Wastewater treatment

Sewage treatment removes solids, then organic matter, then — where required — nutrients, before returning the water to a river or the sea.

Population equivalent: Works are sized in population equivalent, a measure of organic load rather than of people — one food factory can equal a small town.
Why storm overflows exist: Combined sewers carry rainfall as well as sewage. Without a relief point, heavy rain would back sewage up into homes.
Sludge is a resource: Anaerobic digestion produces biogas that can power a large share of the works, and biosolids used as agricultural fertiliser.

Text description

Raw sewage passes through preliminary treatment removing rags and grit, then primary settlement removing settleable solids, then secondary biological treatment by activated sludge where micro-organisms consume dissolved organic matter, then final settlement, then optionally tertiary treatment for nutrients or disinfection, before discharge. Sludge from the primary and secondary stages goes to anaerobic digestion, producing biogas and biosolids.