4UNIT III PRIMARY TREATMENT OF SEWAGE ObjectiveUnit Operation and ProcessesSelection of treatment processesOnsite sanitation - Septic tank, Grey water harvestingPrimary treatment – Principles, functions design and drawing ofscreen,grit chambersprimary sedimentation tanksOperation and Maintenance aspects.
5ObjectivePrimary treatment consists solely separating the floating materials and also the heavy settable organic and inorganic solids. It also helps in removing the oils and grease from the sewage.This treatment reduces the BOD of the wastewater by about 15 to 30%.The operations used areScreening for removing floating papers, rages, cloths, etc.,Grit chambers or detritus tanks for removing grit and sandSkimming tanks for removing oils and grease.
6ObjectivePrimary settling tank is provided for removal of residual suspended matter.The organic solids, which are separated out in the sedimentation tanks in primary treatment, are often stabilized by anaerobic decomposition in digestion tank or incinerated.After digestion the sludge can be used as manure after drying on sludge drying beds or by some other means.
7Unit Operation and Processes Classification of Treatment MethodsThe individual treatment methods are usually classified as:Physical unit operationsChemical unit processesBiological unit processes.
8Unit Operation and Processes Physical Unit Operations:Treatment methods in which the application of physical forces predominates are known as physical unit operations. Most of these methods are based on physical forces, e.g. screening, mixing, flocculation, sedimentation, flotation, and filtration.
9Unit Operation and Processes Chemical Unit Processes:Treatment methods in which removal or conversion of contaminant is brought by addition of chemicals or by other chemical reaction are known as chemical unit processes, for example, precipitation, gas transfer, adsorption, and disinfection.
10Unit Operation and Processes Biological Unit Processes: Treatment methods in which the removal of contaminants is brought about by biological activity are known as biological unit processes.This is primarily used to remove biodegradable organic substances from the wastewater, either in colloidal or dissolved form.In the biological unit process, organic matter is converted into gases that can escape to the atmosphere and into bacterial cells, which can be removed by settling.Biological treatment is also used for nitrogen removal and for phosphorous and sulphate removal from the wastewater.
11Unit Operation and Processes The main contaminants in domestic sewage, to be removed areBiodegradable organicsSuspended Solids (SS)pathogens,with first two having been considered as the performance indicators for various treatment units.In general the objective of the domestic wastewater treatment is to bring downBOD < 30 mg/L and SS < 30 mg/L for disposal into inland water bodies.
12Elements of plant Analysis and Design The important terms used in analysis and design of treatment plants are (CPHEEO, 1993):Flow Sheet: It is the graphical representation of a particular combination of unit operations and processes used in treatment.Process Loading Criteria (or designed criteria): The criteria used as the basis for sizing the individual unit operation or process is known as process loading criteria.
13Elements of plant Analysis and Design Solid Balance: It is determined by identifying the quantities of solids entering and leaving each unit operation or process.Hydraulic profile: This is used to identify the elevation of free surface of wastewater as it flows through various treatment units.Plant Layout: It is spatial arrangement of the physical facilities of the treatment plant identified in the flow sheet
14Onsite sanitationRural areas and the outskirts of the urban areas may have insufficient population and infrastructure to support the sewer system and central treatment plant. Hence, onsite sanitation becomes necessary to maintain hygienic living conditions.For environmentally safe onsite sanitation, satisfactory wastewater management techniques should ensure that:water body used for water supplies are not contaminatedflies and vermin have no access to excretasurface water bodies are not polluted by runoffnuisance conditions such as odour are minimized.
15Onsite sanitationAcceptable onsite sanitation systems, depending on circumstances, include septic tanks and surface percolation extended aeration, alone or following a septic tank; and in some area without running water the pit privy is still used.
16Onsite sanitation – SEPTIC TANK This is basically a sedimentation tank with some degree of solid destruction due to sedimentation and subsequent anaerobic digestion.Designed for 24 h liquid retention time at average daily flow.Considering the volume required for sludge and scum accumulation, the septic tank may be designed for wastewater retention time of 1 to 2 days.The flow and characteristics of the wastewater that can be considered for design of septic tank is presented in the Table.Septic tanks can be made from concrete, masonry or fiberglass. Prior two are of rectangular shape and later is generally of circular shape.
