Sewage Secondary Treatment After Primary Treatment, the Sewage Water Is Subjected to the Next Phase Called Biological Treatment Or Secondary Treatment
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Sewage secondary treatment After primary treatment, the sewage water is subjected to the next phase called biological treatment or secondary treatment. Dissolved and suspended biological materials are removed by indigenous microbial decomposition in a managed environment. In this treatment, BOD is reduced up to 90-95%. Biological treatment is carried out in two important phases: 1. Aerobic phase: oxidation lagoons trickling filters, activated sludge 2. Anaerobic phase: sludge digestion A. Oxidation lagoons /oxidation ponds / stabilization ponds 1. It is outdoor, simple, suspension type aerobic treatment. 2. It is suggested for small societies in rural areas where sufficient low lying land of little real state value is available. 3. It is oldest of the sewage treatment systems used in most of developing countries due to its low cost. 4. In this shallow pond is required which should be maximum of 10 feet depth having inlet and outlet. 5. It consists of three different zones as shown in the following diagram: Requirements of Oxidation pond: 1. Presence of algal growth which produces oxygen. 2. Oxygen is also available from atmosphere. 3. Heterotrophic aerobic and anaerobic microbes for decomposing sewage. 4. Sunlight required for photosynthesis, also has bactericidal properties (UV). Advantages of oxidation pond: 1. Simple 2. Inexpensive 3. Low maintenance Disadvantages of oxidation pond: 1. Effected by environment: low temperature, cloudy sky lowers efficiency 2. Takes longer time(Long retention time) 3. Gives bad odour(H2 S) 4. Not hygienic 5. Just lowers BOD by 25%to 60% B. Trickling Filter Method 1. It is also outdoor, aerobic, relatively simple but of film flow type. 2. It consists of large cylindrical ,concrete tank of diameter:8-16m and depth:2-3m 3. Tank is filled with porous bed of crushed stones,rocks,clinker etc. 4. Void age=total air space in bed/total bed volume x 100=45-55% to prevent clogging 5. Sewage enter through inlet, reaches to sprinkler. As its arms rotate, sewage is sprayed on the porous bed. Below the porous bed ,network of drain pipe is present 6. The treated sewage trickles (so called trickling filter) to the bottom, collects in a depression called slump From there, single main outlet carry treated sewage effluent. 7. Rotating arms of sprinkler spray sewage which helps to saturate the sewage drops with oxygen. 8. As sprinkling is intermittent, sewage gets enough space and time to percolate and system maintain aerobic conditions. (Spraying is not continuous). 9. As sewage trickles, microbes in sewage colonize the surface and form microbial film. 10. The surface becomes covered with aerobic microbial population comprising of bacterial species including Sphaerotilus natans, Beggiatoa, Flavobacterium, Achromobacter, Zooglea and Pseudomonas, microalgae, microfungi and protozoa. 11. In this, slime producing bacteria plays a very important e.g. Zooglea ramigera since it is main microbe,the film is called Zooglea film.With time film becomes thicker and thicker. 12. After sometime, the oxygen and nutrients cannot reach innermost cells and they die and film gets slogged off. This can block the pores and untreated sewage stars accumulating on the top. Then porous bed has to be changed Advantages of trickling filter: 1. Lowers BOD of sewage by 75-85% 2. Efficiency can be increased further by using series of such filters 3. Relatively cheaper 4. Easy to operate 5. Advance than oxidation pond Disadvantages 1. Porous bed has to be changed periodically due to clogging 2. Efficiency effected by environment 3. During spraying aerosols are formed that help in spread of pathogen. C. Activated sludge process 1. It is highly advanced, indoor, aerobic and suspension type. 2. Raw sewage after primary treatment is introduced in large tank called aeration tank. 3. Sewage is vigorously aerated and agitated in the presence of activated sludge. 4. This sludge has been taken from previous run and consists of microbial flocs which are aggregates of microorganism in the slime 5. Agitation and aeration can be brought by two ways: a. From the bottom by bubble diffuser(air injection) b. By mechanical devices called paddle stirrer 6. Microbial activity is maintained at high levels by reintroduction of most of the settled activated sludge, hence names as activated sludge process. 7. Following aerobic microbial decomposers develop biological floc that decomposes organic matters into simple soluble molecules, amino acids, ammonia, phosphorus, nitrates, CO2, H2O etc.: Bacteria: Escherichia, Enterobacter, Achromobacter, Flavobcaterium, Pseudomonas, Zooglea, Microccocus, Sphaerotilus, Beggiatoa, Thiothrix etc. Protozoa: Amoaebe, Spriotrich and Vertcella. Sedentary rotifers Filamentous fungi: Geotrichum, Cephalosporium, Penicillum and Cladodsporium. Yeast: very low number A. Septic tank Septic tank is an anaerobic digestion tank usually employed for the treatment of sewage discharged from the communities or residential quarters where population is less and sufficient land is available. A septic tank is made of metal or concrete that achieves two goals: Sedimentation of solid materials Biological degradation of settled solid materials. As sewage enters the tank, sedimentation of solid materials occur in the bottom of the tank named sludge which undergo anaerobic decomposition through the action of anaerobic bacteria that degrade solid organic compounds to simpler and soluble compounds. The collection pipelines of septic tanks should be checked regularly to avoid any leakage near residential area as it may contaminate the drinking water pipelines if they damaged. Septic tank treatment cannot guarantee elimination of all pathogenic microorganisms. .