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Design criteria for waste water treatment
 

Design criteria for waste water treatment

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Design of racks, screens, grit chamber, aeration units, sedimentation

Design of racks, screens, grit chamber, aeration units, sedimentation
tanks, activated sludge and trickling filter processes, rotating biological
contactors, sludge digesters and drying beds

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    Design criteria for waste water treatment Design criteria for waste water treatment Presentation Transcript

    • Design of Facilities for Physical, Chemical & Biological Treatment of Waste Water Bibhabasu Mohanty Asst. Prof. Dept. of civil Engineering SALITER, Ahmedabad
    • Course Content Design of racks, screens, grit chamber, aeration units, sedimentation tanks, activated sludge and trickling filter processes, rotating biological contactors, sludge digesters and drying beds
    • SLUDGE TREATMENT…
    • Introduction…• Sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes.• Waste water sludge is the mixture of waste water and settled solids.• Depending upon the source it may be primary, secondary, excess activated sludge.
    • Objectives…• To reduce the volume of the material to be handled by removal of liquid portion.• To decompose the organic matter and inorganic compounds for reduction in the total solids.
    • GOALS OF SLUDGE TREATMENT…Volume • Thickeningreduction • DewateringElimination of • If used in agriculture as fertiliser orpathogenic compostgermsStabilisation of • Gas productionorganic • Reduction of dry contentsubstances • Improvement of dewatering • Reduction of odourRecycling of • Nutrients, fertilisersubstances • Humus • Biogas
    • Sludge handling and disposal includes:-  Collection of sludge  Transportation of sludge  Processing of sludge to convert it to a form suitable for disposal  Final disposal of the sludge
    • Composition…• Sludge from plain sedimentation tank- settable solids (raw sludge)• This gray in color contain garbage, fecal solids, debris.• Bad odor.• From sec. settling tank following a trickling filter consists of partially decomposed organic matter.• Dark brown in color, less odor than raw sludge.
    • Sludge types…• Primary sludge  3 to 8 % solids  About 70% organic material• Sec. sludge  Wasted microbes and inert materials  90% organic material• Tertiary sludge  If sec. clarifier is used to remove phosphate, this sludge contain chemical precipitates.
    • Overview Wastewater treatment Primary, secondary, tertiary sludgeProcess water Thickening Stabilisation Biogas Thickening Agriculture Dewatering Disposal site Drying Construction industry Incineration Atmosphere
    • Thickening (volume reduction) by GravityGravity separation, similar to settling tankAdditional mechanic stirring to enhance flocculation andextraction of water and gasSupernatant is introduced to primary clarifier or – if floatablesand grease contents are high – to grid chamberThickened sludge is withdrawn from hopper and introduced tosludge treatmentFor an efficient thickening process the development of gasbubbles must be prevented
    • Gravity ThickenerInflow Scum scimmer Sludge liquor Thickened sludge
    • Thickening by FlotationPre treatment: mostly chemical flocculationSludge is placed in contact with air-saturated water(full flow or recycle pressurization)Air bubbles attach to solid particlesFloating Sludge bubble composite is collected at thesurfaceWater is recovered under a scum baffle and removed
    • Thickening by Flotation
    • Sludge stabilization (mass reduction)• Aerobic digestion• Anaerobic digestionAerobic sludge digestion may be used to treat only  Waste activated sludge  Mixtures of waste activated siudge and primary siudge  Activated sludge treatment plant without primary settling
    • Advantages Volatile solids reduction is equal that obtained anaerobically Lower BOD concentrations in supernatant liquor Production of an odorless, humus-like, biologically stable end Operation is relativeluy easy Lower capital cost
    • Disadvantages A high power cost is associated with supplying the required O2 A digested sludge is produced with poor mechanical dewatering characteristics A useful by-product such as methane is not recovered
    • Process designFactors taht must be considered in designing aerobic digesters include; Solid reduction Hydraulic retention time Oxygen requirements Energy requirements for mixing environmental condition such as pH, temperature.
