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 shows how such measures ultimately offer savings on investments.

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Design of Stormwater Tanks - Recommendations and Layout

  1. 1. GRUNDFOS waStewater StORmwateR taNkSDeSign of Stormwater tankSRecommendations and layout
  2. 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. 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. 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. 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. 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. 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.

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