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Tertiary Treatment

The wastewater treatment process typically includes three stages: primary, secondary and tertiary treatment.

Tertiary treatment provides a final treatment stage to further improve the effluent quality before it is discharged in the environment (sea, river, lake, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process. It is also called “effluent polishing”.

Three main tertiary treatment processes are described here: Filtration, Lagooning, and Nutrient Removal.


Sand filtration removes much of the residual suspended matter. Filtration over activated carbon, also called carbon adsorption, removes residual toxins.


Lagooning provides settlement and further biological improvement through storage in large man-made ponds or lagoons. These lagoons are highly aerobic and colonization by native macrophytes, especially reeds, is often encouraged. Small filter feeding invertebrates such as Daphnia and species of Rotifera greatly assist in treatment by removing fine particulates.

Nutrient removal

Wastewater may contain high levels of the nutrients nitrogen and phosphorus. Excessive release in the environment can lead to a buildup of nutrients, called eutrophication, which can in turn encourage the overgrowth of weeds, algae and cyanobacteria (blue-green algae). This may cause a rapid growth in the population of algae. The algae numbers are unsustainable and eventually most of them die. The decomposition of the algae by bacteria uses up so much of the oxygen in the water that most or all of the animals die, which creates more organic matter for the bacteria to decompose. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies.

Nitrogen removal is achieved through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by the reduction of nitrate to nitrogen gas (denitrification). Nitrogen gas is released in the atmosphere and thus removed from the water. Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiary treatment. Many sewage treatment plants use centrifugal pumps to transfer the nitrified mixed liquor from the aeration zone to the anoxic zone for denitrification. These pumps are often referred to as Internal Mixed Liquor Recycle (IMLR) pumps.

Phosphorus removal is important because an excess of phosphorus can lead to algae growth in many freshwater systems. It is also particularly important for water reuse systems where high phosphorus concentrations may lead to fouling of downstream equipment such as reverse osmosis. Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron, aluminum or lime. Chemical phosphorus removal requires significantly smaller equipment footprint than biological removal, is easier to operate and is often more reliable than biological phosphorus removal. Another method for phosphorus removal is to use granular laterite.

For any additional information on tertiary treatment, please visit the Wikipedia article that was used for this webpage.