    • Anaerobic digestion Sludge held without aeration for 10-90 days Process can be accelerated by heating to 35-40oC These are called High Rate Digesters (10-20 days) Advantages  low solids production  useable methane gas produced Disadvantages  high capital costs  susceptibility to shocks and overloads
    • Basic Components of Digester GasAnaerobic Digesters Digested Sludge Raw Sludge Mixing Heat Exchanger Circulating Pump
    • Anaerobic digestion process Organic acids Complex CH4 and and Organics CO2 H2 Acid producing Methane producing bacteria bacteria (acidogens) (methanogenics)
    • Three Mechanisms Occurring:Hydrolysis Process – conversion of insoluble high molecular compounds (lignin, carbohydrates, fats) to lower molecular compoundsAcidogenesis Process – conversion of soluble lower molecular components of fatty acids, amino acids and sugars (monosaccharide) to lower molecular intermediate products (volatile acids, alcohol, ammonia, H2 and CO2)Methanogenesis Process – conversion of volatile acids & intermediate products to final product of methane and CO2
    • Steps in anaerobic (oxygen-free) digestion:Particulate and complex organics Hydrolysis Soluble simpleorganicsSoluble simple organics Acidogenesis Short organicacids MethanogenesisShort organic acids CH4 & CO2
    • Conventional anaerobic digester High rate anaerobic digester
    • Anaerobic Digester Design Mean Cell Residence Time Volumetric Loading Factor Observed Volume Reduction Loading Factors Based on Populations
    • Sludge dewatering Dewatering aims to reduce the water content further. The sludge can then be handled like a solid. Dewatering can be done mechanically using a filter press (employing pressure or vacuum), or a centrifuge. Also be done using drying beds.
    • Drying beds• Most popular methods.• A drying bed consists of a 30 cm bed of sand with an under-drainage .• Sludge is applied on the sand bed and is allowed to dry by evaporation and drainage of excess water over a period of several weeks depending on climatic conditions.
    • • Bacterial decomposition of the sludge takes place during the drying process while moisture content is sufficiently high.• During the rainy season the process may take a longer time to complete.
    • TRICKLING FILTER PROCESSES…
    •  Trickling filter is an attached growth process i.e. process in which microorganisms responsible for treatment are attached to an inert packing material. Packing material used in attached growth processes include rock, gravel, slag, sand, redwood, and a wide range of plastic and other synthetic materials.
    • Process Description The wastewater in trickling filter is distributed over the top area of a vessel containing non-submerged packing material. Air circulation in the void space, by either natural draft or blowers, provides oxygen for the microorganisms growing as an attached biofilm.
    •  The organic material present in the wastewater metabolised by the biomass attached to the medium. The biological slime grows in thickness as the organic matter abstracted from the flowing wastewater is synthesized into new cellular material.
    • Flow Diagram for Trickling Filters Recirculation= A portion of the TF effluent recycled through the filter Recirculation ratio (R) = returned flow (Or)/ influent flow (Q) Or Recycle Final clarifierQInfluent Primary Wast clarifier sludg Trickling filter
    • Advantages simplicity of operation resistance to shock loads low sludge yield low power requirements
    • Disadvantages relatively low BOD removal (85%) high suspended solids in the effluent (20 -30 mg/L) little operational control
    • Types of FiltersTrickling filters are classified as high rate or low rate,based on the organic and hydraulic loading applied to theunit.S.No. Design Feature Low Rate Filter High Rate Filter Hydraulic loading, 1. 1-4 10 - 40 m3/m2.d Organic loading,kg 2. 0.08 - 0.32 0.32 - 1.0 BOD / m3.d 3. Depth, m. 1.8 - 3.0 0.9 - 2.5 0.5 - 3.0 (domestic wastewater) up to 8 for 4. Recirculation ratio 0 strong industrial wastewater.
