Activated sludge Process Technology for Sewage Treatment Plant

Activated sludge is a process for treating sewage and industrial wastewater using air and a biological flock composed of bacteria and protozoa. This process is one part of Sewage water treatment Plant (STP). The process involves introduction of air or oxygen into a mixture of primarily treated sewage or industrial wastewater combined with organisms to develop a biological flock which reduces the organic content of the sewage. A Conventional Activated Sludge (CAS) system commonly includes an aeration tank, which is used for biological degradation, and a secondary clarifies (sedimentation tank), where the sludge is separated from the treated wastewater.

  

This is the most common and oldest bio-treatment process used to treat municipal and industrial wastewater. Typically, wastewater after primary treatment i.e. after removal of suspended impurities, is treated in an activated sludge process based on biological treatment system comprising of aeration tank followed by secondary clarifies. The first step of a CAS system is the aeration tank, in this step the wastewater is mixed with air to activate micro-organisms. While digesting and breaking the wastewater, the organisms collide with each other, resulting in larger particles called flocks, and these flocks have a larger capacity to degrade the biological components of the wastewater. The aeration tank is a completely mixed or a plug flow bioreactor where specific concentration of biomass, measured as mixed liquor suspended solids (MLSS) or mixed liquor volatile suspended solids (MLVSS) is maintained along with dissolved oxygen (DO) concentration to effect bio-degradation of soluble organic impurities, measured as biochemical oxygen demand (BOD) or chemical oxygen demand (COD).

The aeration basin is followed by a secondary clarifies or settling tank. During this step, micro-organisms with their adsorbed organic materials get settled. Water from the clarifies is transported to installations for disinfection and final discharge or to other tertiary treatment units for further purification. The aerated mixed liquor from the aeration tank overflows by gravity to the secondary clarifies unit to separate out the biomass and allow clarified, treated water to the downstream filtration system for finer removal of suspended solids. The surplus micro-organisms can easily be channeled to any of the sludge treatment solutions where energy can be recovered from the bio solids. This additional step closes the energy cycle of the wastewater treatment plant allowing it to run independent of fossil fuel. The separated biomass is returned to the aeration tank by means of return activated sludge (RAS) pump. Excess biomass (produced during the bio-degradation process) is wasted to the sludge handling and de watering facility. Another part of the micro-organisms is fed back into the aeration tank to keep the load of micro-organisms at a sufficient level for the biological degrading processes to continue.

Advantages

·         Good quality effluent

·         Can sustain seasonal variation

·         Self-sustaining system

·         Freedom from flies spread and odor nuisance due to high degree of treatment

·         Small land area requirement

·         Easily maintained mechanical work

·         Low installation cost (Cost effective)

·         Less land requirement than UASB

Disadvantages

– High energy consumption

– Foaming, particularly in winter season, may adversely affect the oxygen transfer, and hence performance

– Requires elaborate sludge digestion /drying/disposal arrangement

– More land requirement than SBR & MBBR

– Nitrogen and Phosphorous removal require additional anoxic tank and more       internal recirculation