What is tertiary treatment?
Tertiary treatment: Advanced treatment after the secondary biological stage
In short
Tertiary treatment is the third, polishing stage of wastewater treatment, after primary settling and secondary biological treatment. It removes what those stages leave behind: fine suspended solids, phosphorus and nitrogen, pathogens and trace contaminants, using filtration, chemical precipitation, disinfection, membranes, activated carbon or advanced oxidation. It is required where permit limits are strict or the water is reused.
Klarifi tracks 320 membrane and filtration projects and 52 water reuse projects in published US city capital improvement plans, and 420 US plants that disinfect with ultraviolet light.
Primary, secondary, tertiary
Primary treatment settles solids and removes about half of the suspended solids and a third of the BOD. Secondary treatment uses bacteria to bring BOD and suspended solids down to the federal standard of 30 mg/L. Tertiary treatment goes beyond that standard, typically to below 10 mg/L of each and often below 5 mg/L, and targets pollutants that secondary treatment was never designed for.
The main tertiary processes
Filtration: deep-bed sand and anthracite filters, cloth-media disc filters and membranes remove the fine solids that escape the clarifiers. Phosphorus removal: alum or ferric salts precipitate phosphorus, which is then filtered out, reaching 0.1 mg/L or lower. Nitrogen polishing: denitrification filters with an added carbon source. Disinfection: ultraviolet light, chlorine or ozone.
Water reuse and trace contaminants take advanced treatment: reverse osmosis, granular activated carbon, ion exchange, and advanced oxidation with ozone or with ultraviolet light and hydrogen peroxide.
What drives tertiary projects
Low phosphorus limits on lakes and slow rivers; water reuse, where states such as California, Florida, Texas and Arizona require filtration and high-level disinfection for irrigation and considerably more for potable reuse; limits on metals and toxics; and emerging contaminants including PFAS, which conventional treatment does not remove.