The processing of wastewater for the removal or reduction of contained solids or other undesirable constituents. It is divided into three steps: primary, secondary, and tertiary. Primary treatment uses screens and sedimentation tanks to remove most materials likely to float on the water or settle on the bottom. Secondary treatment uses a biological process to consume organic materials in the waste and disinfect the effluent. Tertiary treatment removes additional nutrients, suspended solids, and other pollutants.