18Onsite sanitation – SEPTIC TANK The inlet and outlet are baffled so that the floating matter and grease will be retained in the tank.Heavy solids settle at the bottom of the tank, where the organic fraction will decompose following anaerobic pathway. The production of biogas may interfere with the sedimentation of the solids.Every septic tank should be provided with the ventilation pipe with the top of the pipe covered with suitable mosquito proof wire mesh.The top of the pipe should extend to at least 2 m above the highest building height present in the vicinity of 20 m from the septic tank.
19Onsite sanitation – SEPTIC TANK Construction Details of the Septic Tank
20Onsite sanitation – SEPTIC TANK Post treatment can be achieved by aerobic treatment or subsurface disposal.Diffused air aeration with solids recycling (extended aeration), sand filter or synthetic media filter (attached growth process) can be used for treatment of septic tank effluent.Filter bag equipment and hypochlorite addition will also be suitable for treatment. However, frequent replacement of filter bag and hypochlorite addition makes it costly.
21Design Features of Septic Tank The tank should be large enough to provide space forsedimentation of solidsdigestion of settled sludgestorage of sludgescum accumulated between successive cleaning.
22Design Features of Septic Tank Sewage flow: The flow of sewage is considered to be proportional to the number of fixture units discharging simultaneously. One fixture unit is treated as equivalent to the flow of 10 L/min. This is equivalent to the discharge generated from one water closet (WC) when flushed.The number of fixtures discharging simultaneously depends on the population served. For example for the population of 5 persons, number of fixtures will be one and probable peak discharge will be 10 L/min.Similarly for population of 10, 20, and 30 numbers of fixtures will be 2, 3, and 4, and probable peak discharge will be 20 L/min, 30 L/min, and 40 L/min, respectively.
23Design Features of Septic Tank Detention time: The detention time of 12 to 36 hSewage flow:Q = 40 – 70 litres/capita/day (from WC)Q = 90 – 150 litres/capita/day (from WC+Sullage)Sludge withdrawal: The sludge is withdrawn at a frequency of 6 months to 3 years in large tank. For small tank it can be 2 to 3 years.Length to Width ratio: L = 2 to 3 WWidth >= 0.9mDepth: 1.2 to 1.8m
24Design Features of Septic Tank Free Board: 0.3mRate of sludge accumulation – 30 litres/person/yearInlet & Outlet Baffles:Baffle/Tees should extend upto top level of scum (22cm above top sewage line) & stop below the bottom of the covering slab (min 7.5cm)Inlet – 30cm below the top sewage lineOutlet – penetration 40% of depth of sewageOutlet invert level – 5 to 7.5cm below inlet invert level
25Design Features of Septic Tank Other details of Septic Tank 1. Septic tanks are provided with water tight cover, along with ventilation pipe extending up to 2.0 m above the highest building in the 20 m radius. 2. Inlet and outlet pipes are located on opposite walls with baffle to avoid exit of floating matter.
26Onsite sanitation Pit Privy Pit privies still exist in large number in rural areas particularly in developing countries and underdeveloped countries.A typical privy consists of a pit of about 1.0 m2 by 1.25 m deep, lined with rough boards on the sides and covered with a reinforced concrete slab.A concrete riser supports the seat and ventilator pipe conveys odours through the roof. The slab rests on the concrete curb to which the house is bolted.Earth is banked around the curb to prevent surface runoff from entering the pit.
27Onsite sanitation Pit Privy For average family size such privy will serve for about 10 years.Cleaning is not practical and new privy should be dug once the old is full.The house, slab, and the curb can be moved to the new location.Pit privies with heavy use are often lines with concrete and have an access door at the rear of the unit.This permits the contents to be removed and hauled to a municipal treatment plant or suitable disposal site.
28Onsite sanitation Aqua privy In an aqua privy, urine and faeces are dropped into a water- tight tank which stores and decomposes the excreta in the absence of oxygen (i.e. anaerobically) as in a septic tank.It differs from the septic tank as regards the method of entry of the contents, and from the pit latrine in that the sludge is easier to remove.The aqua privy may be located above ground level or partly above and partly below.
