Chemical Process
Presence of carbonaceous matter in the sewage

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

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Chemical Process
Unsaturated oils compared to saturated oils have

Lower melting point & lower reactivity to oxygen
Higher melting point & lower reactivity to oxygen
Lower melting point & higher reactivity to oxygen
Higher melting point & higher reactivity to oxygen

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