The purpose of tertiary treatment is to provide a final treatment stage to further improve the effluent quality before it is discharged to the receiving environment. 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. The various tertiary treatments are as follows:
Filtration: Sand filtration removes much of the residual suspended matter. Filtration over activated carbon also called carbon adsorption, removes residual toxins
Lagoons or ponds: Lagoons or ponds provide 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.
Biological nutrient removal: Biological nutrient removal is regarded by some as type of secondary treatment process and by others as a tertiary treatment process. Waste water may contain high levels of the nutrients nitrogen and phosphorous. Excessive release to the environment can lead to a built-up of nutrients, called eutrophication which can in turn encourage the overgrowth of weeds, algae and cyanobacteria. This may cause an algal bloom. Nitrogen is removed through the biological oxidation of nitrogen from ammonia to nitrate followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water. Phosphorous can be removed biologically in a process called enhanced biological phosphorous removal.
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