How are magnets used in the treatment sewage?

Chlorine gas may be added to the treated effluent to disinfect it (kill pathogens) prior to discharge. This is usually a tertiary or quaternary treatment.

to encourage bacteria to grow

A process for the purification of mixtures of human and other domestic wastes; the process can be aerobic or anaerobic or both (facultative).

No. Electromagnets are usually made of a core (some type of metal that can be magnetized such as iron) and a coil. The coil is put around the core, and if current flows through the coil the core will...

bar magnets