Up-flow Anaerobic Sludge Blanket

UP-FLOW ANAEROBIC SLUDGE BLANKET

The Up-flow Anaerobic Sludge Blanket (UASB) reactor maintains a high concentration of biomass through the formation of highly settleable microbial sludge aggregates. The wastewater flows upwards through a layer of very active sludge to cause anaerobic digestion of organics of the wastewater. At the top of the reactor, three phase separation between gas-solid-liquid takes place. Any biomass leaving the reaction zone is directly recirculated from the settling zone. This process is suitable for both soluble wastewaters as well as wastewaters containing particulate matter. A typical UASB reactor consists of an upflowing treatment tank provided with a feed inlet distribution system at the tank bottom. A gas solid separator device is provided at the top to help provide a quiescent zone at the top of the reactor. The wastewater enters the tank at the bottom and flows upward through the sludge bed which is formed during the process. The sludge bed develops, the microorganisms capable of flourishing in an oxygen deficient environment. The sludge bed (blanket) traps the suspended organics of the upmoving wastewater. The suspended solids trapped in the sludge bed are degraded by the anaerobic and anaerobically working facultative bacteria producing methane and carbondioxide (biogas). The biogas produced during anaerobic decomposition helps in providing gentle mixing and stirring of the biomass, thereby increasing the efficiency of decomposition, reducing the BOD and suspended solids of the wastewater. The methane or biogas is collected at the top of the tank in a gas collector from where it can be withdrawn for use as a by-product. Water from the sludge mixture enters a settling tank where sludge settles down and flows back into the bottom of the reactor. After an initial start-up period followed by granulation a sludge blanket is formed in the lower part of the reactor. UASB operates as a suspended growth system. In this system microbes attach themselves to each other or to small particles of the suspended matter of sewage to form granules and ultimately the sludge blanket. The gas produced in the process causes sufficient agitation to keep the sludge fully mixed. Retention of the bacteria containing sludge in the reactor is the most important feature of the UASB process. The bacteria in the sludge continue to perform their function of treating the incoming influent wastewater. The continuous activity of bacteria helps reducing the retention time to 6-8 hours. The treated effluent is collected in gutters and discharged out of the reactor. The sludge is periodically discharged into drying beds to be used as soil enricher. Methane gas generated can be used as gas for domestic or industrial use. It may also be used for generating electricity after dehydration and cleaning.