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Which of the following is put into Anaerobic sludge digester for further sewage treatment?
In secondary sewage treatment, microbial flocs settle in the settling tank to form ACTIVATED SLUDGE. A small part is recycled as inoculum into the aeration tank, while the major remaining part is pumped into ANAEROBIC SLUDGE DIGESTERS to produce biogas! Mark Activated sludge (Option 4).
Municipal wastewater undergoes sequential stages of purification: 1. Primary Treatment: Physical removal of large and small particles via sequential filtration (floating debris) and sedimentation (settled solids form primary sludge). 2. Aeration Tank (Secondary Treatment): Effluent from primary settling is vigorously agitated in aeration tanks where aerobic microbes form flocs (masses of bacteria associated with fungal filaments), dramatically consuming dissolved organic matter and slashing the Biochemical Oxygen Demand (BOD). 3. Settling Tank & Activated Sludge: Once BOD is sharply lowered, the effluent flows into a settling tank where the microbial flocs settle under gravity to form activated sludge. 4. Anaerobic Digestion: A minor aliquot of activated sludge is recycled back into the aeration tank as inoculum. The major bulk of activated sludge is pumped into large anaerobic sludge digesters.
In the anaerobic digester, obligate and facultative anaerobic bacteria (including methanogens like *Methanobacterium*) digest the bacterial and fungal biomass of the activated sludge. This anaerobic decomposition generates a flammable gas mixture (biogas consisting predominantly of methane, carbon dioxide, and hydrogen sulphide).
Diagnose every choice: Know exactly why the right option works and why distractors fail.
Incorrect. Primary sludge comprises raw settled solids from primary physical treatment, not the biological sediment fed to anaerobic digesters.
Incorrect. Floating debris is removed by sequential filtration during the very first step of primary treatment.
Incorrect. Primary effluent is passed into the aeration tank for aerobic digestion, not anaerobic digesters.
Correct. Activated sludge sedimented in settling tanks is pumped into anaerobic sludge digesters for methane fermentation.
Primary sludge is non-biological raw grit/settled solids. Activated sludge is the biomass-rich aerobic floc sediment formed after secondary treatment, which is specifically transferred to anaerobic digesters for biogas production.
"Activated sludge from secondary settling tanks is pumped into anaerobic sludge digesters for further treatment."
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