UNIT III PRIMARY TREATMENT OF SEWAGE

1. UNIT III PRIMARY TREATMENT OF SEWAGE

2. UNIT III PRIMARY TREATMENT OF SEWAGE

3. UNIT III PRIMARY TREATMENT OF SEWAGE

4. UNIT III PRIMARY TREATMENT OF SEWAGE • Objective • Unit Operation and Processes • Selection of treatment processes • Onsite sanitation - Septic tank, Grey water harvesting • Primary treatment – Principles, functions design and drawing of • screen, • grit chambers • primary sedimentation tanks • Operation and Maintenance aspects.

5. Objective • Primary 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 are • Screening for removing floating papers, rages, cloths, etc., • Grit chambers or detritus tanks for removing grit and sand • Skimming tanks for removing oils and grease.

6. Objective • Primary 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.

7. Unit Operation and Processes Classification of Treatment Methods The individual treatment methods are usually classified as: • Physical unit operations • Chemical unit processes • Biological unit processes.

8. Unit 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.

9. Unit 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.

10. Unit 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.

11. Unit Operation and Processes The main contaminants in domestic sewage, to be removed are • Biodegradable organics • Suspended 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 down • BOD < 30 mg/L and SS < 30 mg/L for disposal into inland water bodies.

12. Elements 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.

13. Elements 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

14. Onsite sanitation • Rural 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 contaminated • flies and vermin have no access to excreta • surface water bodies are not polluted by runoff • nuisance conditions such as odour are minimized.

15. Onsite sanitation Acceptable 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.

16. Onsite 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.

17. Onsite sanitation – SEPTIC TANK

18. Onsite 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.

19. Onsite sanitation – SEPTIC TANK Construction Details of the Septic Tank

20. Onsite 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.

21. Design Features of Septic Tank The tank should be large enough to provide space for • sedimentation of solids • digestion of settled sludge • storage of sludge • scum accumulated between successive cleaning.

22. Design 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.

23. Design Features of Septic Tank • Detention time: The detention time of 12 to 36 h • Sewage 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 W Width >= 0.9m Depth: 1.2 to 1.8m

24. Design Features of Septic Tank • Free Board: 0.3m • Rate of sludge accumulation – 30 litres/person/year • Inlet & 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 line Outlet – penetration 40% of depth of sewage Outlet invert level – 5 to 7.5cm below inlet invert level

25. Design 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.

26. Onsite 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.

27. Onsite 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.

28. Onsite 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.

29. Onsite sanitation Aqua privy

30. Onsite 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.

31. Onsite 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.

32. Onsite sanitation Bore Hole Latrine

33. Onsite 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 4 - 6 m is usual. • Augured holes, 300-500 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.

34. Onsite 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.

35. Dug well Latrine • It 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.

36. screen Primary treatment – Principles, functions design and drawing of screen

37. Functions of screen • The 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.

38. drawing of screen

39. screen • Larger 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

40. Design of screen • The gaps between the bars may vary between 25 and 50 mm • The 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.

41. Design of screen • Bar 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 0.375 m/sec. • Also the velocity should not rise to such extent as to dislodge the screenings from the bars.

42. Design of screen • The 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

43. Design 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 and • depth may vary from 25 mm to 75 mm, but sizes less than 8 x 25 mm are normally not used.

44. Design 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 = 0.231 m3/Sec Maximum Flow = 30 MLD = 0.347 m3/Sec Minimum flow = 12 MLD = 0.139 m3/Sec

45. Design 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.

46. Design 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

47. Design 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

48. Design 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)30 + 9 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,

49. Design of screen - Example 1 Velocity of flow in screen chamber = 0.231 / 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 0.009 + 31 x 0.03 = 1.2 m Liquid depth at average flow = 0.5 / 1.2 = 0.416 m Provide free board of 0.3 m Hence, total depth of the screen = 0.416 + 0.3 = 0.716 m, say 0.75 m

50. Design 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 0.416 + 1.2) = 0.246 m V = (1/n) R2/3 S1/2 S1/2 = V.n / R2/3 = 0.462 x 0.013 / (0.246)2/3 S1/2 = 0.0153 S = 0.000234 Bed slope is nearly 1 in 4272 m