How does a Wastewater Treatment Plant Work?


The sewage enters the Plant and the coarse material (e.g. sticks, toilet paper, etc.) is removed by a bar screen. The sewage is then passed into a Primary Clarifier where its velocity is slowed. Heavy material sinks to the bottom of the Clarifier and is pumped to the Anaerobic Digester. Light material (e.g. grease) floats to the surface where it is skimmed off. It too is pumped to the Anaerobic Digester. The Suspended and dissolved material passes through the Primary Clarifier to the Aeration Tanks.

The Aeration Tanks contain extremely large numbers of microorganisms that are used to consume the dissolved / suspended material. This is the same process that would occur in nature, but is accelerated by concentrating the microbial population in the tank and maintaining optimum growth conditions. In this manner, organic waste is removed in hours rather than weeks.

As the microorganisms grow, they excrete a sticky waste product that causes them to cling together. Upon reaching the Secondary Clarifier , the flocculated (clumped) microorganisms have sufficient weight to cause them to fall out of suspension via gravity. The organisms are then scraped from the bottom of the Secondary Clarifier . They can then either be returned to the Aeration Tanks to eat more material, or pumped to the Anaerobic Digester if the population becomes too great and must be removed from the system (to prevent the organisms from starving and/or cannibalizing each other).

By the time the effluent reaches the end of the Secondary Clarifier, over 95% of the Suspended Solids and Biochemical Oxygen Demand (BOD) have been removed. However, the effluent, even though it may appear quite clear, may contain many pathogenic organisms. For this reason, chlorine is added to the discharge to disinfect it prior to it reaching the lake. Chlorine can also be harmful to the environment, so sulfur dioxide is added to neutralize the chlorine prior to it entering the waterway.

The Anaerobic Digester is a large tank that is completely void of oxygen and is maintained at 35 o C. The microorganisms inside the tank break down the large, complex organic material into smaller, simpler compounds. The process occurs in two stages. In the first stage, organic material is consumed by a group of organisms collectively called "Acid Formers". As their name suggests, they convert the organic material into organic acids. A second group of microbes then attack the organic acids and convert them to even simpler compounds. The most important byproduct of this process is methane. The methane gas is collected in a floating roof in the Secondary Digester and is burned in the boiler to maintain the temperature of the Primary Digester. Any excess methane is used to heat the building thereby reducing the Plant's dependance on the costly purchase of natural gas. The digested sludge exiting the digester system then supplied to local farmers, free of charge, to be used as high quality, organic fertilize.