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51.

Give a flow chart of sewage treatment.

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52. What is ‘Secondary treatment” of sewage ?

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Differences between primary sludge and activated sludge

Primary sludge	Activated sludge
1. The settled or sedimented solid during the primary treatment of sewage forms the primary sludge	1. The sediments formed by the bacterial flocs from the effluent during the primary treatment, constitute the activated sludge.
2. Microorganisms are not used.	2. Microorganisms are used.