    •  Hydraulic loading rate is the total flow including recirculation applied on unit area of the filter in a day. Organic loading rate is the 5 day 20°C BOD, excluding the BOD of the recirculant, applied per unit volume in a day. Recirculation is generally not adopted in low rate filters. A well operated low rate trickling filter in combination with secondary settling tank may remove 75 to 90% BOD and suitable for treatment of low to medium strength domestic wastewaters.
    •  The high rate trickling filter, single stage or two stage are recommended for medium to relatively high strength domestic and industrial wastewater. The BOD removal efficiency is around 75 to 90%. Single stage unit consists of a primary settling tank, filter, secondary settling tank and facilities for recirculation of the effluent. Two stage filters consist of two filters in series with a primary settling tank, an intermediate settling tank which may be omitted in certain cases and a final settling tank.
    • Process Design Generally trickling filter design is based on empirical relationships to find the required filter volume for a designed degree of wastewater treatment. NRC equations commonly used. NRC (National Research Council of USA) equations give satisfactory values when there is no re- circulation, the seasonal variations in temperature are not large and fluctuations with high organic loading.
    •  NRC equations: These equations are applicable to both low rate and high rate filters. The efficiency of single stage or first stage of two stage filters, E2 is given by E2= 100 1+0.44(F1.BOD/V1.Rf1)1/2 For the second stage filter, the efficiency E3 is given by E3= 100 [(1+0.44)/(1- E2)](F2.BOD/V2.Rf2)1/2
    • where E2= % efficiency in BOD removal of single stage or first stage of two-stage filterE3=% efficiency of second stage filterF1.BOD= BOD loading of settled raw sewage in single stage of the two-stage filter in kg/dF2.BOD= F1.BOD(1- E2)= BOD loading on second-stage filter in kg/dV1= volume of first stage filter, m3 Rf1= 1+RV2= volume of second stage filter, m3 (1+R/10)2 R=recycle ratioRf1= Recirculation factor for first stage, F=recirculationR1= Recirculation ratio for first stage filter factorRf2= Recirculation factor for second stage,R2= Recirculation ratio for second stage filter.
    • Q. Problem: Design a low rate filter to treat 6.0 Mld of sewage of BOD of 210 mg/l. The final effluent should be 30 mg/l and organic loading rate is 320 g/m3/d. Solution: Assume 30% of BOD load removed in primary sedimentation i.e., = 210 x 0.30 = 63 mg/l. Remaining BOD = 210 - 63 = 147 mg/l. Percent of BOD removal required = (147-30) x 100/147 = 80% BOD load applied to the filter = flow x conc. of sewage (kg/d) = 6 x 106 x 147/106 = 882 kg/d To find out filter volume, using NRC equation E2= 100 1+0.44(F1.BOD/V1.Rf1)1/2
    •  80 = 100 Rf1= 1, (no recirculation) 1+0.44(882/V1)1/2 V1= 2704 m3 Depth of filter = 1.5 m, Filter area = 2704/1.5 = 1802.66 m2, and Diameter = 48 m Hydraulic loading rate = 6 x 106/103 x 1/1802.66 = 3.33m3/d/m2 < 4 hence o.k. Organic loading rate = 882 x 1000 / 2704 = 326.18 g/d/m3 which is approx. equal to 320
    • ACTIVATED SLUDGE PROCESSES…
    •  The most common suspended growth process used for municipal wastewater treatment is the activated sludge process.
    • Activated sludge plant involves: 1.wastewater aeration in the presence of a microbial suspension, 2.solid-liquid separation following aeration, 3.discharge of clarified effluent, 4.wasting of excess biomass, and 5.return of remaining biomass to the aeration tank.
    • Process The process involves air or oxygen being introduced into a mixture of primary treated or screened sewage or industrial wastewater combined with organisms to develop a biological floc which reduces the organic content of the sewage. The combination of wastewater and biological mass is commonly known as mixed liquor. In all activated sludge plants, once the wastewater has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated supernatant is run off to undergo further treatment before discharge.
