Tertiary wastewater treatment is the third and final cleaning stage that water goes through at a treatment plant, designed to polish the water to a quality safe enough for release into rivers, lakes, oceans, or the ground. Sometimes called “effluent polishing,” it picks up where the first two stages leave off, targeting the finer contaminants that primary and secondary treatment can’t fully remove. Not every treatment plant includes this step, but it’s increasingly common as water quality standards tighten.
How It Fits Into the Treatment Sequence
To understand tertiary treatment, it helps to know what comes before it. Primary treatment is mostly physical: large debris is screened out and solids are allowed to settle in tanks. Secondary treatment is biological. Bacteria and other microorganisms consume dissolved organic material like sugars, fats, and proteins from human waste, food waste, soaps, and detergents. These microorganisms clump the remaining material into a sludge that can be separated from the water.
After secondary treatment, the water is cleaner but still contains elevated levels of nutrients (mainly nitrogen and phosphorus), fine suspended particles, residual bacteria, and trace chemicals. Tertiary treatment addresses all of these through a combination of filtration, nutrient removal, and disinfection. If disinfection is used, it is always the final step before the water is discharged.
Filtration: Removing What’s Left
The most straightforward part of tertiary treatment is filtration, which catches fine suspended solids that made it through secondary treatment. Sand filters are the most common type. Water passes through layers of sand (and sometimes gravel or activated carbon), and particles get trapped in the gaps between grains. Membrane filtration is a more advanced option that pushes water through synthetic membranes with extremely small pores, capable of catching not just particles but some dissolved contaminants as well.
Constructed wetlands serve a similar filtering role in some systems. These are engineered marshlands planted with reeds and other vegetation where water flows slowly through gravel or sand beds. The plants and their root systems support communities of microorganisms that break down pollutants, while the substrate physically traps particles. They’re a lower-energy alternative to mechanical filtration, though they require more land.
Nitrogen and Phosphorus Removal
Excess nitrogen and phosphorus in discharged water feed algal blooms in rivers and coastal areas, which deplete oxygen and kill aquatic life. Removing these nutrients is one of the primary goals of tertiary treatment, and each requires a different approach.
Nitrogen removal relies on biology. Specialized bacteria first convert ammonia into nitrate in oxygen-rich conditions (a process called nitrification), then a different group of bacteria converts nitrate into harmless nitrogen gas in oxygen-free conditions (denitrification). Treatment plants create these alternating conditions by cycling aeration on and off in dedicated tanks, or by routing water through separate zones. In constructed wetlands, the same effect is achieved through intermittent aeration or tidal flow patterns that expose microorganisms to changing oxygen levels.
Phosphorus is harder to remove biologically. Plant uptake and microbial absorption account for only a small fraction. Instead, most plants rely on chemical precipitation: iron or aluminum salts are added to the water, which bind to dissolved phosphorus and form solid particles that settle out or get caught by filters. In constructed wetlands, engineers have experimented with filter materials that have a high capacity to bind phosphorus, including iron oxide, limestone, zeolites, shells, and industrial byproducts like blast furnace slag and fly ash. Iron oxide mixed into sand beds has shown promise for long-term phosphorus capture through both chemical binding and biological uptake by plants.
Disinfection Methods
Disinfection kills or inactivates the bacteria, viruses, and parasites that remain after filtration and nutrient removal. Three methods dominate.
- Chlorine is the oldest and most widely used. It’s effective and inexpensive, with typical doses ranging from 5 to 20 milligrams per liter depending on how much organic matter remains in the water. The tradeoff is that chlorine requires a relatively long contact time and can form harmful byproducts when it reacts with residual organic compounds, so many plants add a dechlorination step afterward.
- UV light works by damaging the DNA of microorganisms so they can no longer reproduce. It requires only a short contact time, leaves no chemical residue, and produces no byproducts. The U.S. Public Health Service sets a minimum UV dosage for disinfection equipment to ensure consistent pathogen inactivation.
- Ozone is a powerful oxidizer that destroys pathogens on contact. It’s faster than chlorine and also breaks down some chemical contaminants, but the equipment is more expensive to install and operate.
Trace Chemical Contaminants
A growing concern in wastewater treatment is the presence of micropollutants: pharmaceuticals, hormones, industrial chemicals, and compounds like PFAS that pass through conventional treatment largely intact. Tertiary treatment can reduce many of these, but the results vary widely depending on the method and the specific contaminant.
Membrane filtration with very small pore sizes (nanofiltration) performs better than larger-pore membranes, particularly for pharmaceuticals and fluorinated compounds. In one systematic evaluation, removal rates above 70% were achieved for 9 different contaminants, while 22 others were only partially removed and 7 showed low removal. Hormone-disrupting compounds proved especially stubborn, with nanofiltration offering no statistically significant improvement over coarser filtration for that class of pollutant.
Advanced oxidation is a newer approach specifically targeting these resistant chemicals. The process generates highly reactive molecules, primarily hydroxyl radicals, that are powerful enough to break apart organic contaminants that resist conventional treatment. These radicals attack chemical bonds indiscriminately, breaking pollutants down into simpler, less harmful compounds and, in some cases, fully converting them to carbon dioxide and water. Advanced oxidation can be driven by combinations of ozone, UV light, and hydrogen peroxide, making it adaptable to different plant designs. It’s increasingly used as a final polishing step where micropollutant standards are strict.
Why Tertiary Treatment Matters
The practical difference between secondary and tertiary treatment shows up in what’s discharged. Secondary-treated water still contains enough nitrogen and phosphorus to trigger ecological damage downstream, enough pathogens to pose a health risk, and enough suspended solids to cloud receiving waters. Tertiary treatment brings these levels down to the point where the discharged water is safe for ecosystems and, in some configurations, clean enough for agricultural irrigation or even indirect reuse in drinking water systems.
The cost is real. Tertiary systems require additional infrastructure, energy, and chemicals. But as population growth increases the volume of wastewater and as regulators tighten limits on nutrient discharge and emerging contaminants, tertiary treatment is shifting from an optional upgrade to a baseline expectation for municipal plants in many regions.