30Onsite sanitation Aqua privy The contents of the tank have no contact with the ground.Unlike the septic tank, the aquaprivy does not require much water; but each day a quantity equivalent to that of the added cleansing water has to be evacuated from it, and therefore provision must be made for a means of draining off effluent.This effluent should not be allowed to run into open fields or gardens.Aqua privies may be recommended whenever the supply of water is limited, although they do not always work satisfactorily. Mosquitoes have been known to breed in the vicinity of this type of latrine.
31Onsite sanitation Bore Hole Latrine A bored-hole latrine is a hole drilled in the ground to receive and store the excreta.It is similar to a basic pit latrine, but pit is a hole bored in a soil auger, either manually or mechanically.It is suitable for stable, permeable soil, free of stones, and where the groundwater is deep beneath the surface.However, bore-hole latrines do present sanitary and health hazards, and expert advice should be sought before they are constructed.
33Onsite sanitation Bore Hole Latrine It consists of a hole covered by a one seat latrine box.Borehole latrines have an augured hole instead of a dug pit and may be sunk to a depth of 10 m or more, although a depth of m is usual.Augured holes, mm in diameter, may be dug quickly by hand or machine in areas where the soil is firm, stable and free from rocks or large stones.While a small diameter is easier to bore, the life of the pit is very short. For example a 300-mm diameter hole with 5 m deep will serve a family of five people for about two years.
34Onsite sanitation Bore Hole Latrine The small diameter of the hole increases the likelihood of blockage, and the depth of augured hole increases the danger of groundwater contamination. Even if the hole does not become blocked, the sides of the hole become soiled near the top, making fly infestation probable.However, borehole latrines are convenient for emergency or short-term use, because they can be prepared rapidly in great numbers, and light portable slabs may be used.The holes should be lined for at least the top 0.5 m or so with an impervious material such as concrete or baked clay. Improved type of bore hole latrine will also avoid fly nuisance and odour.
35Dug well LatrineIt is similar to that of bored-hole latrine but only difference is in the diameter of the whole.In dug well privy 75 cm x 75 cm x 360 cm pit is excavated, which is lined with honey comb brick work or stone work, to absorb the liquid waste.In conservancy system the human excreta from unsewered area are collected in dug well type latrine.
36Primary treatment – Principles, functions design and drawing of screen
37Functions of screenThe function of the bar screen is to prevent entry of solid particles/ articles above a certain size; such as plastic cups, paper dishes, polythene bags, condoms and sanitary napkins into the STP. (If these items are allowed to enter the STP, they clog and damage the STP pumps, and cause stoppage of the plant.)The screening is achieved by placing a screen made out of vertical bars, placed across the sewage flow.
39screenLarger STPs may have two screens: A coarse bar screen with larger gaps between bars, followed by a fine bar screen with smaller gaps between bars.In smaller STPs, a single fine bar screen may be adequate.If this unit is left unattended for long periods of time, it will generate a significant amount of odor: it will also result in backing of sewage in the incoming pipelines and chambers
40Design of screenThe gaps between the bars may vary between 25 and 50 mmThe screen chamber must have sufficient cross-sectional opening area to allow passage of sewage at peak flow rate (2.5 to 3 times the average hourly flow rate) at a velocity of 0.8 to 1.0 m/s,(The cross-sectional area occupied by the bars of the screen itself is not to be counted in this calculation.)The screen must extend from the floor of the chamber to a minimum of 0.3 m above the maximum design level of sewage in the chamber under peak flow conditions.
41Design of screenBar screen is a set of inclined parallel bars, fixed at a certain distance apart in a channel.These are used for removing larger particles of floating and suspended matter.The wastewater entering the screening channel should have a minimum self-clearing velocity m/sec.Also the velocity should not rise to such extent as to dislodge the screenings from the bars.
