Sewage treatment
- 1. Sewage treatment DR AMRUT SWAMI DEPT. OF COMMUNITY MEDICINE
- 3. sewage Sewage is waste water from community, containing solid & liquid excreta, derived from houses, street & yard washings, factories & industries. Sullage: Waste water which does not contain human excreta.
- 4. Amount of sewage that flows in sewers depends upon: Habits of the people Time of the day The average amount of sewage which flows through the sewerage system in 24 hours is called the “dry weather flow”
- 5. Health aspects Environmental problems: Creation of nuisance, unsightliness & unpleasant odours Breeding of flies & mosquitoes Pollution of soil & water supplies Contamination of food Increase in the incidence of disease
- 6. Composition of sewage 99.9 % - water 0.1 % - Solids (Organic & inorganic) (suspension & solution) Offensive odour is because of decomposition of organic matter 1 gm of faeces contains 1000 million of E. coli, 10 to 100 million of faecal streptococci & 1 to 10 million spores of Cl. Perfringens.
- 8. Aims of sewage purification To “stabilize” the organic matter so that it can be disposed off safely To convert the sewage water into an effluent of an acceptable standard of purity which can be disposed off in to land, rivers or sea.
- 9. “strength” of sewage Biochemical Oxygen Demand (BOD): Amount of oxygen absorbed by a sample of sewage during a specified period, generally 5 days, at a specified temperature, generally 20º C for the aerobic destruction or use of organic matter by living organisms. If the BOD is 300 mg/L & above – strong 100 mg/L – weak
- 10. Chemical Oxygen Demand (COD): measures the oxygen equivalent of that portion of the organic matter in a sample which is susceptible to oxidation by a strong chemical oxidiser. Suspended Solids: 100 mg/L – weak 500 mg/L – strong
- 11. Decomposition of organic matter 1. Aerobic process: Most efficient method of reduction of sewage Requires continuous supply of free dissolved oxygen Organic matter is broken down into simpler compounds like CO2, water, ammonia, nitrites, nitrates & sulphates by the action of bacterial organisms including fungi & protozoa
- 12. 2. Anaerobic process: Effective when sewage is highly concentrated & contains plenty of solids. End products of the decomposition are methane, ammonia, CO2 & H2. Reactions are slower & mechanism of decomposition extremely complex.
- 13. Modern sewage treatment Primary treatment: Solids are separated from the sewage by screening & sedimentation & subjected to anaerobic digestion
- 14. Modern sewage treatment Secondary treatment: Effluent is subjected to aerobic oxidation
- 16. 1. Screening Metal screen – to intercept large floating objects like pieces of wood, rags, masses of garbage & dead animals – to prevent clogging. Screen consists of vertical or inclined steel bars set at 5 cm apart. Screenings are removed from time to time, either manually or mechanically & disposed off by trenching or burial.
- 17. 2. Grit chamber Narrow chamber of 10 to 20 meters in length. Maintain a constant velocity of 1 foot per second, with a detention period of 30 seconds to 1 minute. Function is to allow settlement of heavier solids like sand & gravel, which is removed periodically & disposed off by plain dumping or trenching.
- 18. 3. Primary sedimentation Large rectangular tank. Sewage flows very slowly at 1 – 2 feet per minute. Sewage spends 6 – 8 hours in the tank. Sedimentation of suspended matter – 50-70% solids settle down. 30-40% reduction in the number of coliform organisms.
- 19. Organic matter which settles down is called as ‘sludge’ & removed mechanically. Fat & grease rise to the surface – ‘scum’ – removed from time to time & disposed off. Microorganisms present in the sewage attack complex organic solids & break them down into simpler soluble substances & ammonia.
- 20. If sewage contains organic trade wastes, it is treated with chemicals like lime, aluminium sulphate & ferrous sulphate.
- 21. Secondary treatment One of the following methods: 1. Trickling filter method 2. Activated sludge process
- 22. 1. Trickling filter method Bed of crushed stones, 1-2 m deep & 2-30 m in diameter, depending on size of population. Effluent from primary sedimentation tank is sprinkled uniformly on the surface of the bed by a revolving device. Device consists of hollow pipes having row of holes. Pipes keep rotating, sprinkling the effluent in a thin film on the surface of the filter.
- 23. A very complex biological growth consisting of algae, fungi, protozoa & bacteria occurs over the surface & down through the filter – known as zoogleal layer. As the effluent percolates through the filter bed, it gets oxidized by the bacterial flora in the zoogleal layer – biological action. Do not need rest pauses, because wind blows freely through the beds supplying the oxygen needed by zoogleal flora.
