Sep 30, 2026Technology
Wastewater Treatment: Primary, Secondary and Tertiary
A plain-language guide to the three stages of wastewater treatment and what each one removes.
Primary treatment
Primary treatment is physical: screening removes large solids, and settling tanks allow suspended solids to drop out as sludge. It reduces solids and floating debris but leaves dissolved pollutants in the water.
Secondary treatment
Secondary treatment is biological. Microorganisms break down dissolved and suspended organic matter, the same principle behind the activated sludge process that has shaped wastewater treatment for a century.
Tertiary treatment
Tertiary treatment removes what remains, such as nutrients (nitrogen and phosphorus), residual solids, and pathogens, preparing water for discharge or reuse. In advanced plants, recovered heat and treated water are reused to cut cost and reach zero discharge.