42Design of screenThe slope of the hand-cleaned screens should be between 300 and 450 with the horizontal and that of mechanically cleaned screens may be between 450 and 800.The submerged area of the surface of the screen, including bars and opening should be about 200% of the c/s area of the extract sewer for separate sewers and 300% for combined sewers
43Design of screen Clear spacing of bars for hand cleaned bar screens - 25 to 50 mm mechanically cleaned bars - 15 mm to 75 mm.The width of the bars facing the flow may be 8 mm to 15 mm anddepth may vary from 25 mm to 75 mm, but sizes less than 8 x 25 mm are normally not used.
44Design of screen - Example 1 Design a bar screen chamber for average sewage flow 20 MLD, minimum sewage flow of 12 MLD and maximum flow of 30 MLD. Solution: Average flow = 20 MLD = m3/Sec Maximum Flow = 30 MLD = m3/Sec Minimum flow = 12 MLD = m3/Sec
45Design of screen - Example 1 Assume: 1) manual cleaning and angle of inclination of bars with horizontal as 30o. 2) size of bars 9 mm x 50 mm, 9 mm facing the flow. 3) A clear spacing of 30 mm between the bars is provided. 4) velocity of flow normal to screen as 0.3 m/sec at average flow.
46Design of screen - Example 1 Net submerged area of the screen opening required = (0.231 m3/Sec) / 0.3 m/sec = 0.77 m2 Assume velocity of flow normal to the screen as 0.75m/sec at maximum flow, hence Net submerged area of screen opening =(0.347 m3/Sec ) / 0.75 m/sec = 0.46 m2 Provide net submerged area = 0.77 m2
47Design of screen - Example 1 Net submerged area of the screen opening required = (0.231 m3/Sec) / 0.3 m/sec = 0.77 m2 Assume velocity of flow normal to the screen as 0.75m/sec at maximum flow, hence Net submerged area of screen opening =(0.347 m3/Sec ) / 0.75 m/sec = 0.46 m2 Provide net submerged area = 0.77 m2
48Design of screen - Example 1 Gross submerged area of the screen When ‘n’ numbers of bars are used the ratio of opening to the gross width will be [(n+1)30] / [(n+1) x n] ≈ 0.77 (for 20 to 30 number of bars) Gross submerged area of the screen = 0.77 / 0.77 = 1 m2 Submerged vertical cross sectional area of the screen = 1.0 x Sin 30 = 0.5 m2 = c/s area of screen chamber,
49Design of screen - Example 1 Velocity of flow in screen chamber = / 0.5 =0.462 m/sec This velocity is greater than the self cleansing velocity of 0.42 m/sec Provide 30 numbers of bars. Gross width of the screen chamber = 30 x x 0.03 = 1.2 m Liquid depth at average flow = 0.5 / 1.2 = m Provide free board of 0.3 m Hence, total depth of the screen = = m, say 0.75 m
50Design of screen - Example 1 The size of the channel = 1.2 m (width) x 0.75 m (depth) Calculation for bed slope: R = A/P = (0.416 x 1.2) / (2 x ) = m V = (1/n) R2/3 S1/2 S1/2 = V.n / R2/3 = x / (0.246)2/3 S1/2 = S = Bed slope is nearly 1 in 4272 m
51Design of screen - Example 1 Head loss through the screen, h, when screen is not clogged. h = β (W/b)4/3 hv Sin θ = 2.42 (9/30)4/3 [(0.462)2/(2 x 9.81)] Sin 30 = 2.65 x 10-3 m = m = 2.65 mm For half clogged screen, the head loss can be worked out using opening width as half Thus, b = 30/2 = 15 mm And h = 6.67 x 10-3 m = 6.67 mm < 15 mm
52Design of screen - Example 1 However, provide 15 mm drop of after screen. If this head loss is very excessive, this can be reduced by providing bars with rounded edges at upstream, or by reducing width of bars to 6 to 8 mm, or by slight reduction in velocity. Except for the change in shape of bars in other cases the channel dimensions will change.