    •  Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new wastewater entering the tank. This fraction of the floc is called return activated sludge (R.A.S.). Excess sludge is called surplus activated sludge(S.A.S.) or waste activated sludge(W.A.S). S.A.S is removed from the treatment process to keep the ratio of biomass to food supplied in the wastewater in balance. S.A.S is stored in sludge tanks and is further treated by digestion, either under anaerobic or aerobic conditions prior to disposal.
    • Advantages Diverse; can be used for one household up a huge plant Removes organics Oxidation and Nitrification achieved Biological nitrification without adding chemicals Biological Phosphorus removal Solids/ Liquids separation Stabilization of sludge Capable of removing ~ 97% of suspended solids The most widely used wastewater treatment process
    • Disadvantages Does not remove color from industrial wastes and may increase the color through formation of highly colored intermediates through oxidation Does not remove nutrients, tertiary treatment is necessary Problem of getting well settled sludge Recycle biomass keeps high biomass concentration in aeration tanks
    • Types of Activated Sludge ProcessesPlug Flow wastewater is routed through a series of channels constructed in the aeration basin. Wastewater Flows to tank & is treated as it winds its way through the tank. As the wastewater goes through the system, BOD and organics concentration are greatly reduced.
    •  Variations to this method include:  adding return sludge and/or in decreasing amounts at various locations along length of the tank;  wastewater BOD is reduced as it passes through tank,  air requirements and number of bacteria required also decrease accordingly.
    • Complete Mix wastewater may be immediately mixed throughout the entire contents of the aeration basin (mixed with oxygen and bacteria). This is the most common method used today. Since the wastewater is completely mixed with bacteria and oxygen, the volatile suspended solids concentration and oxygen demand are the same throughout the tank.
    • Contact Stabilization Microorganisms consume organics in the contact tank. Raw wastewater flows into the contact tank where it is aerated and mixed with bacteria. Soluble materials pass through bacterial cell walls, while insoluble materials stick to the outside. Solids settle out later and are wasted from the system or returned to a stabilization tank. Microbes digest organics in the stabilization tank, and are then recycled back to the contact tank, because they need more food.
    •  Detention time is minimized, so the size of the contact tank can be smaller. Volume requirements for the stabilization tank are also smaller because the basin receives only concentrated return sludge, there is no incoming raw wastewater. Often no primary clarifier before the contact tank due to the rapid uptake of soluble and insoluble food.
    • Extended Aeration Used to treat industrial wastewater containing soluble organics that need longer detention times. This is the same as complete mix, with just a longer aeration. Advantage - long detention time in the aeration tank; provides equalization to absorb sudden/temporary shock loads. Less sludge is generally produced because some of the bacteria are digested in the aeration tank. One of the simpler modifications to operate.
    • Design Consideration The quality or characteristics of raw waste water to be treated. The desired quality or characteristics of effluent or treated waste water. The type of reactor that will be used. Volumetric and organic loading that will be applied to the reactor.
    •  Amount of O2 required and the aeration system will provide to supply O2 and to support mixing. The quantity of sludge that will be generated and wasted for its further management. Besides these nutrient requirements of microbes, environmental conditions under which plant operated.
    • Design stepsThe design computations require the determination of: Volume or dimensions of the aeration tank Amount of O2 required and power needed for aeration Quantity of sludge that will produced for particular waste and treatment conditions  Volume and dimensions of sec. settling tank
    • Design criteria No of aeration tanks, N= min. 2 (small plants) = 4 or more (large plants) Depth of waste water in tank= 3-4.5 m (usually) = 4.5-7.5 m (diffuse aeration) = 1-6 m (surface aeration) Freeboard= 0.3-6 m (diffuse aeration) = 1-1.5 m (surface aeration) Rectangular aeration tank L:B= 5:1 and B:D=3:1 to 4:1 (depends on the aeration system)
    •  Air requirement:I. 20-55 m3 of air/Kg of BOD removed for diffuse aeration when F/M => 0.3II. 70-115 m3 air/Kg of BOD removed for diffuse aeration when F/M <= 0.3 Power required for complete mixing : 10-14 kW/1000 m3 of tank volume for surface aeration system
    • ROTATING BIOLOGICALCONTRACTORS (RBC)…
    • Rotating Biological Contactors,commonly called RBC’s, are used in wastewater treatment plants(WWTPs). The primary function of these bio-reactors at WWTPs is the reduction of organic matter.
