What are the byproducts of wastewater? Do they have any commercial value? [2 MARKS]

Naming: 1 Mark
Commercial value: 1 Mark

By-products of wastewater treatment are sludge and biogas. Sludge is produced when solids wastes like faeces settle at the bottom of the tank during sedimentation. When this sludge is decomposed by anaerobic bacteria, biogas is produced.
Sludge contains nitrogen, phosphorus and organic matter and is used as agricultural manure. Biogas is low-cost energy utilized by homes. Hence both have high commercial utility.