Explain the Secondary treatment for wastewater

Secondary Treatment: to remove soluble BOD that escapes the primary treatment and to provide added removal of suspended solids.  Biological processes (activated sludge (aeration) or trickling filter are the most common one chosen at larger plants). Typically same treatment is provided in the receiving waters but these treatment processes speed up the breakdown of degradable organic pollutants. 

Typically more than 85% of BOD and suspended solids are removed.  However, it does not remove significant amounts of nitrogen, phosphorous or heavy metals or pathogenic bacteria and viruses. Typical Standards: 20 mg/l BOD₅, 20 mg/l suspended solids (for discharges to rivers flowing south and flows into Gulf of Mexico).