From there the water flows to a primary clarifier/settling basin. Here there are some organic solids that if given sufficient time can settle out. After that (primary) clarifier the water enters biological treatment. Biological treatment of wastewater represents one of the least expensive methods to treat large volumes of water both industrial and municipal. The reduction in mass of pollutants in a biological wastewater treatment system is performed by a mixed community of microorganisms. The community possesses the capabilities to utilize constituents within the waste stream for energy and sources of carbon. Those microorganisms are referred to as activated sludge. The term "activated" simply refers to the fact that the microbes are alive and actively utilizing the organic and some of the inorganic constituents as sources of food (substrate). The main objectives for treatment are 1) Convert soluble organic matter to biomass (i.e. more microorganisms) 2) degrade insoluble organic matter and 3) and transform soluble inorganic matter. Design of these facilities requires and understanding of the dynamics within the microbial community (BTW, feel free to ask design questions, especially those that are seeking to build schools, centers, etc. that are totally self sufficient). SRT is the main design criteria used for sizing tanks, determining aeration requirements, etc. SRT values typically range from 3 to 10 days depending upon the goals of the plant. If I have a flowrate of 1 m3/hr and I have a tank that is 1 m3 in size, the volume of water (1m3) will be in that tank for 1 hour. So how do you achieve SRTs longer than the amount of time that the water is in the tank? The sludge is pumped (i.e. recirculated) through the system allowing for the microbes to be in the system longer than the water.

Activated sludge is primarily made up of Prokaryotes (Bacteria with a few Archaea) and Eukaryotes (water mites, rotifers, and other predators) Currently an abundance of research is focused on identifying microorganisms and their corresponding "function' within a wastewater sludge; however, those techniques usually require that sludge be removed form the plant and cultivated in the lab. Removal from one environment to another necessarily means that some dynamics within the microbial ecosystem will change.

For extended information on activated sludge go to
Presentation: http://home.no.net/rbines/environmental.ppt

One of the primary goals of secondary treatment is to remove nutrients (phosphorus and nitrate). All permits (NPDES) that are given (by EPA) with the intention of the WWTP discharging to a water body have requirements limiting the amount of nutrients in the water. Eutrophication is caused by the excessive discharge of nutrients to a water body. This effect can best be illustrated by a lake turning into a wetland/swamp. In secondary treatment ammonia is used by the microbial community and turned into nitrate. The nitrate is then used as a terminal electron acceptor (analogous to the way humans use oxygen) and it is converted to nitrogen gas, which escapes from the system by volatilization. The process of phosphorus removal is slightly more complex, but will be expounded upon for those specifically interested. (In other words, if you have questions about that I will address it then.)