53Operation and Maintenance aspects OF SCREEN Check and clean trap at frequent intervalsRemove both settled solids (at bottom) and the floating greaseDo not allow solids to get washed out of the trapDo not allow oil and grease to escape the trapRedesign the trap if solids and grease escape on a regular basis, despite good cleaning practices
54grit chamberPrimary treatment – Principles, functions design and drawing of GRIT CHAMBER
55grit chamberGrit chamber is the second unit operation used in primary treatment of wastewater and it is intended to remove suspended inorganic particles such as sandy and gritty matter from the wastewater.The grit chamber is used to remove grit, consisting of sand, gravel, cinder, or other heavy solid materials (inorganic - that have specific gravity much higher than those of the organic solids in wastewater) by the process of sedimentation due to gravitational forces and to pass forward the lighter organic material.
56Principle of grit chamber Based on the process of sedimentation in which the organic matter present in sewage which is having specific gravity greater than that of water (ie 1.0) is made to settle.In still sewage these particles will tend to settle down by gravity whereas in flowing sewage they are kept in suspension because of turbulence in water. Hence as soon as turbulence is retarded by offering storage to sewage these impurities tend to settle down at the bottom of the tank offering such storage.
57Principle of grit chamber Settling of discrete particles (Type I sedimentation or settling).Discrete or granular particles are those which donot change their size, shape and weight.Flocculated particles are those which change their size, shape and weight and thus lose their identity.
58Functions of grit chambers Grit chambers are providedto protect moving mechanical equipment from abrasion and abnormal wearavoid deposition in pipelines, channels, and conduitsreduce frequency of digester cleaning.
59Functions of grit chambers Separate removal of suspended inorganic solids in grit chamber and suspended organic solids in primary sedimentation tank is necessary due to different nature and mode of disposal of these solids.Grit can be disposed off after washing, to remove higher size organic matter settled along with grit particleswhereas, the suspended solids settled in primary sedimentation tank, being organic matter, requires further treatment before disposal.
60Horizontal Velocity in Flow Though Grit Chamber The settling of grit particles in the chamber is assumed as particles settling as individual entities and referred as Type – I settling. The grit chamber is divided in four compartments as inlet zone, outlet zone, settling zone and sludge zoneL – Length of the settling zone, H – Depth of the settling zone,v – Horizontal velocity of wastewaterVo – Settling velocity of the smallest particle intended to be removed in grit chamber.
61Compartments of grit chamber Horizontal Velocity in Flow Though Grit ChamberCompartments of grit chamberZone – I: Inlet zone: This zone distributes the incoming wastewater uniformly to entire cross section of the grit chamber.Zone – II: Outlet zone: This zone collects the wastewater after grit removal.Zone – III: Settling zone: In this zone settling of grit material occurs.Zone – IV: Sludge zone: This is a zone where settled grit accumulates.
62Design of grit chambers Now, if Vs is the settling velocity of any particle, thenFor Vs >= Vo these particles will be totally removed,For Vs < Vo, these particles will be partially removed,Where, Vo is settling velocity of the smallest particle intended to be removed.The smallest particle expected to be removed in the grit chamber has size 0.2 mm and sometimes in practice even size of the smallest particle is considered as 0.15 mm. The terminal velocity with which this smallest particle will settle is considered as Vo. This velocity can be expressed as flow or discharge per unit surface area of the tank, and is usually called as ‘surface overflow rate’ or ‘surface settling velocity’
63Design of grit chambers Now for 100 percent removal of the particles with settling velocity Vs >= Vo, we haveDetention time = L/v = H/VoOr L/H = v/Vo …………(1)To prevent scouring of already deposited particles the magnitude of ‘v’ should not exceed critical horizontal velocity Vc, and the above equation becomesL / H = Vc / Vo
64Design of grit chambers The critical velocity, Vc, can be given by the following equationβ= constant, = 0.04 for unigranular sand= 0.06 for non-uniform sticky materialf = Darcy –Weisbach friction factor = 0.03 for gritty matterg = Gravitational acceleration,S = Specific gravity of the particle to be removed (2.65 for sand), andD = Diameter of the particle, m
65Design of grit chambers The grit chambers are designed to remove the smallest particle of size 0.2 mm with specific gravity around For these particles, using above expression the critical velocity comes out to be Vc = m/sec.
