Question

Explain Activated sludge method with the help of diagram

Explain activated sludge.

Answer 1

Principle -Here ,the adequate amount of O₂ or air is passed through sewage ,containing aerobes ,complete aerobic oxidation takes place .If this aeration is carried out with sludge from previous oxidation process ,the oxidation is faster .This sludge is known as activated sludge .
Process : The process involves the mixing of sedimented sewage with activated sludge and then it is sent to aeration tank. Microorganisms should be provided with nutrients such as N and P which are supplied in form of urea .The other nutrients such as K,Mg,Ca are generally present in waste .
The efficiency of activated sludge is determined by pH ,temperature and redox potential.
It is kept for 5-6 hrs in order to have complete oxidation ,whereby C gets converted to CO2 ,N to NH3 and then to nitrites , nitrates .At least 0.5 ppm O2 must be present and pH is maintained between 6.5-9 .
After aeration ,the effluent is settled in settling tank ,where the sludge is settled and clean ,liuid is drawn off .A part of settled sludge can be used further for fresh batch of sewage and the process continues .
The remaining sludge is used for i)Land spreading ii)Dumping in sea iii)Digestion process : where sludge is kept in closed tank in absence of air for almost a month ,it gives methane which can be used as a fuel ( 400-600 Litres of fuel is generated per Kg of sludge ) .
It is the most versatile method and BOD removal is upto 90-95 % .