- Treat the <u>sewage</u> before it is released into the water body.
Explanation:
The correct form of the word to fill the given sentence would be 'sewage' as it is in the singular form which agrees with the pronoun and the verb followed by it that is singular('it is'). It is the term used to denote the 'suspended water and solid waste transported by sewers to be disposed of or processed.' The large treatment plants are installed in every city for treatment of this wastewater in several stages prior to exempting them into a waterbody in order to prevent the water bodies from being polluted by these wastes. Thus, the correct form is 'sewage.'