66Settling Velocity of the Particles Settling velocity of any discrete particle depends on its individual characteristics and also on the characteristics of the fluid. Assuming particles to be spherical, the settling velocity of any particle, Vs, can be given by the following formula:R = Reynold’s Number = Vs.D/νν = Kinematic viscosity of the fluid
67drawing of grit chambers Horizontal flow grit chamber
68Horizontal Flow Rectangular Grit Chamber A long narrow channel is used in this type of grit chamber. The wastewater moves through this channel in more or less plug flow condition with minimal mixing to support settling of the particles.Higher length to width ratio of the channel is used to minimize mixing. For this purpose a minimum allowance of approximately twice the maximum depth or 20 to 50% of the theoretical length of the channel should be given for inlet and outlet zones.
69Horizontal Flow Rectangular Grit Chamber The width of this channel is kept between 1 and 1.5 m and the depth of flow is normally kept shallow. A free board of minimum 0.3 m and grit space of about 0.25 m is provided.For large sewage treatment plant, two or more number of grit chambers are generally provided in parallel. The detention time of 30 to 60 seconds is recommended for the grit chamber.
70Control of Velocity Through the Grit Chamber With variation in sewage flow received at treatment plant, it is important that velocity of the wastewater in the grit chamber should be maintained nearly constant.Otherwise when flow is lower, deposition of not only inorganic solids but also organic solids will occur in grit chamber due to lowering of velocity.With flow higher than average, when the velocity will exceed the critical velocity, scouring of already deposited grit particles will occur leading to failure of performance..
71Control of Velocity Through the Grit Chamber Hence for proper functioning, the velocity should not be allowed to change in spite of change in flow in the grit chamber.This can be achieved by provision of proportional weir or Parshall flume at the outlet end of grit chamber.The shape of the opening between the plates of a proportional weir is made in such a way that the discharge is directly proportional to liquid depth in grit chamber. As a result the velocity of water in the chamber will remain constant for all flow conditions.
73Disposal of GritConsiderable quantities of grit will be collected at the sewage treatment plant, about to 0.2 m3/ML. Quantity of grit will be more particularly for combined system. Necessary arrangement should be made at the treatment plant for collection, storage and disposal of this grit matter. The grit collected can be disposed in the following manner:In large treatment plant, grit is incinerated with sludgeIn the past, grits along with screening was dumped into sea.Generally, grit should be washed before disposal to remove organic matter.Land disposal after washing is most common.
74Design of Grit ChamberDesign a grit chamber for population with water consumption of 135 LPCD. Solution Average quantity of sewage, considering sewage generation 80% of water supply, is = 135 x x 0.8 = 5400 m3/day = m3/sec Maximum flow = 2.5 x average flow = x 2.5 = m3/sec Keeping the horizontal velocity as 0.2 m/sec (<0.228 m/sec) and detention time period as one minute.
75Design of Grit ChamberLength of the grit chamber = velocity x detention time = 0.2 x 60 = 12.0 m Volume of the grit chamber = Discharge x detention time = x 60 = 9.36 m3 Cross section area of flow ‘A’ = Volume / Length = 9.36/12 = m2 Provide width of the chamber = 1.0 m, hence depth = m Provide 25% additional length to accommodate inlet and outlet zones.
