipped with a mer arrangement outlet. tual capacity of the sewer system but in some cases skimmer and/or a screen (Fig. 31). Screens can be in- that will detain serve the purpose (see Figure 30). stalled either vertically or horizontally, with or with- most floatable When storm water inflow ceases and the emptying out build-in skimmers. Depending on the design of material in the period is initiated, the stored water should be guid- emergencY oUtlet the emergency outlet, one type may be preferred stormwater tank. ed back to the sewer system where it is conveyed to over another. Independent of installation, the ma- Spillway the treatment plant. This must be done in a man- Sending untreated wastewater to the recipient is terial captured on the screen must be automatical- ner to ensure the sewer system is not overloaded. not desirable. The stormwater tank’s function is ly returned to the detained water in the tank with Using a controlled outlet provides the opportunity to accumulate polluting material and reduce the which it will be conveyed to the wastewater treat- to control and regulate the emptying of the stored number of overflows from the tank. It is however ment plant. This is to minimise the need for service water in consideration of the actual load and ca- not economically feasible to construct stormwater of the tank. pacity of the sewer system. In advanced control sys- tanks that are large enough to cope with all storm- tems, where wastewater flows in citywide systems water events and an evaluation based on the water Depending on the water quality goal for the receiv- are optimised continuously, other parameters such quality goals of the recipient. The risk for overflows ing water, other arrangements, such as flow regula- as the water levels in other storage tanks can de- from a tank with a certain storage volume is based tors to minimise erosion of the recipient or equip- SUmP anD rUnoff channel cide the outflow pumping operation of the specific on statistical data for 10, 50 or 100 year storm events ment for disinfection, must also be considered at tank. Using pumps for a coordinated control of the as well as the duration curves. the emergency outlet. The sump and runoff channel must be constructed sewer system thereby enables that advanced con- in a way so the detained water with suspended so- trol strategies can be used to optimise and manage The storm water in excess of the storage volume When the water in the tank reaches a level where it lids is conveyed to the outlet pump or gravity outlet wastewater flows in the entire system. However the needs to be directed safely and in a controlled man- spills over the overflow weir, a backwater effect in independent of outlet capacity. It is important that outflow capacity and pipe dimension should still be ner out of the tank, instead of causing a hydraulic the tank will occur. This level, h, can be estimated by the sump contains an edge/weir that the sand can designed and constructed so the outlet structure is overload of the sewer system. The stormwater tank the following equation based on the water flow, Q, fall over. This will prevent sand accumulating at the capable of guiding suspended solids out of the tank must therefore be equipped with an overflow ar- and width of the weir, B. The coefficient C is specific bottom plate around the outlet and behind the without settling and thus blocking the outlet. rangement, such as a spillway or a pump, which can for the dimension and design of the weir and must cleaning equipment. at least discharge an amount corresponding to the be chosen accordingly in some cases it must be de- Moreover, the use of pumps or motor valves gives maximal inflow to the tank (Fig. 6). termined experimentally. The sump can furthermore increase operation time the opportunity to refine cleaning of the tank by re- of the RainJet and hence cleaning time. This can be ducing the outflow. This reduction should normally Positioning of the overflow arrangement should be done if the RainJet is equipped with a water inlet be done in the last period of the cleaning process at done so that a short cut from the inlet to the emer- Q = C x B x h x √2 x g x h pipe that draws in water from the sump in a level low water level. Setting the actual volume of water gency outlet is avoided. This can be done by plac- below the bottom plate figure 33. during flow reduction must be done in relation to ing them far a part from each other, with hydrau- The expected backwater level should be estimated the total storage volume. The volume left during lic streamlines taken into considerations. Where and it must be taken into account when placing If the RainJet does not have a water inlet pipe and cleaning must be negligible compared to the total possible, the inlet and emergency outlet should be the overflow arrangements in order not have water must be stopped before the water level is below the volume in order to cope with any possible stormwa- placed in different sections of the tank. spilling over the sides of the tank. This means that bottom plate, some of the suspended material in ter event that can take place during this process. the tank walls must be higher than the maximum the remaining water will have time for settling at During the filling of the tank, the intention is that water level, thus a certain volume of the tank can- the bottom of the tank (Figure 33 left). By a careful control of the outlet, the RainJet works suspended solids settle at the bottom of the tank. not be used for storage. In addition to this back- as a continuous flush cleaning system that can be When using pumps to cope with the overflow, it is water level a certain safety margin is also included As the slope of the tank for practical and economi- Figure 32: tuned to the actual conditions and demands com- therefore not advisable to pump from the bottom when placing the weir, making the storage volume cal reasons is not steep enough, the passive grav- pared to single flush systems. This gives an oppor- of the tank, as large amounts of settled matter will even less as the difference between weir and edge ity water flow cannot drag the settled material to Sketch showing the tunity to run an optimal cleaning process where the be flushed out to the recipient, and this is to be of the tank is increased. the sump which means that the tank will not be additional storage exact energy needed to obtain complete cleaning avoided. Instead, pumps with large flows and most cleaned completely. volume gained with is matched specifically. This is obtained through a often low heads should be installed to pump from To utilise the entire volume of the tank, tip-weirs a tip-weir installed predefined cleaning time calibrated according to the uppermost volume of the stored water. can be installed. This will make it possible for the at the emergency the actual demands. water to backup to the top of the tank, thus giving outlet in contrary additional time for settling of material, before an to having a weir eventual overflow occurs (Fig. 32). overflow. Figure 30: H Additional storage A water brake can dissipate the outflow mo- (Qdim.Hdim) volume during Max. backup Lost storage Lost storage filling mentum. The water flow is guided tangential- water level volume volume ly into the volute of the hydro brake where a Start vortex will form. The resulting high peripheral Start overflow velocities will create an air-filled core which overflow will result in increased back pressure that Storage Storage reduces the discharge flow to the designed volume To recipient volume To recipient maximum limit. Q
