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Anaerobic Treatment PreetiBala Adatiya Lecturer School of Biotechnology DAVV Anaerobik Arıtma Biyoteknolojisi Anaerobic Treatment The use of microbes in the absence of oxygen for the stabilization of organic material by conversion to methane, carbon dioxide, new biomass and inorganic products. Anaerobic treatment is most suitable for wastewaters with COD concentrations in the high strength range (>2000 mg/l) Anaerobik Arıtma Biyoteknolojisi Anaerobic Digestion Process Complex Organics Acid producing bacteria (acidogens) Organic acids and H2 CH4 ve CO2 Methane producing bacteria (methanogenics) Anaerobic Digestion Process Anaerobik Arıtma Biyoteknolojisi Three Mechanisms Occurring: Hydrolysis Process – conversion of insoluble high molecular compounds (lignin, carbohydrates, fats) to lower molecular compounds Acidogenesis Process – conversion of soluble lower molecular components of fatty acids, amino acids and sugars (monosaccharides) 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 Anaerobik Arıtma Biyoteknolojisi ANAEROBIC SLUDGE DIGESTION Anaerobic digestion is one of the oldest process used for the stabilization of sludges. It involves the decomposition of organic and inorganic matter in the absence of molecular oxygen. Anaerobik Arıtma Biyoteknolojisi Process description: In the anaerobic digestion process, the organic material in mixture of primary settled and biological sludges is converted biologically. Under anaerobic conditions, to a variety of and products including methane (CH4) and carbondioxide. The process is carried out in an airtight reactor. Sludge, introduced continuously or intermittently, is retained in the reactor for varying periods of time. The stabilized sludge, withdrawn from the reactor, is reduced in organic and pathogen content. Anaerobik Arıtma Biyoteknolojisi The two types of commanly used anaerobic digesters are identified as; Standard-rate High-rate Anaerobik Arıtma Biyoteknolojisi Standard-Rate Digester: In the standard-rate digestion process; the contents of digesters are usually unheated and unmixed. Detention times for the standard-rate process vary from 30 to 60 days. They are usually carried out as a single-stage process. The functions of digestion, sludge thickening, and supernatant formation are carried out simultaneously. As a result of digestion, the sludge stratifies by forming a supernatant layer above the digesting sludge and becomes more mineralized. As a result of the stratification and the lack of mixing, not more than 50% of the volume of a standard-rate single-stage digester is used. Because of these limitations, the standard-rate process is used to small installations. Anaerobik Arıtma Biyoteknolojisi High-Rate Digester: In the high-rate digestion process; the contents of digesters are heated and mixed completely. The required detention time for high-rate digestion is typically 15 days or less with the exception of higher loading rates and improved mixing, there are only a few difference between the primary digester in a conventional two-stage process and a single-stage high-rate digester. Anaerobik Arıtma Biyoteknolojisi Two-stage digestion; frequently, a high-rate digester is coupled in series with a second digestion tank. The first tank is used for digestion and is heated and equipped wirh mixing facilities. The primary function of the second stage is to separate the digested solids from the sopernatant, however additional digestion and gas production may occur. Process design: Anaerobik Arıtma Biyoteknolojisi An anaerobic digester is well mixed with no liquid solids separation. Consequently, the bioreactor can be treated as a continuous stirred tank reactor (CSTR) in wich the HRT (hydraulic retention time) and SRT (sludge retention time or sludge age) are identical, the quantity of methane gas can be calculate as below: VCH4 (L/day) = 0,35 (L/gas)*(E*Q*S0-1,42*Px) Anaerobik Arıtma Biyoteknolojisi AEROBIC SLUDGE DIGESTION Aerobic 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 Anaerobik Arıtma Biyoteknolojisi 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 Anaerobik Arıtma Biyoteknolojisi 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 description: Anaerobik Arıtma Biyoteknolojisi Aerobic digestion is similar to the activated-sludge process. As the supply of available substrate (food) is depleted, the microorganisms begin to consume their own protoplasm to obatin energy for cell maintenance reactions when this occurs, the microorganisms are ssaid to be in the endogenous phase. C5H7NO2 + 7O2 5CO2 + NO3- + 3H2O + H+ Cell tissue is oxidized aerobically to CO2 and H2O and ammonia. The ammonia form this oxdation is subsequently oxidized to nitrate. Anaerobik Arıtma Biyoteknolojisi An actuality, only about 75 to 80 percent of the cell can be oxidized, the remaining 20 to 25 percent is composed of inert component. A pH drop can occur when ammonia is oxidized to nitarte if the alkalinity of the wastewater is insufficient, chemical addition may be required. Two variation of the process are commonly used: conventional aerobic digestion (with air) high-purity oxygen aerobic digestion Aerobic digestion with air is the most commonly used process. Process design: Anaerobik Arıtma Biyoteknolojisi Factors 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.