76Design of Grit ChamberThe length of the grit chamber = 12 x 1.25 = 15.0 m Provide 0.3 m free board and 0.25 m grit accumulation zone depth, hence total depth = = 1.33 m and width = 1.0 m Size of the Grit Chamber 15m x 1m x (1.03m+0.3m) Freeboard
77Design of Aerated Grit Chamber Design aerated grit chamber for treatment of sewage with average flow of 60 MLD. Consider the peak factor of 2. Solution: 1. Average flow = 60 MLD = m3 /sec, and Peak flow = x 2.0 = m3/sec 2. Volume of grit chamber Provide two chambers to facilitate periodic cleaning and maintenance Provide detention time = 3.0 min Volume of each tank = x 3 x 60 /2 = m3
78Design of Aerated Grit Chamber 3. Dimensions of aeration basin: Provide depth to width ratio of 1: 1.2 Provide depth = 3.0 m hence width = 1.2 x 3.0 = 3.6 m Length = / (3 x 3.6) = m Increase length by 20% to account for inlet and outlet conditions. Total length = x 1.2 = m. 4. Determine the air-supply requirement Consider 0.3 m3 /min.m of length air supply Air Requirement = x 0.3 = 4.17 m3 /min Provide air swing arrangement at 0.5 m from floor
79Design of Aerated Grit Chamber 5. Quantity of grit : Consider grit collection m3/103 m3 Volume of grit = x 60 x 60 x 24 x x 10-3 = 1.8 m3/d 6. Check for surface overflow rate (SOR) Settling velocity of the smallest particle = 2.4 cm/sec, Actual SOR in the grit chamber = / (2 x 3.6 x ) = m/s = 1.67 cm/sec, which is less than the settling velocity of the smallest particle hence design is safe.
80Principle of sedimentation tanks Based on the process of sedimentation in which the suspended organic solid matter present in sewage which is too light to be removed by grit chamber is removed.In still sewage these particles will tend to settle down by gravity whereas in flowing sewage they are kept in suspension because of turbulence in water. Hence as soon as turbulence is retarded by offering storage to sewage these impurities tend to settle down at the bottom of the tank offering such storage.
81Principle of sedimentation tanks Settling of Flocculated particlesFlocculated particles are those which change their size, shape and weight and thus lose their identity.Due to involvement of many unknown parameters under settling of light weight, sticky, and non regular shaped particles, the classical laws of sedimentation as applicable in grit removal are not valid and this settling is called as flocculant settling.
82function of primary sedimentation tanks The purpose and function of the clarifier is threefold:Allow settling of light organic suspended solids coming out of the Grit chamber to the bottom of the clarifierTo thicken the solids, in order to produce a thick underflowTo produce clear supernatant water, in the overflow from the clarifierThe clarifier tank is only a passive device: All the above actions occur due to gravity.
83design of primary sedimentation tanks The primary sedimentation tank generally removes 30 to 40% of the total BOD and 50 to 70% of suspended solids from the raw sewage.The flow through velocity of 1 cm/sec at average flow is used for design with detention period in the range of 90 to 150 minutes.This horizontal velocity will be generally effective for removal of organic suspended solids of size above 0.1 mm.Effluent weirs are provided at the effluent end of the rectangular tanks, and around the periphery in the circular tanks.
84design of primary sedimentation tanks Weir loading less than 185 m3/m.d is used for designing effluent weir length (125 to 500 m3/m.d). Where primary treatment follows secondary treatment, higher weir loading rates can be used.The sludge collection hopper is provided near the centre in circular tank and near the influent end in rectangular tanks.A baffle is provided ahead of the effluent weir for removal of floating matter. This scum formed on the surface is periodically removed from the tank mechanically or manually.
85design of primary sedimentation tanks Primary sedimentation tanks can be circular or rectangular tanks (Figure 16.2) designed using average dry weather flow and checked for peak flow condition.The numbers of tanks are determined by limitation of tank size. Two tanks in parallel are normally used to facilitate maintenance of any tank.The diameter of circular tank may range from 3 to 60 m (up to 45 m typical) and it is governed by structural requirements of the trusses which supports scrapper in case of mechanically cleaned tank.
86design of primary sedimentation tanks Rectangular tank with length 90 m are in use, but usually length more than 40 m is not preferred.Width of the tank is governed by the size of the scrappers available for mechanically cleaned tank.The depth of mechanically cleaned tank should be as shallow as possible, with minimum 2.15 m.The average depth of the tank used in practice is about 3.5 m. In addition, 0.25 m for sludge zone and 0.3 to 0.5 m free board is provided.The floor of the tank is provided with slope 6 to 16 % (8 to 12 % typical)for circular tank and 2 to 8% for rectangular tanks.