  • 14. 26 Stormwater tankS Stormwater tankS 27 mon pump to feed several ejectors, or outlet pumps Figure 35: ! if these are necessary. By installing equipment in the runoff channel, the amount of equipment in- Slanting benches in stalled at the bottom plate is minimised and this a wall placed runoff means far less additional flow resistance and fewer channel. Note that places for unwanted sedimentation. the benches start below the bottom Installation of equipment in the sump should plate so the material be done in a way so suspended solids carried to in the tank can fall the sump do not accumulate and block the out- into the sump and let. This requires that the distance between indi- get flushed away. vidual pumps as well as pumps and walls should be at least equal to the free passage of the pump. However, larger impurities might be dragged into the tank depending on upstream measurements, Figure 33: However, if the RainJet is able to flush until the wa- to the outlet. It is furthermore important that the which should also be taken into account during po- ter level has dropped below the bottom plate, it will benches are constructed so they have a clearance sitioning. The effect of first stop when the remaining water is present in height up to the bottom plate (Figure 35). This will having an inlet the sump (Figure 33 right). The limited volume of allow solids that are dragged to the sump to fall into Self cleaning of the rUnoff channel water pipe on the water left in the sump will give a relatively short de- the runoff channel and get flushed away. Without pump is that clean- tention time, limiting the time for settling of mate- this clearance height solids might be flushed back As no actual flushing by the RainJet takes place in Increasing the outlet flow will yield a higher bottom ing can proceed rials. Together with the slope of the sump, this will up on the bottom plate and deposit at the walls be- the runoff channel it can be compared to a sewer shear stress in the runoff channel as expected. The until the tank is ensure that all suspended materials are conveyed hind the RainJet. pipe, and therefore it must be designed to be self magnitude of the outlet flow needed to obtain the practically empty so back to the sewer system. cleaning. This ensures that all material dragged necessary shear stress in the sump depends among the entire bottom The actual tank shape and positioning of the sump down from the bottom plate will be conveyed to the other things on the slope. This therefore implies plate can be The sump can be positioned in different ways in the provides the basis for how cleaning equipment pump sump or gravity outlet and removed from the that the slope and outlet flow somehow should completely cleaned. tank. For rectangular tanks it is often placed at one should be installed for efficient cleaning. Choosing stormwater tank. be harmonised to ensure an efficient cleaning. The end of the tank walls or traversing the tank at the one or the other positioning of the sump in a cer- RainJet will furthermore recirculate a lot of water middle. For circular tanks it can be positioned at a tain tank shape depends, among other things, on Self cleaning of sewers is often linked to water ve- during the emptying period, which also will aid in tank wall or at the centre of the tank (Fig. 34). the desired accessibility of equipment and piping, locity and for pipes in sewer systems self cleaning is flushing the sump. construction costs and maximal depth of construc- obtained with a water velocity above 0.8 m/s. Thus Independent of sump layout, effective removal of tion. keeping this velocity as a minimum in the runoff At a small outlet flow there will be a risk that mat- material in the runoff channel is best obtained if channel of the stormwater tank will ensure that de- ters collected in the sump will build up and ulti- benches are constructed to mimic hydrodynamic Another important issue when designing the sump tained material is conveyed away from the sump. mately block the outflow pipe due to a too low flow patterns, thus reducing areas where sedimen- is the fact that there must be space reserved for the shear stress in the channel. Therefore it is crucial to tation can take place and helping to guide water installation of equipment such as RainJets, a com- Water speed is not a completely reliable measure for design the sump with a high enough outlet flow to the ability of water to carry material, as the mean make sure that matters collected in the sump are speed for different cross sections and diameters will effectively conveyed to the sewer system through give different carrying abilities. On the other hand, the outlet pipe or pump. self cleaning of the runoff channel can be expressed Figure 34: as the shear stress (τ) acting at the bottom surface Also during final cleaning, when the water falls Figure 36: as explained above (see paragraph about bottom from the bottom plate and into the sump, the self Sump positioning plates on page 17). cleaning will be enhanced as the turbulence cre- Bottom shear in different tank ated by this action will keep material in suspension stress in the sump shapes. Left: wall To illustrate how the outlet flow influences the self- in the water while it flows to the outlet. with a slope of 8% placed sump in a cleaning ability during final cleaning, simulations at different outlet rectangular and varying this parameter have been made. The simu- flows. circular tank. Right: lations show the bottom shear stress in the runoff From left 50 l/s, traversing sump in channel which is emptied by gravity through an 200 l/s and 400 l/s. a rectangular tank Sump outlet pipe. However the same would be the case if and centre sump in a pump has been used for the purpose. a circular tank. Sump Wall Shear 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 [pa] Sump Sump
  • 15. 28 Stormwater tankS Stormwater tankS 29 Figure 37: oPen/cloSeD tank configUrationDifference in maximal Stormwater tanks can either be constructed as open layout of RainJets in the tank, such that these can depth, depending on or closed structures. Choosing one or the other con- be placed to minimise dead zones around pillars.where the outlet from figuration will be a balance between several factors, the sump is placed. which usually end up decided by location of the When a covered tank is filled, washed down and 4% tank. Some of the things to consider when choosing emptied with wastewater, there must be effective Difference in depth of either of the configurations are described below. ventilation that ensures the installation with a construction 0.18 change in air volume that matches the changes in Open tanks water volume, so the construction is not damaged Open tanks offer the possibility to easily control by pressure differences, and which also ensures conditions in the tank, offer simple inspection of that gases do not accumulate in critical concentra- the cleaning process in the tank, and a relatively tions of a toxic or explosive character. 4% uncomplicated access to the tank. Ventilation to remove toxic or explosive gases is implicit in the The ventilation system should of course be installed design, although dependant on depth. Besides that to ensure that the speed of air exiting the system there are typically few or no obstructions, such as does not create an unreasonable level of noise, DePth of conStrUction Wall-placed sump pillars to support a cover that will interfere with the which in built-up areas may well be regulated. The runoff channel must be designed taking into cleaning jets. This can be done by building ventilation tracts or The higher the shear stress that can be obtained consideration the water velocity of the arriving openings of a sufficient size between the tank and in the runoff channel the more certain one can be water. If the velocity of the water running to the However, there are also consequences which can in- surroundings, and by ensuring that placement of that material will be carried away. However, the runoff channel is too high and the flow cannot be clude inconvenience from noise and odours that af- these create as little disturbance as possible. It may actual slope of the channel will be a compromise absorbed by the channel, a reflection wave might fect surrounding areas, and the open tank requires be necessary to equip the ventilation with noise re- between the possible depth and the economics of arise which will flush water back up on the bottom closing off the tank with a security fence, to ensure duction to ensure that operation does not exceed construction, compared to the ability to obtain self plate, possibly depositing solids. that people and animals do not fall into the tank. noise levels. cleaning. On this basis, it could be tempting to create the run- Closed tanks The equipment installed in the tank is exposed to The maximal depth of construction might set the off channel very wide or deep. However, creating a Closed or covered tanks offer the possibility for re- various influences, depending