- 24. Zoogleal layer lives, grows & dies. The dead matter sloughs off, breaks away & is washed down the filter. It is a light green, flocculent material called ‘humus’. Oxidized sewage is now led into the secondary sedimentation tanks or humus tanks
- 25. 2. Activated sludge process “Heart” of the process is ‘aeration tank’ The effluent from primary sedimentation tank is mixed sludge drawn from the final settling tank (known as activated sludge or return sludge, rich culture of aerobic bacteria) Proportion of activated sludge to incoming effluent is 20 to 30 %.
- 26. Mixture is subjected to aeration in the aeration chamber for 6-8 hours. Aeration is accomplished by mechanical agitation or by forcing compressed air continuously from the bottom of the aeration tank. During this process, organic matter of the sewage gets oxidized into CO2, nitrates & water with the help of aerobic bacteria in the activated sludge.
- 27. Typhoid & cholera organisms are destroyed. This plant occupies less space, requires skilled operations. Best suited for larger cities.
- 28. Secondary sedimentation Oxidized sewage is detained for 2-3 hours. Sludge that is collected is called as ‘aerated sludge’ or ‘activated sludge’ because it is fully aerated. It is inoffensive, rich in bacteriae, nitrogen & phosphates – if dehydrated, it is valuable manure.
- 29. Part of the activated sludge is pumped back into the aeration tanks in the activated sludge process & rest pumped into the sludge digestion tanks for treatment & disposal.
- 30. Methods of sludge disposal
- 31. 1. Sludge digestion If incubated under favourable conditions of temperature & pH, it undergoes anaerobic auto-digestion in which complex solids are broken down into water, CO2, methane & ammonia. Volume is reduced. Takes 3 – 4 weeks or longer Residue is inoffensive, sticky & tarry mud which will dry readily & form excellent manure. Methane gas released can be used for heating & lighting purpose
- 32. 2. Sea disposal Sea coast towns & cities can dispose of sludge by pumping it into the sea.
- 33. 3. Land Sludge can be disposed of by composting with town refuse.
- 35. 1. Disposal by dilution Disposal into water sources like rivers & streams. Effluent is diluted & impurities are oxidized by the dissolved oxygen in water. Effluent must be rendered free from pathogenic organisms by chlorination. It is recommended that effluent should not have more than 30 mg/L of suspended solids & BOD should not be more than 20 mg/L
- 36. 2. Disposal on land If suitable land is available the effluent can be used for irrigation purpose. E.g., the Okhla Sewage Treatment Plant in Delhi
- 37. Other methods of sewage disposal 1. Sea outfall 2. River outfall 3. Oxidation ponds 4. Oxidation ditches
- 38. 1. Sea outfall Sea coast towns & cities may dispose off their sewage by discharging it into the sea. Purification takes place by dilution & solids get slowly oxidized. Drawback – offensive solid matter may be washed back to the shore & create public nuisance.
- 39. 2. River outfall Raw sewage should never be discharged into rivers. It should be purified first.
- 40. 3. Land treatment (sewage farming) After grit removal, screening & short period of settlement, sewage may be applied to land, if sufficient & suitable (porous soil) land is available. Land is first laid into ridges & furrows. Sewage is fed into furrows intermittently & crops are grown on the ridges. Crops which do not come in contact with sewage & likely to be eaten raw are found suitable.
- 41. Drawbacks: May not be possible to operate during rainy season. Have to use alternate method. If not managed properly, farms will stink – “sewage sickness” – because of lack of sufficient aeration & rest pauses to the land.
- 42. 4. Oxidation pond Known as – waste stabilization pond, redox pond, sewage lagoons, etc. Open, shallow pool 1 to 1.5 m deep with an inlet & outlet. To qualify as oxidation pond, there must be presence of Algae, certain type of bacteria which feed on decaying organic matter & sunlight
- 43. Organic matter is oxidized by bacteria to simple chemical compounds like CO2, ammonia & water.
- 44. The algae with the help of sunlight, utilize CO2, water & inorganic minerals for their growth – mutual beneficial biological balance between algae & bacteria.
- 45. Oxygen needed for oxidation is derived from atmosphere but mostly from the algae which liberates oxygen under the influence of sunlight.
- 46. Sunlight is important factor in functioning of oxidation pond. Oxidation ponds are mainly aerobic during sunshine hours & some hours of the night. In remaining hours of the night, bottom layers are mainly anaerobic. Effluent may be treated for land irrigation or discharged into water sources after appropriate treatment.
- 47. 5. Oxidation ditches Use of mechanical rotors for extended aeration Land required is less as compared to oxidation ponds. These are low-cost treatment methods for the purification of sewage.