Question 5: What is sludge? Explain how it is treated.
Answer:
The waste water (or sewage) is then passed into a sedimentation tank and allowed to stand there for a while. Most of the solid organic matter settles down on the slopping bottom of the sedimentation tank in the form of a sludge. Thus, the solid part of sewage is called sludge.
The sludge is taken out from the bottom of first sedimentation tank and put into a large, closed tank called digester tank. In the digester tank, many types of anaerobic bacteria decompose the organic matter present in sludge to produce biogas (also called sewage gas). The anaerobic bacteria do not need oxygen of air to carry out the process of decomposition. That is why the digester tank is a closed tank. The biogas produced here can be used as a fuel directly or it can be used to generate electricity. The digested sludge left after the removal of biogas is taken out, dried and used as a manure. The wastewater treatment gives us two useful products: biogas and sludge. The biogas is used as a fuel whereas sludge (or rather ‘digested sludge’) is used as a manure. The use of dried sludge as a manure returns the nutrients to the soil.
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