Chemical Process
Parathion and Malathion are

Pesticides
Tranquilisers
Plasticisers for unsaturated polysters
Pain-relieving drugs (analgesic)

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Chemical Process
Presence of carbonaceous matter in the sewage

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

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