87design of primary sedimentation tanks The scrappers are attached to rotating arms in case of circular tanks and to endless chain in case of rectangular tanks.These scrappers collect the solids in a central sump and the solids are withdrawn regularly in circular tanks.In rectangular tanks, the solids are collected in the sludge hoppers at the influent end, and are withdrawn at fixed time intervals.The scrapper velocity of 0.6 to 1.2 m/min (0.9 m/min typical) is used in rectangular tank and flight speed of 0.02 to 0.05 rpm (0.03 typical) is used in circular tank.
88design of primary sedimentation tanks Inlets for both rectangular and circular tanks are to be designed to distribute the flow equally across the cross section.Scum removal arrangement is provided ahead of the effluent weir in all the PST.The surface overflow rate of 40 m3/m2.d (in the range 35 to 50 m3/m2.d) is used for design at average flow.At peak flow the surface overflow rate of 80 to m3/m2.d could be used when this PST is followed by secondary treatment.
89design of primary sedimentation tanks Lower surface settling rates are used when waste activated sludge is also settled in the PST along with primary solids. In this case the surface overflow rate of 24 to 32 m3/m2.d and 48 to 60 m3/m2.d are used for average and peak flow conditions, respectively.The weir loading rate less than 185 m3/m.d is used for designing effluent weir length (in the range 125 to m3/m.d). Weir loading rate up to 300 m3/m.d is acceptable under peak flow condition.
90design of primary sedimentation tanks Higher weir loading can be acceptable when primary treatment is followed by secondary treatment.As such the weir loading rate has very less impact on the overall performance of sewage treatment plant when secondary treatment is provided after primary treatment.The detention time in PST could be as low as 1 h to maximum of 2.5 h. Providing detention time of 1.5 to 2.5 h at average flow is a common practice.
94drawing of circular sedimentation tank The figure shows only some part of the tank floor (1). A bucket-shaped sludge collection pit(2) collects the sludge that is swept by the squeegees.The collected sludge is pumped out through the outlet port(3) and outlet pipe(4) In the center of the pit, an RCC pillar (5) is provided. A bush housing (6) is mounted on this pillar. The housing contains a bush, which provides a frictionless support to the rotating rake (7).
95Design of primary sedimentation tank Design the primary sedimentation tank to treat wastewater with average flow rate of 10 MLD and peak flow of 22.5 MLD. Solution Assume surface settling rate = 40 m3/m2.d Therefore, the surface area of the tank = 10 x 106 / 40 x 103 = 250 m2 Check for peak flow condition: The SOR at peak flow = 22.5 x 103 /250 = 90 m3/m2.d < recommended value at peak flow.
96Design of primary sedimentation tank Assume width = 6.0 m Therefore theoretical length = 250/6 = > 40 m Hence, provide two tanks in parallel Total length of each tank = 41.66/2 + 2 (inlet) + 2 (outlet) = say m Flow rate x detention time = depth x surface area = volume of tank or Flow / Surface area = depth / detention time = Surface settling rate Provide detention time of 1.5 h Liquid depth required = 40 x 1.5 / 24 = 2.5 m
97Design of primary sedimentation tank Flow through velocity = (0.116 m3/sec) / (2 x 2.5 x 6) = m/sec < 1 cm/sec hence O.K. At peak flow, the flow through velocity = 22.5 x 103/(2 x 6 x 2.5) = 750 m/d = m/sec. (Horizontal velocity should be checked for non-scouring velocity i.e. less than 0.06 m/sec.) Provide total depth = (free board) (space for sludge) = 3.25 m Weir loading rate = 10 x 103 / 12 = m3/m.day > 185 m3 /m.day
98Design of primary sedimentation tank Length of weir required = 10 x 103 / 185 = m Hence, provide about 27.1 m of weir length for each tank. This can be provided by two effluent collection channels across the width at outlet end offering total 24.0 m and side weir of total 1.55 m on each side.
99Operation and Maintenance aspects. If properly designed, engineered and constructed, clarifiers call for very little attention in terms of operation and maintenance.Indeed, the unmechanized (hopper-bottom) settling tanks may be said to be zero- maintenance units.Some parts of the mechanical rake (such as the motor, gearbox etc.) call for only routine maintenance.The sacrificial rubber squeegees sweeping the floor of the clarifier need to be checked and replaced, possibly once in two years.