Water flow is slowed to allow heavy solids to settle. Later, they are removed. Scrapers collect more solid matter and any grease remaining at the top of the water. Air is mixed with wastewater, along with bacteria and other microorganisms. These organisms consume harmful organic matter, A sedimentation tank allows the solid wastes and microorganism to settle and separate from wastewater.
Chlorine is added to wastewater as a disinfectant before it is discharged.
In both primary and secondary treatment, solids are removed for further processing to digestion tanks. They are left alone to decompose. When stabilized, these biosolids meet all state, local and federal regulations for utilization on land application sites. Three main by-products are created by wastewater treatment biosolids, methane gas and water. Biosolids can be composted and used to fertilize land, as discussed below.