What is tertiary treatment?
Tertiary treatment: Advanced treatment after the secondary biological stage
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 435 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.
Tertiary treatment questions
What is the difference between secondary and tertiary treatment?
Secondary treatment is the biological stage that removes dissolved organic matter and brings BOD and suspended solids to about 30 mg/L. Tertiary treatment follows it and polishes the effluent further with filtration, nutrient removal and disinfection, or prepares it for reuse.
Is tertiary treatment required?
Not everywhere. The federal minimum for publicly owned plants is secondary treatment. Tertiary processes are required when the NPDES permit carries limits that secondary treatment cannot meet, usually for phosphorus, nitrogen or very low suspended solids, or when the effluent is reused.
Does tertiary treatment remove PFAS?
Conventional tertiary filtration does not. Removing PFAS takes granular activated carbon, ion exchange resin or reverse osmosis, which are found at drinking-water plants and potable reuse schemes far more often than at wastewater plants.
Can tertiary treated wastewater be reused?
Yes. Filtered and disinfected effluent is widely used for irrigation, cooling and industrial supply. Potable reuse adds advanced treatment such as reverse osmosis and advanced oxidation on top of it.
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