on whether the tank basis for how the sump should be designed; wheth- wide channel will lower the water velocity during ducing problems due to noise and odour for sur- is open or closed. Closed tanks are generally better er the channel should slant to one of the sides of the final emptying of the channel, which can favour rounding areas, avoiding aesthetic inconveniences, protected against many influences, whereas equip- the sump or to the middle (Figure 37). sedimentation in the sump. Making it too deep will and they can therefore be placed in urban areas ment in open tanks are exposed to weather such as on the other hand increases construction costs. As without putting up security fences to keep people sunshine, rain, snow, frost, heat and cold. For several tanks placed together or a tank split in a result, the width and depth of the channel is a and animals out. Furthermore, they can be designed many sections, basically three possibilities of the compromise between the ability to absorb the flow so that it blends neutrally into the surroundings by These extremes affect the installed equipment. outlet location exist, as shown in Figure 38, which arriving from the bottom plate, still being able to placing it underground. This means that attention must be given to these will result in different construction depths. keep an appropriate velocity, and keeping a reason- influences, to ensure sensors, pumps (including able construction price. Covering the tank can, however, have the conse- chains and cables) can handle drying out in direct As exemplified in Figure 37, the rightmost sump quence that maintaining a visual control of condi- sun and still be able to function. Pumps can stick configuration in Figure 38 gives the smallest possi- Centre sump tions in the tank is often difficult. Solving such an or rust. They must also be able to handle frost, or ble construction depth when using identical slopes During cleaning of a circular tank, the water circu- issue requires installing hatch covers. These must be protected against frost damage. Alternatively, in the sumps. lation created will force the water to arrive auto- enable viewing into the tank, allow for servicing of sensors can be used without wired contact, such as matically at the sump due to the outlet flow. In the machinery, and provide service access for the en- ultrasound or radar. Frost can also limit operation, However, even though it is possible to minimise the middle of the circulating water volume, the veloc- tire tank (see also the section about Service Access, for example with icing-over of the tank or the freez- construction depth by placing the outlet centrally ity will approach zero, which results in suspended page 30). ing of equipment. Regular inspection and testing of in the sump, other factors might influence this as solids effectively settling at the centre of the tank screens and sluice gates are also necessary. well. The factors that should be considered Inlet for are, where the sump is placed. Normally a cover also requires supports in the form example, pipework, material and equipment need- of pillars in the tank. These supports create friction ed, placement of the adjacent sewer system as well Independent of whether pumps or gravity are used for water circulation during cleaning and make a as interconnection between other tank sections for emptying the tank, the bottom of the sump thorough cleaning around these structures diffi- Inlet when choosing the outlet placement. What Inlet be will should preferably be designed with benches to cult. Placement of these ought then to be known the best design must be evaluated in each case. mimic natural flow patterns and thus prevent sedi- in the design phase for the specification of size and mentation inside the sump. Outlet Inlet Inlet Inlet Figure 38: Outlet Inlet Inlet Different possibilities for Outlet Inlet placement of the Outlet outlet from the stormwater tank. Outlet Outlet Outlet Outlet Outlet Outlet Outlet Outlet Outlet
  • 16. 30 Stormwater tankS Stormwater tankS 31 oBStacleS Service acceSS • How is heavy equipment to be lift- ed? An efficient cleaning of the tank requires a design Regardless of whether the tank is built with or with- • Should the lifting equipment be per- with a minimum of hydraulic losses in order to min- out a cover, the design ought to include the possibil- manently installed or mobile? imise the number of dead zones in the tank, and ity of access to the tank. This makes it possible for: • Should pumps be secured to the also minimise the energy required to obtain clean- bottom of the tank or installed on ing. Pillars, columns, ladders or other obstacles • General inspection of the tank and cleaning auto couplings? protruding from the tank floor, walls or cover will quality • At what level is the tank placed increase the losses and create possible dead zones • Cleaning out the tank manually of small stones above or below terrain? in the tank, and these are areas where undesirable and the like, which may have slipped by • How far from the tank edge should sedimentation during cleaning will take place. Be- • Servicing mechanical equipment and other mechanical equipment be placed? sides altered flow patterns the dynamic cleaning installations, such as RainJets, pumps, grates and • Is vehicle access to the bottom of with the sweeping effect of the water jet is also im- spillways the tank required? peded in this case which will compromise cleaning • Should the tank configuration be at the corners of the tank (Fig. 39). Access to the tank is normally by steps or a ladder, open or closed? normally mounted permanently in the tank (Fig. If obstacles in the tank cannot be avoided, these 40). Thought should be given to whether the fixed Selecting a solution must of course be must be taken into consideration during the layout installation will inevitably capture organic matters, based upon ensuring a good environ- of cleaning equipment in order to obtain the opti- fibres, hair, cloths, etc, which can be washed in with ment for working and maintenance mal cleaning process given the actual conditions. storm water. This can make it unpleasant, danger- around the tank. ous and even impossible to use the steps or ladder In some cases the design of the stormwater tank until they are cleaned. This needs to be thought into the de- requires that the pump and ejector of the RainJet sign from the very first draft, ensur- must be separately installed. The piping connect- An alternative is to use hinged ladders, or ladders ing that the handling of mechanical ing the two parts will create friction losses in the mounted to floats, meaning the ladder moves in equipment is dealt with at the start, system if mounted directly on the tank floor. There- accordance with a rising or falling water level. The and how this influences design of the fore this piping should be mounted 15 - 20 cm above ladder is thus kept out of the wastewater and can bottom plate, the placement of clean- the floor, which will allow detained material to be therefore be used without cleaning in advance. ing equipment and the effectiveness flushed underneath the pipes and further on to of cleaning. the sump. However supports for the piping will still When designing a facility with a stormwater tank, gather some material during emptying that cannot the handling of mechanical equipment for instal- be flushed away. lation and service should also be thought into the Figure 40: design, with the following points taken into consid- The connecting pipes of the RainJet could ultimate- eration: Different possibilities for installation of ladders at a stormwater tank. ly be moulded into the tank floor, which will reduce Top ones are fixed installations that during storage will be sub- completely friction losses and dead zones caused by merged and collect fibrous material from the detained water. the additional piping and supports and also places where material can get stuck. Bottom examples are installations that are able to keep the ladder out of the water either manually by raising the ladder by a hinge or by means of a flotation device mounted on the ladder. Summary of design criteria for physical layout of tank for Figure 39: cleaning equipment Velocity contour plot 0.15 m above the tank bottom showing the effect on