    • A fixed growth biological treatment processes used to consume organic matter (BOD) from wastewater.Consists of 2-6 m diameter disks, closely spaced on a rotating horizontal axis.Disks are covered with a biofilm.The disks are only partially submerged in wastewater.
    •  As the disk rotates the biofilm is exposed to the wastewater only part of the time. The rotation in and out of the wastewater serves to vary the feeding cycle of the bacteria and microorganisms that make up the biofilm. The shaft rotates about 1-10 rpm (slowly).
    • Advantages/Disadvantages Advantages Disadvantages Short contact periods  Need for covering units Handles a wide range of installed in cold climate to flows protect against freezing Easily separates biomass from waste stream  Shaft bearings and Low operating costs mechanical drive units Short retention time require frequent maintenance Low sludge production Excellent process control
    • Flow Diagram of an RBC
    • Design Criteria No of modules = 4-5 Dia of flat discs = 2-6 m Thickness of flat disc = up to 10 mm Discs spacing = 30-40 mm Speed of rotating shaft = 1-10 rpm Disc submergence = 40% of dia Thickness of bio-film = 2-4 mm
    •  Organic loading = 3-10 gm BOD/m2 of disc surface area Hydraulic loading = 0.02-0.16 m3/m2-d Sludge production = 0.5-0.8 Kg/Kg BOD removed Hydraulic retention time = 0.5 -2.0 h
    • RACKS & SCREENS...
    • screen is a device with openings for removing bigger suspended or floating matter in sewage which would otherwise damage equipment or interfere with satisfactory operation of treatment units.
    • Figure Definition sketch for types of screens used in wastewatertreatment
    • Design ConsiderationVelocity The velocity of flow ahead of and through the screen varies and affects its operation. The lower the velocity through the screen, the greater is the amount of screenings that would be removed from sewage. However, the lower the velocity, the greater would be the amount of solids deposited in the channel.
    •  Hence, the design velocity should be such as to permit 100% removal of material of certain size without undue depositions. Velocities of 0.6 to 1.2 mps through the open area for the peak flows have been used satisfactorily. Further, the velocity at low flows in the approach channel should not be less than 0.3 mps to avoid deposition of solids.
    • Head loss Head loss varies with the quantity and nature of screenings allowed to accumulate between cleanings. Head loss through screens mainly depends on: Size and amount of solids in waste water Clear openings between bar  Method of cleaning and its frequency Velocity of flow through the screens
    • The head loss through clean flat bar screens is calculated from the following formula: h = 0.0729 (V2 - v2) where, h = head loss in m V = velocity through the screen in mps v = velocity before the screen in mps
    • Another formula often used to determine the head loss through a bar rack is Kirschmers equation: h = b (W/b)4/3 hv sin qwhere h = head loss, mb = bar shape factor (2.42 for sharp edge rectangular bar, 1.83 for rectangular bar with semicircle upstream, 1.79 for circular bar and 1.67 for rectangular bar with both u/s and d/s face as semicircular).W = maximum width of bar u/s of flow, mb = minimum clear spacing between bars, mhv = velocity head of flow approaching rack, m = v2/2gq = angle of inclination of rack with horizontal
    • The head loss through fine screen is given by h = (1/2g) (Q/CA)where, h = head loss, m Q = discharge, m3/s C = coefficient of discharge (typical value 0.6) A = effective submerged open area, m2
    • GRIT CHAMBER...
