Which among the following are the by-products of wastewater treatment?

The correct option is C

Biogas and sludge

When wastewater is allowed to settle in a large tank, solids like mud, faeces settle at the bottom of the tank. This is called sludge. It is transferred to a separate tank where it is decomposed by anaerobic bacteria. Biogas is produced during this process. Dried sludge can be used as a manure.