hydraulic flow patterns when the tank has pillars at the tank floor to support the cover. Besides altered flow patterns the dynamic cleaning with Based on a summary of the elements described above, the following list extracts some of the most important layout criteria. the sweeping effect of the water jet is also impeded in this case. This list is not exhaustive, as the final layout should also carefully evaluate actual conditions onsite. • Design the tank dimensions or individual sections so it matches the performance of the cleaning equipment • Consider water capacity, direction/positioning and number of the inlet(s) Velocity • Consider the amount of sand and suspended solids in the incoming water Contour 1 0.500 • Create benches in corners and along the walls to mimic natural hydraulic conditions 0.450 0.400 • Create a smooth bottom surface as this will increase cleaning performance 0.350 0.300 • Avoid obstructions of any kind to minimise dead zones that will favour sedimentation 0.250 0.200 • Design a sump in connection with the outlet 0.150 0.100 • Design the sump, connections and outlet system so it is self cleaning 0.050 0.000 • Let the RainJets draw in water from the sump to increase flushing time [m s^-1] • Consider intensity of outlet, retention and emptying time • Consider duty strategy for tank operation
  • 17. 32 Stormwater tankS Stormwater tankS 33 rain JetS 10 m 10 m Thrust force needed 650 N Thrust force delivered 300 N Figure 42: Two types of RainJets are available for cleaning The jet length of the ejector is determined by the Considerations of using RainJets alone stormwater tanks; the RainJet Water/Air (wa) and the RainJet Water/Water (ww) (Fig. 41). first stage. A higher flow through the first stage of the ejector results in a greater velocity and head 6m 6m Jet length 4 Suitable or in combination thus a longer reaching water jet. For the RainJet 10 m with a mixer. Thrust Not The WA ejector is self aspirating and introduces air into the jet flow. By introducing air into the mixing WA a second stage can be mounted to increase the thrust force produced by the first stage of the ejec- 8m 8m force 7 sufficient zone, odour levels can be minimised, because for- tor. Here a secondary water flow will similar to the 6m mation of anaerobic zones in the tank will be limit- RainJet WW be dragged into the ejector pipe. This 10 m ed. This is necessary if water is detained in the tank additional intake of water will increase the total 10 m Thrust force needed 650 N for a medium-long period during which biological water flow from the ejector, and thus increase the 8m Thrust force delivered 650 N activity can take place and use up all available oxy- thrust force. 6m gen. With the jet provided by the RainJet WA, this 6m type of ejector is designed to clean tanks with a length/width ratio of ~ 3 (long, narrow tanks). conSiDerationS for Sizing 10 m Jet length 7 Too long When sizing RainJets it is important to consider if 20 m 20 m Thrust The RainJet WW has been developed for the clean- ing of wider stormwater tanks than the WA ejector. the main purpose is cleaning or mixing. When sizing for cleaning purposes, the maximum jet length is 6m force 4 Suitable The WW ejector cleans tanks with a length/width the critical parameter, whereas for mixing purposes 10 m ratio ~ 2. Contrary to the WA RainJet, the WW Rain- the thrust force is the critical parameter. However, For illustration of the effective jet length produced by the specific RainJet, the water jet has been shown to penetrate the 10 m wall even though this would not possible in reality Jet does not introduce air into the water, but uses the application will almost always demand both 20 m all energy to drag in a secondary water flow. It will parameters, and there will most often be a discrep- 6m increase the total ejector water flow and create a ancy between them as shown on Figure 42 and Fig- 6m powerful jet. As energy is not used to introduce air ure 43. Satisfying both parameters will often result Thrust force needed 650 N 10 m into the water, the WW ejector has a better mixing in a trade off with one of them. Thrust force delivered 650 N efficiency than the WA ejector with the same power 8m 8m input. However, the length of the water jet will be Thrust force shorter than for the WA ejector due to difference in In cases where the thrust force is insufficient for the 6m Jet length 4 Suitable construction. For that reason the WW ejector is rec- application but the jet dimension fits, two methods ommended when the retention time of the water is can be used to solve this issue. Thrust short to medium, or when odour problems will not 8m force 4 Suitable become an issue. Either a larger RainJet with a sufficient thrust force can be chosen even though the jet length will be When the ejector works, the pump provides the pri- too long for the tank as long as the jet width still fits mary water flow in the first stage of the ejector. The the tank. Or installation of one or more mixers to FACTS: In some cases it is also possible to move the ejector first stage is constructed with a reduction nozzle in- deliver the remaining thrust force to meet what is forward in the tank so it is adjusted to the jet ratio side the ejector pipe or as a nozzle in itself. Accord- needed in the application can be a possibility. With Maximum thrust: of desired equipment (see Fig. 43 bottom (right)). ing to the law of mass conservation and Bernoulli’s the last method the mixers must however be turned A rule-of-thumb is that with the maximum However in these cases it must be checked that the principle, this reduction creates a depression over off when the water level decreases and the RainJet thrust force delivered by a RainJet WA or RainJet can deliver a sufficient volume of water to the nozzle that in the RainJet WA will drag down air must then be able to produce sufficient thrust for WW, a maximum volume of approximately be able to overcome the necessary bottom displace- and in the RainJet WW will drag in a secondary wa- mixing the water at this point. Consideration of 1,500 m³ pr. RainJet can be mixed. However, ment forces to drag settled material to the sump ter flow. The primary water flow mix with the air or whether to install additional equipment should it depends on the specific application such as (see Figure 19). Furthermore, it must be carefully the secondary water flow in the ejector and create however be done before choosing this solution. tank shape, suspended solid concentration, considered whether flow conditions in the tank will a strong jet stream for cleaning and the thrust force tank physical design, and so on, and must be be changed and favour that sediment builds up be- for mixing the detained water. evaluated for each application. hind the ejector instead of reaching the sump. As can be seen on Figure 45, pushing the ejector for- ward in the tank means that more energy will be Jet length anD wiDth wasted at the tank wall instead of being used for Figure 41: mixing and later on cleaning. This wasted energy There can be situations where the thrust force pro- results in a large area behind the ejector having a The principles by duced by the RainJet is sufficient to mix the water low water velocity, where sedimentation of mate- which a RainJet WA but where the jet stream cannot reach the furthest rial will be favoured. (left) and RainJet end or cover the entire tank floor (see Figure 43), be- WW (right) work. cause of the physical layout of the tank. This can be However pushing the ejector slightly forward as mixed Secondary caused by too steep a slope, an unfavourable ratio shown on the simulation in the middle does in this mixed Secondary water water flow water water flow between width/length, or result from the direction case not affect flow velocities significantly. This and air of the slope and position of the sump and outlet. could then be a solution for cleaning a tank that has a minor mismatch between actual required clean- These situations can be handled by splitting the ing surface and the optimal length to width ratio depression zone depression zone tank into different zones or moving the ejector for- of the ejector. ward to obtain the ideal relation between the per- primary water flow primary water flow formance of a RainJet and the tank width/length ratio.
