Chemical Process
Presence of carbonaceous matter in the sewage

Reduces sulphate ions to sulphides causing obnoxious smell
Increases the quantity of chlorine used for its purification
All of these
Causes reduction in its dissolved oxygen content thereby endangering the life of aquatic creatures

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Chemical Process
Enzymes are

Catalyst for temperature sensitive reactions
Derived from living organisms
All of these
Proteins with high molecular weight (around 10, 000)

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