    • Grit chambers are basin to remove theinorganic particles to prevent damage to the pumps, and to prevent their accumulation in sludge digesters.
    • Types of Grit Chambers Mechanically cleaned Manually cleaned In mechanically cleaned grit chamber, scraper blades collect the grit settled on the floor of the grit chamber. The grit so collected is elevated to the ground level by several mechanisms such as bucket elevators, jet pump and air lift. Manually cleaned grit chambers should be cleaned at least once a week. The simplest method of cleaning is by means of shovel.
    • Aerated Grit Chamber An aerated grit chamber consists of a standard spiral flow aeration tank provided with air diffusion tubes placed on one side of the tank. The grit particles tend to settle down to the bottom of the tank. Settling rates dependant upon the particle size and the bottom velocity of roll of the spiral flow.
    • Design criteria Recommended for horizontal flow and aerated grit chamber. Flow= maximum Detention time= 30-90 s (usually 60 s) Flow through velocity, vh= 0.2-0.4 m/s (usually 0.3 m/s) Settling velocity= 0.016-0.021 m/s for 0.2 mm dia particle = 0.01-0.015 m/s for 0.15 mm dia particles Liquid depth= 1-1.5 m Length= 3-25 m Quantity of grits= 0.022-0.075 m3/1000 m3 of flow
    • Determination of settling velocityTransition law: The design of grit chamber is based on removal of grit particles with minimum size of 0.15 mm and therefore Stokes law is not applicable to determine the settling velocity of grit particles for design purposes. v2 = 4g(ρs-ρw)d 3 CDρw
    • Where:g= acceleration due to gravity (assume 9.81 m/s2)ρw= density of water (1000 Kg/m3)ρs= density of solid particles (normally of specific gravity 2.65=2.65*1000=2650 Kg/m3)d= dia of particlesCD= coefficient of drag force depends on flow condition
    • AERATION UNITS...
    •  Unit process in which air and water are brought into intimate contact. The contact time and ratio of air to water must be sufficient for exchange sufficient oxygen.AdvantagesProviding O2 for purification and improving overall quality.CO2 reduction-reduces the corrosion.Raising the pH.VOC removalEffective method for bacterial control
    • Methods of aerationDiffused aerationSpray aerationTurbine aerationSurface aeration
    • Diffused aeration Providing maximum water surface per unit volume of air. Air bubbles brought with water in a mixing or contact chamber. A common way to aerate water is via diffused air. Air is pumped through some sort of diffuser to generate small bubbles.
    •  Usually gas is injected into the bottom of the aeration tank and is allowed to rise to the surface in an open tank. The rising bubbles transfer oxygen to the water, as well as transport bottom water to the surface. The bubbles raising through water create turbulence. Untreated water is allowed to enter the tank from top and exit from bottom.
    • Efficiency of diffused aeration can be improved:Fine bubbles (0.2 cm dia) as compared to coarse bubble (2.5 cm dia)By increasing water depth (9-15 ft)By improving the basin geometry (width to depth ratio not exceed 2)By increasing the retention time (10-30 min)
    • Typical diffused aeration system looks like:
    • There are a large variety of diffuser types. For example ceramicplates
    • These plates are arranged on manifolds at the bottom ofaeration tanks as shown here.
    • Other types of diffusers include coarse aerators
    • Again, these diffusers would be arranged by a manifoldon the bottom of an aeration tank.
    • To determine the oxygen transfer rate in these diffusedaeration systems, first define the pressure differencefrom top to bottom of the tank.At the surface:Psurface 14.7(1 0.032 Alt)Alt = altitude in thousands feet above sea level Psurface has units of psi
    • 62.4 H Pbottom Psurface (psi) 144H = depth of tank (depth of discharge point) in feet.