  • 18. 34 Stormwater tankS Stormwater tankS 35 Figure 43: recommendation How to for inStallation and overcome poSitioning unfavourable length/width Installation of the RainJet will always be done within the ratios stormwater tank whereas the accompanying pump can be either wet or dry installed. Which is the preferred installa- tion type is individual from project to project. If a wet instal- lation is the choice for the pump it has, besides permanently fixation to the ejector, also the possibility to be coupled to the ejector with an auto coupling (Fig. 45). For stormwater tanks, various different shapes exist and are still being projected. The following provides a guide of how to place RainJets in rectangular and circular tanks and gives ideas about how many RainJets preferentially should be in- stalled. However it is beneficial to have a certain freedom in the RainJet installation following commissioning in order to adjust it according to the actual conditions in the tank. Figure 45: As shown in figure 42 and figure 43, various situ- Fixed (left) and ations are possible. Sometimes the jet length fits, auto coupling (right) sometimes the thrust force. The situations illu- The final selection of RainJets Selection anD Sizing mounting on the strate the intricacy in sizing RainJets and the con- Inquiries for RainJets are handled based ejector of a RainJet siderations that should be made. as well as their exact positioning WA and WW. must be project-specifically on customer needs. For selection and Different methods of adjusting both parameters exist. However, adjusting one of them might affect handled in each case to obtain sizing of equipment, please find your the other and vice versa. This must be thoroughly local Grundfos sales office at: considered to obtain the most suitable equipment the best possible solution for for the solution. the specific tank. www.grundfos.com Figure 44: rectangUlar tankS CFD simulation Velocity Contour 1 Dimensioning criteria for RainJets showing how The following key points should be considered when dimensioning RainJets for rectangular tanks. streamlines and 0.500 possible areas for sedimentation in 0.450 • The bottom slope should be between 1-2 % floor with a the stormwater 0.400 towards the outlet. RainJet Wa RainJet WW slope of 1 % tank change due to 0.350 • The length/width ratio of the tank if a certain Tank Tank different positioning Area 0.300 type of RainJet is preferred or recommended. Dimensions Dimensions of the RainJet. [m2] (3:1) (2:1) Contour plots are 0.250 • Base dimensioning on jet length and thrust shown 15 cm above force. 10x3 30 8x4 0.200 the bottom plate 121x4 50 10x5 with the ejector 0.150 • In tanks with pillars or similar two or more having equal RainJets should be used and placed to avoid 15x5 75 12.6x6 0.100 performance in dead zones and resulting sedimentation 0.050 problems. 18x6 100 14x7 each case. 0.000 21x7 150 17x8.5 • Taking in water from a sump or controlling [m s^-1] the outlet capacity can increase operation 24.5x8 200 20x10 time for flushing. 27.5x9 250 22.5x11 • Installation should be made so it can be 30x10 300 24.5x12 trimmed according to actual conditions. 31.5x10.5 350 26.5x13
  • 19. 36 Stormwater tankS Stormwater tankS 37 PoSitioning circUlar tankS When designing a rectangular tank the with SUmP PlaceD at the wall W RainJets can be placed either at one end or in the middle of the tank depending on For circular tanks with the sump placed at the wall, the tank length and sump positioning (Fig. the tank should be treated as a rectangular tank 46). The bottom slope of the tank should obeying the length/width ratio of the RainJets. The L decrease between 1 – 2 % towards the tank floor must as for rectangular tanks have a slope W/2 W/2 sump. of 1 – 2 % towards the outlet/drainage channel. For the RainJet WA, the appropriate ratio For diameters larger than one RainJet can handle, between length and width should be close the tank can be split up into two or more zones. W to ~3. The RainJet WW generates a wider For example, when using two RainJets they should jet compared to the RainJet WA, therefore be placed in an angle of around 20° to the centre the RainJets WW are more suitable for axis of the tank. Another example of placement of 3 L rectangular tanks with a length/width ra- RainJets are also shown in Figure 49. L W/2 tio of ~2. The jet length and width must 2 W/2 correspond to the tank dimensions for effective cleaning. In longer or wider tanks where one RainJet L does not fulfil the dimensional require- ments, the RainJets can be placed ahead or besides each other (fig. 47). For wide tanks partitions between different sec- tions might be installed when RainJets are placed next to each other. This divides the tank into more sections that will fill suc- Figure 48: cessive as one section at a time is filled be- fore overflow into the next section takes Principle for place- place. This will minimise the need for ment of RainJet(s) cleaning the whole tank every time, if only in circular storm- a sub-volume of the tank becomes filled water tanks with Figure 46: (see Figure 6). sump placed at the wall Principle for Figure 47: placement of Rain- Jets in rectangular Principle for stormwater tanks W 2W placement of more than one RainJet W in wide or long L stormwater tanks W/2 W/2 grundfos WebCapS W go to the grundfos online WebCapS, L Computer aided product Selection 2L program, to access more than 180,000 grundfos products complete with data and Cad drawings. L W/2 W/2 www.grundfos.com/webcaps
  • 20. 38 Stormwater tankS Stormwater tankS 39 circUlar tankS with centre SUmP This placement offers the advantages of a jet stream Placing the RainJet close to the sump: Placing the RainJet close to the tank wall: that follows the flow direction very smoothly, which This placement has several advantages, such as This placement has the advantages of offering easy Dimensioning criteria for RainJets per the principle of leverage requires a comparable the possibility of flushing the tank until completely access to the RainJet/pumps as they are close to The following key points should be considered when low energy input. If the suction pipes of the Rain- empty, the absence of obstructions on the bottom the tank wall which makes cabling, lifting chains dimensioning RainJets for circular tanks with centre Jets are not connected to the sump, flushing until plate, and no need for further piping. The short and guide pipes easy accessible, thus with the draw placed sump. the tank is completely empty is not possible. More- suction pipe results in a low pressure loss, possi- back that when the RainJet is installed on the bot- over the positioning of the RainJet on the bottom bly reducing the pump input power. However, the tom plate it obstructs the water flow. This position- number of plate obstructs the water flow, and this placement jet flow doesn’t follow the flow direction tightly, ing requires least necessary energy to accelerate the tank diameter complicates the demands for the secure mounting and thus greater energy is required to get the fluid fluid as per the principle of leverage. The RainJets RainJets* of cabling, for example. In all, this placement offers moving, because of inefficient leverage. Again, this can be installed without additional suction pipes, D<5m ~1-2 good cleaning of the periphery at the cost of risk- placement complicates the demands for the secure but then this placement does not allow flushing 5 m > D > 15 m ~2-3 ing quality issues in the centre of the tank, if no mounting of cabling, for example. In all, this place- until completely empty. To be able to clean until the additional piping or suction is made in the sump, ment ensures good cleaning in the middle of the tank is completely empty suction pipes from the D > 15 m ~3-4 enabling the RainJet to run until