Using existing tankage the process begins at the head of the bioselectors in the rapid mix tank. The city has the ability to return mixed liquor at flows of six times the influent flow to the rapid mix tank, but normal practice is one to two times influent flow. This flow is mixed with the influent flow and causes a rapid depletion of the soluble COD available as a food source for the filamentous micro-organisms. Plant operators are also able to manipulate D.O. levels in the bioselectors by running or shutting off aeration equipment. The bioselectors consist of four 310,000 gallon tanks, which were at one time anoxic basins, and now contain piping with fine bubble membrane type diffusers for aeration.
Aeration is provided by three centrifugal blowers.
This technology did not work for the city and a major modification and replacement project was the installation of a new aeration system within the existing tankage. There are two aeration basins which hold 2.8 millon gallons each. Each aeration basin has four zones that the wastewater flows through as it makes its circuit around the aeration basin. Three of the zones in each aeration basin are piped for air and also use fine bubble membrane type diffusers for aeration. Plant operators have the ability to open and shut off grids in the three zones to maintain D.O. at desired levels.