Municipal wastewater (raw sewage) enters a municipal wastewater treatment plant where it then undergoes primary treatment where the heavier solids are allowed to settle out. These heavier solids are then sent to either an aerobic or anaerobic digester where the organic solids are converted by micro-organisms into a more stable form. The cleaner wastewater resulting from primary treatment then undergoes secondary treatment where chemicals are commonly added to precipitate out phosphorus in chemical compounds, and to promote the coagulation or aggregation of other solids that still remain in the wastewater. These compounds and aggregates also settle out and they too are sent to the digester.
Although the water that results after secondary treatment is quite clean it is still subjected to chlorination (disinfection) before it is discharged to the surface water outlet. This third stage is often referred to as tertiary treatment.
The solids sent to the digester are then retained in the digester for a time
period that is long enough for the micro-organisms to break-down or convert
the organic solids to a more stable form. When that conversion process has been
completed the treated solids or 'biosolids' are then transported directly to
a land application site or they are sent to a storage facility (e.g. during
the winter when land application is not permitted).