the tank is com- tank, with the risk of possible quality issues at the RainJet must be connected to the sump. In all, this * numbers can only be used as a rule of thumb and a pletely empty or the outlet capacity is so great that periphery. placement offers good cleaning of the periphery at careful evaluation of the exact numbers must always be the tank will be emptied before the material settles, the cost of risking quality issues in the centre of the done for each specific installation after the RainJet has stopped working. tank. • The bottom slope should be around 3-4 % towards However the RainJets can also be placed close to the the centre sump to optimise the required number of sump or close to the tank wall instead. This place- RainJets. ment gives raise to some considerations which are laYoUt StrategY • Use tank diameter as a guideline to decided the re- outlined below. Whether to choose one over the quired number of RainJets to install other placement for the RainJets should be evalu- When a given tank must be equipped with RainJets Different variations of tank layouts exist and it not • Base dimensioning of the RainJet on the required ated for each specific installation. for cleaning, the principle way of thinking is to di- feasible to show positioning of RainJets in all of thrust force vide the tank into equal rectangular sub-volumes/ them here. However, as an example of the variety, • In tanks with pillars or similar two or more RainJets areas with a length/width ration of either ~1:2 or the layout shown in figure 51 below illustrates this. should be used and placed to avoid dead zones and ~1:3; depending on type of RainJet to use. The num- resulting sedimentation problems 0.4-0.6 x R ber of sub-areas that fits into the tank will then Here, the RainJets have been positioned to create • Taking in water from a sump or controlling the out- correspond to the number of RainJets to install a circular movement of the water based on consid- let capacity can increase operation time for flushing (Fig. 51). erations of bottom slope and sump position. The • Installation should be made so it can be trimmed ac- RainJets have been positioned at an angle so the jet cording to actual conditions. 30° When doing this, it is important to take the bot- streams create smooth flow conditions without in- tom slope and sump placement in to account when terfering with each other. -6 0 Positioning splitting the tank into sub-areas. If direction of the ° The jet length for the circular tank is less significant slope and position of the sump have not been de- than in the rectangular tanks due to water inertia cided, the division of the tank into sub-areas can that is created during mixing and cleaning which then aid in deciding this based on best coverage will create a circular motion (Fig. 49). and cleaning of the tank. Sump For circular tanks with a centre sump the RainJet 0.4-0.6 x R 40m 40m should be placed at a distance of 0.4-0.6 times m m radius from the edge of the tank (Fig. 50). In the 37 37 direction of flow, the RainJet should be placed at an angle of 30-60° to the centre axis of the tank. The Figure 50: 40m 40m slope of the bottom should be between 3-4% from m m the edge to the centre. Principle for positioning of RainJets in a circular tank 37 37 with centre sump. R denotes radius. Figure 51: Figure 49: Ratio 1:3 At the left the 0.4-0.6 x R Exemplification principle of water of how to divide motion in a Velocity Vector 1 Ratio 1:3 the tank area into circular tank. 0.500 sub-compartments Ø25 Ø25 when planning 0.375 30° m m layout and equip- At the right 12.5 -6 velocity streamline m ping of stormwater 0° plot from a CFD 0.250 tanks. In the circular simulation of a Ø25 m Ø25 m tank the ejectors 0.125 circular tank with could also be placed 12.5 m closer to the centre centre sump 0.000 [m s^-1] where the arrow angled as shown in heads indicate flow Figure 49 to cover direction. Sump Ratio 1:2 the tank area. 0.4-0.6 x R Ratio 1:2
  • 21. 40 Stormwater tankS Stormwater tankS 41 Risk of airpockets Figure 52: Regardless which solution is chosen, it is important to keep a degree of freedom in the installation at Placement of the ejector. This is in regards to the angle of the Risk of airpockets RainJet(s) in circular ejector during final adjustment of the equipment stormwater tanks at commissioning based on the actual conditions with trap sump. in the tank. Using adjustable flanges or other initia- tives are therefore necessary. mUltiPle eJectorS SUPPlieD BY the Same PUmP throUgh a manifolD For some applications it could be beneficial to sup- ply multiple ejectors with one single pump. In this case all ejectors must be connected via a common manifold. This can be an issue if space is limited in the sump, if protruding RainJets in the tank are re- The traversing sump will in this design work as an PiPe inStallation quested or because of accessibility to equipment. DeSign of manifolD Figure 54: obstacle to the flow and thereby a catch where sus- However this solution will raise the issue of having pended solids preferentially will settle and be fur- Installation of pipes between the pump and ejec- one as opposed to many pumps in the installation. When choosing the solution where more ejectors How to design a ther conveyed out of the tank. tor can be handled either by placing the pipes above are supplied through a manifold, there are different manifold without the floor or moulding them into the floor. Using a common manifold to feed all RainJets with things to consider for maximum performance from the risk for getting Furthermore piping from one central pump placed one pump has both pros and cons. As a benefit, the all RainJets. airpockets. at the tank wall feeding several RainJets can be in- Piping placed on the bottom plate with mechanical maintenance cost of the system will decrease as stalled in the sump without obstructing the flow supports must be placed with sufficient clearance to less mechanical equipment is installed in the tank. For the components and configuration of the pipe unnecessarily on the bottom plate, and without allow material to pass unhindered underneath; typ- A drawback is that flexibility of system adjustment work, eccentric reducers must be used compared to creating unintended places for sedimentation of ically this will be at a height so the piping connects decreases, as branch pipes are welded in a fixed po- standard narrowings, in addition to placing any off- gross material. directly to the ejector. However, the supports placed sition to the manifold, so if tank drawings do not branching pipes at the top of the manifold (Fig. 54). on the bottom plate will increase the hydraulic correspond to the actual conditions it can be hard This design will prevent the possibility for accumu- PiPing anD manifolD losses and create areas for possible sedimentation. to adjust the system. This drawback can to some lation of air at the top of the pipes, which will other- Furthermore if large impurities get trapped under extent be overcome as described above by using ad- wise reduce hydraulic capacity of the manifold. When placing the ejector and pump of the Rain- the piping this will create an area where material justable flanges or other initiatives for final adjust- Figure 55: Jet separately either to push ejector units forward, can build up. On the other hand, as piping is freely ment of the equipment at commissioning. Obtaining equal performance of the different ejec- to feed several ejectors with one single pump or available it is fairly easy to disassemble if the piping tors on the manifold requires that they have the Different layouts to place the pump dry installed, additional piping becomes clogged due to large impurities carried to However using one pump per ejector will give com- same water flow and hence the same headloss. where