    • Mechanical Aeration Basically there are two types of mechanical aeration.Turbine Aeration: In this system coarse bubbles are injected into the bottom of the tank and then a turbine shears the bubbles for better oxygen transfer. Efficiency of turbine aerators is generally higher than diffused aeration.
    • Surface Aeration:In this case a mixing device is used to agitate the surface so that there is increased interfacial area between liquid and air.There are many different proprietary types of surface aerators .
    • Common surface aerators
    • Design consideration for mechanical aerators is usuallybased on Eckenfelder and Ford equation. C w Cl T 20 N N0 (1.02) 9.17 Notice that there is no depth consideration for mechanical aeration.
    • Where as: N = actual transfer rate (lb-O2/hr) N0 = manufacturer specified transfer rate ( lb/hr) for clean water, 20oC, zero DO. Cw = saturation value for oxygen for wastewater under operating conditions. 9.17 = saturation DO for clean water, 20oC. Cl = the design oxygen concentration in the aeration basin. T = Temp. α = oxygen transfer correction factor for waste water
    • SEDIMENTATION TANKS...
    • Solid liquid separation process in which a suspension is separated into two phases –Clarified supernatant leaving the top of the sedimentation tank (overflow).Concentrated sludge leaving the bottom of the sedimentation tank (underflow).
    • Purpose of SettlingTo remove coarse dispersed phase.To remove coagulated and flocculated impurities.To remove precipitated impurities after chemical treatment.To settle the sludge (biomass) after activated sludge process / tricking filters.
    • Principle of Settling Suspended solids present in water having specific gravity greater than that of water tend to settle down by gravity as soon as the turbulence is retarded by offering storage. Basin in which the flow is retarded is called settling tank. Theoretical average time for which the water is detained in the settling tank is called the detention period.
    • Types of SettlingType I settling (free settling)Type II settling (settling of flocculated particles)Type III settling (zone or hindered settling)Type IV settling (compression settling)
    • Design parameters for settling tank Overflow rate Solids loading DetentioTypes of settling Depth m3m2/day kg/m2/day n time Average Peak Average PeakPrimary settling only 25-30 50-60 - - 2.5-3.5 2.0-2.5Primary settling followed by 35-50 60-120 - - 2.5-3.5secondary treatmentPrimary settling with 25-35 50-60 - - 3.5-4.5 -activated sludge returnSecondary settling for 15-25 40-50 70-120 190 2.5-3.5 1.5-2.0trickling filtersSecondary settling foractivated sludge (excluding 15-35 40-50 70-140 210 3.5-4.5 -extended aeration)Secondary settling for 8-15 25-35 25-120 170 3.5-4.5 -extended aeration
    • Design DetailsDetention period: for plain sedimentation: 3 to 4 h, and for coagulated sedimentation: 2 to 2.5 h.Velocity of flow: Not greater than 30 cm/min (horizontal flow).Tank dimensions: L:B = 3 to 5:1. Generally L= 30 m (common) maximum 100 m. Breadth= 6 m to 10 m. Circular: Diameter not greater than 60 m. generally 20 to 40 m.
    • Depth 2.5 to 5.0 m (3 m).Surface Overflow Rate: For plain sedimentation 12000 to 18000 L/d/m2 tank area; for thoroughly flocculated water 24000 to 30000 L/d/m2 tank area.Slopes: Rectangular 1% towards inlet and circular 8%.
    • Problem: Design a rectangular sedimentation tank to treat 2.4 million litres of raw water per day. The detention period may be assumed to be 3 hours.
    • Solution: Raw water flow per day is 2.4 x 106 L . Detention period is 3h.Volume of tank = Flow x Detention period = 2.4 x 106 x 3/24 = 300 m3Assume depth of tank = 3.0 m.Surface area = 300/3 = 100 m2L/B = 3 (assumed). L = 3B. 3B2 = 100 m2 i.e. B = 5.8 m L = 3B = 5.8 X 3 = 17.4 mHence surface loading (Overflow rate) = 2.4 x 106 = 100 24,000 L/d/m2