Chemical Process
Hydrocynic acid (HCN) is used as an insecticide for

Citrus fruits
Controlling poultry lice
Controlling timber degradation by ants
Potato beetle

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Chemical Process
Presence of carbonaceous matter in the sewage

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

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