one pump must be installed between the units. the tank and thereby restore flow to the ejector. plete freedom for final adjustment during commis- This is most easily obtained by making the manifold feeds several ejec- sioning if this should be necessary. Although sev- symmetrical if this is possible. Doing this will also tors. In the left ex- The additional pipe will increase head losses of the On the other hand, moulding the pipes in to the eral pumps increase maintenance costs, the safety simplify calculations and design of the system. amples the ejectors system and result in increased head and decreased tank floor has the advantages that hydraulic loss- of having more pumps is an advantage in case of must have equal flow (H2, Q2), and thus, a new pump duty point es and additional areas for sedimentation due to pump breakdown. However, as the pump on the However, the intention in some cases could also be performance. In the (DPreduced). The increased headloss should be accoun- protruding piping is complete avoided. However ejector manifold can be mounted either on an auto for different performance from each ejector due to examples at the ted for when selecting the pump for the ejector. a drawback is the fact that if piping moulded into coupling or dry-installed, this safety issue is a mat- tank layout and thus jet length requirements (Fig. right, the ejectors As the water volume and hence the flow cleans the the tank floor becomes clogged it can be difficult ter of estimate. 55). Obtaining different performance from the ejec- must have different stormwater tank, a pump with the original flow (Q1) to clean and restore flow to the ejector. In circular tors requires that the piping is laid out accordingly, performance. and the required head (H2) must be chosen for the tanks, piping placed across the flow direction will, at which can be a source for some rather complex application to have the correct pump duty point low water levels, generate a substantial hindrance. calculations. (DPrequired) (Figure 54). Figure 53: What to be aware H of when placing additional piping between the pump DPRequired and ejector. H2 DPReduced Requirerd pump H1 for the RainJet DPNormal Original pump for the RainJet Q2 Q1 Q
  • 22. 42 Stormwater tankS Stormwater tankS 43 tUBe anD channel tankS When emptying the tank after the stormwater EXAMPLE: event, the velocity of the water flowing down the For a symmetrical manifold each RainJet must receive 50 l/s at 4.1 mWC to give the required jet length Another type of tank also commonly used for storm tank can be too low to drag the settled material to and thrust force. Calculate the pump performance when feeding both ejectors through a manifold water detention is the tube or channel tank (Fig. the wastewater treatment plant for cleaning. This with the following characteristics. 56). Without going into details this type of tank is can be overcome by installing a cleaning system basically an over-dimensioned pipe made of a series that pumps water from the lowest end of the tank of connected precast elements of concrete or plas- to the highest end where it will increase the water Q = 50 l/s tic. Tube tanks are often placed in-line as part of the flow down the tube tank. 20 m sewer system with a constant flow also during dry H = 4.1 mWC All pipes are in stainless steel and the flange weather conditions, which implies that the bottom Properly designed, the additional water flow pro- 10 m of the ejector is DN100. The resistance number of the tube should be designed to handle small and vided by the pump will increase the bottom shear of the fittings are as given below: large flows. stress above the critical value of around 3-4 N/m2 DN100 and ensure that settled material at the bottom of C 1 3m 1: Eccentric reducer (ζ = 0.5) Tube tanks should if possible be constructed with a the channel will be dragged to the outlet pump B 2 slope that will provide self-cleaning with dry weath- from where it is transferred for treatment. A 3 5m 2: 90° bend (ζ = 0.5) er conditions and keeping in mind the maximum 1m back water level in the system during stormwater 3: T-piece (ζ = 3) events. However, if it is not possible to obtain self- DN200 cleaning of the tube tank, flushing must be used as sedimentation of material will occur during the de- tention period. As the flow is being split into two in the T-piece the pump flow must equal 2 x 50 l/s = 100 l/s. To find the head of the pump suitable for the system the pipe losses must be calculated and added to the head of the ejector. For these calculations the following equations have been used: ѵ2 ν : average velocity [m/s] Hsingle losses = ζ 2g g : gravitational acceleration [m/s2] f : friction number [-] L x ѵ2 Hpipe = f x L : distance [m] Rh 2g Rh : hydraulic radius [m] Q 4x Q : flow [m3/s] ѵ= Π x d2 d : diameter [m] Calculation of the headlosses is split into three sections. The headlosses of pipework and single losses for each section have been calculated using WebCAPS (www.grundfos.com). Section A) from the pump to T-piece where the flow is 100 l/s in a DN200 pipe: H = 1.57 m Section B) after the T-piece to eccentric reducer where the flow is 50 l/s in a DN200 pipe: H = 0.331 m Section C) after the eccentric reducer to ejector where the flow is 50 l/s in a DN100 pipe: H = 1.53 m Figure 56: If the chosen pipe configuration gives unacceptable headlosses, other dimensions could be used and the calculation must be repeated. Inflow Sketch of a channel tank The total head of the pump for the system can then be found by adding the losses of the three sections to- for storm water gether with the head of the ejector itself. Outflow detention Htotal = 1.57 m + 0.331 m + 1.53 m + 4.1 m = 7.53 m The hydraulic characteristic of the pump must hence be: Q = 100 l/s, H = 7.53 m
  • 23. 44 Stormwater tankS Stormwater tankS 45 recommendation for controlS and Set pointS Automatic operation and the control strategy of from the entire sewer system. Collecting these data With a steady water level for a certain period of time, ous flush, dragging all material to the sump for con- the stormwater tank can be achieved when con- can help in getting an overview over which tanks the RainJet might be run intermittently or continu- veyance to the sewer system. This time period should trolling and monitoring the action of the different are full or where space is available, if more water ously for resuspension, avoidance of sedimentation, be set according to actual experiences based on condi- components in the tank based on water levels. Data should enter the sewer system. and to limit odour problems. This should however only tions in the tank. on water levels can be obtained from any type of be initiated below the critical duty level as water lev- level switches/devices installed in the tank and control StrategY els above this set point might be critical for overflows, Between oPeration can be used to determine if the level is increasing should another inflow take place. or decreasing. A controller gathering the data must To obtain an effective automatic cleaning of the Because of the impact of the environment on the in- be able to control the interactions between the in- stormwater tank it is not only sufficient to select DecreaSing water level stalled equipment, it can be an advantage to install as stalled equipment, with the benefit of obtaining the correct equipment. It is also important to have it part of the control system function tests and test runs efficient cleaning, maximum hydraulic relief of the connected to an intelligent control system manag- With decreasing water level, the action of the Rain- of pumps, sensors, and so on, to ensure full function. sewer system and a reduction of the risk of pollu- ing the filling, depletion and cleaning process. Wa- Jet is strictly controlled by the different water