A wastewater treatment plant separates solids from the liquid, and consists of two basic stages: primary treatment and secondary treatment.
In the primary treatment stage, larger solids are removed from wastewater by settling.
Secondary treatment is a large biological process for further removal of the remaining suspended and dissolved solids. Secondary treatment removes up to 85% of the remaining organic material through a biological process of cultivating and adding sewage microorganisms to the wastewater. This process is accomplished in a trickling filter or an aeration tank.