In our last post on Wastewater Treatment, we covered Pretreatment and Primary Treatment. Today we’ll finish off the treatment process with Secondary and Tertiary Treatment.
Secondary treatment is designed to substantially degrade the biological content of the sewage which are derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. Bacteria and protozoa consume the 
biodegradable soluble organic contaminants (such as sugars, fats, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are either fixed-film or suspended-growth systems.
A fixed-film system utilizes trickling filters, biotowers, and rotating biological contactors, which allows biomass to grow while the sewage passes over its surface. The biomass is then removed from the flow. A suspended-growth systems include activated sludge, where biomass is mixed with the sewage. A suspended-growth system can be operated in a smaller space than trickling filters that treat the same amount of water. However, fixed-film systems are better 
able to cope with drastic changes in the amount of biological material and can provide higher removal rates for organic material and suspended solids than suspended growth systems.
Finally, there is tertiary treatment. The purpose of tertiary treatment is to provide a final treatment stage to further improve the effluent quality before it is 
discharged, whether into the sea, a river, lake, or into the ground. If a wastewater treatment plant utilizes disinfection, it is always the final process. It is also called “effluent polishing.” Other tertiary processes include filtration, lagooning (allowing the effluent to settle while stored in a man-made lagoon), and nutrient removal, which removes nitrogen and phosphorus from the effluent.