levels. Not everything can be integrated into the controls, and tion due to overflow. ter levels are used to control operation of the tank Above the critical duty level equipment will never run. it can therefore be necessary occasionally to visually where predefined set points based on variations Between critical and noncritical duty level the RainJet check the tank for foreign items as well as checking Constant water inflow and outflow from the in water levels define the actual control strategy. can be run continuously or intermittently to loosen overflow grates and the functionality of ventilation, to stormwater tank could well be essential. This can The principle of control using set points is sketch in up the settled material that can form relatively solid ensure the required level of operation of the tank. be difficult to achieve when the water is conveyed figure 57. sediments. When the water level falls to the noncriti- by gravitation, due to differences in water levels. cal level the RainJet should be running continuously to However, pumps controlled by frequency convert- Figure 57 is a description of the principles of Rain- keep organic and inorganic matter in suspension. ers can tightly control the flow and keep it constant, Jet use and consequently does not contain defined independent of water levels. This tight control will levels. The different levels depend on inertia when Close to when the tank is empty the RainJet still runs, especially during outflow from the stormwater filling and emptying the tank, and as a general but the pump/valve for effluent can be discontinued tank ensure a constant hydraulic load in the sewer rule adjustments according to the actual operat- for a while, giving the RainJet time to make a continu- system and consequently a constant flow at the ing conditions should be possible. The levels should downstream wastewater treatment plant securing be defined based on the actual cleaning process, treatment conditions. but also out of consideration of minimising energy consumption. The controller must therefore be de- The operational data from the stormwater tank signed in a way so it is possible to adjust the set should not only be used in the tank but also ulti- points at a later stage. mately be transmitted to a central place where the entire sewer system can be monitored. This infor- increaSing water level mation helps provide a current status of the system and can additionally be used to plan pre-emptive With increasing water levels, the RainJet should not maintenance of the installed equipment. be in duty. This is to enhance sedimentation of or- ganic and inorganic matter in the tank in case an Collecting and connecting data from more tanks overflow happens. When inflow ceases and until in the same or different watersheds can be useful capacity is available in the sewer system, a steady for controlling water flows and avoiding overflow water level in the stormwater tank is maintained. Level Duty at increasing Duty at steady Duty at decreasing water level water level water level Overflow level Off Off Off Figure 57: Critical duty level Control scheme for RainJets. Blue ar- On Off On rows indicate slope/ Off trend/gradient of Noncritical the water level in duty level the tank. The text in the circles indicate specific duty mode of the RainJet based On on variations in Off On Off water levels. Tank empty
  • 24. 46 Stormwater tankS Stormwater tankS 47 equipment for Stormwater tankS Grundfos offers a variety of products for stormwater tank operation. Below is an overview of the products and specifications. More information about the products is available on www. grundfos.com/water-utility Product: Product: Product: Product: RainJet WA/WW SL, SLV S-range Controls and monitoring Dedicated Controls: User friendly display, plug and play commissioning and all the right functions. Supplied as modules or ready-made panels. applications are typically sewage network pumping stations controlling up to six pumps. MP204: electronic motor protection. Product types: SL1, SLV Dry-running, motor temperature, over- and Product types: under voltage or current, power factor and alarm Pump principle: SV, S1, S2, S3, S4 log. Product types: Submersible semi-open, single channel or Super RainJet wa, RainJet ww Vortex wastewater pumps. Pump principle: Super Vortex impeller or channel measured energy consumption. impeller pumps. with or with out built-in controller. Principle: CUE: Stainless steel versions available. RainJets, with or without intake of air, for auto- Flow, Q: max. 87,5 l/s Frequency converters for three-phase pumps. matic and hygienic cleaning of tanks used for Flow, Q: max. 1800 l/s adjustment of the pump performance to the Head, H: max. 44 m temporary storage of storm- or wastewater. demand. Head, H: max. 110 m Liquid temp.: max. 40°C together with sensors, the CUe offers various Area: max. 300 m2 Liquid temp.: 0°C to +40°C Discharge diam.: DN 65 to DN 150 / 2 ⁱ⁄₂” to 6” control modes. Thrust: max. 1500 N Discharge diam.: DN 80 to DN 800 Product: Product: Product: Product: AFG/AMD/AMG SEV, SE1 KPL/KWM Remote Management Product types: Grundfos Remote Management: AFG, AMD, AMG Product types: SeV, Se1 Product types: Internet-based remote monitoring & control for Pump principle: kPL/kwm pump systems. Pump principle: Horizontal gear driven flowmakers. Submersible or dry installed single channel or Complete system overview. Direct and gear driven mixers. Super Vortex wastewater pumps. Stainless steel Pump principle: versions available. alarms on SmS and email to on duty staff Axial thrust: Submersible axial-flow pump. Controls and protection systems available Full historical event log AFG 998-6632 N Flow, Q: max. 11600 l/s Flow, Q: max. 80 l/s Graphical display of trend data and raw data AMD, AMG 160-3931 N Head, H: max. 45 m Head, H: max. 20 m download Liquid temp.: +5°C to +40°C Liquid temp.: 0°C to +40°C Liquid temp.: 0°C to +40°C monthly summation reports Installation depth: Max. 20 m Discharge diam.: DN 65 to DN 150 / 2 ⁱ⁄₂” to 6” Column pipe diameter: 700 to 1600 mm tools for service optimisation
  • 25. 48 Stormwater tankS Stormwater tankS 49 bibliography Solids in sewers characteristics, effects and control of sewer solids and associated pollutants (2004); grundfos WebCapS Scientific and Technical Report No. 14; edited by R. M. Ashley et al; ISBN: 1 900222 91 4; IWA Publishing Urban and Highway Stormwater Pollution, Concepts and Engineering (2010); Thorkild Hvitved-Jacobsen , Jes Vollertsen and Asbjørn Haaning Nielsen, ISBN: 978-1-4398-2685-0, CRC Press 2010 go to the grundfos online WebCapS, Stormwater collection systems design handbook (2001); editor in chief Mays, Lary W.; ISBN 0-07-135471-9; Computer aided product Selection program, McGraw-Hill toaccess more than 180,000 grundfos products complete with data and Cad drawings. Danish literature: Afløbsteknik (2002); Linde et al; ISBN 87-502-0827-6; Polyteknisk Forlag Lærebog i Hydraulik (2003); Brorson, M and Larsen, T; ISBN 87-7307-691-0; Aalborg Universitetsforlag Pumpe ståbi (2000); Ingeniøren Bøger; Publisher Flening, Søren; ISBN 87-571-2296-2; Ingeniøren Vejledning nr. 71, Regnbetingede udledninger - katalog over teknologier til reduktion af effekter i miljøet (2006); DANVA’s Arbejdsgruppe Om Regnbetingede Udledninger, ISBN 87-90455-61-4; DANVA www.grundfos.com/webcaps
  • 26. Being responsible is our foundation thinking ahead makes it possible innovation is the essencegrUnDfoS a/Spoul due Jensens Vej 7dk-8850 Bjerringbrotel: +45 87 50 14 00 9824 0511www.grundfos.comthe name grundfos, the grundfos logo, and the payoff Be–think–innovate are registrated trademarksowned by grundfos management a/S or grundfos a/S, denmark. all rights reserved worldwide.
  • 27. 50 Storm water tankS Storm water tankS 51

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