In September of 1990, a submersible mixer was installed in the oxidation ditch at the Grand Coulee, Washington, wastewater treatment facilities. This mixer makes it pos- sible to maintain mixing with the brush rotor aerators off, thereby creating anoxic conditions in the ditch. The poten- tial advantages of this process modification include en- hanced nutrient removal, filamentous sludge bulking control, and energy savings. The process is based on a strategy that Maximizes the use of influent soluble BOD (SBOD) under anoxic conditions. Under anoxic conditions, only non- filamentous bacteria use nitrate in place of oxygen, and thus the process “selects” against filamentous organisms, while at the same time reducing total nitrogen and reducing aeration energy requirements. Preliminary empirical obser- vations at Grand Coulee have been encouraging; however, the key to realizing the full potential of this process and making widespread application practical and cost-effective is the development of an automatic control system.

The focus of this proposal will be to conduct the basic research necessary to investigate the feasibility of using oxidation-reduction potential (ORP) as an on-line process variable in the automated control of the anoxic mixing system. The automated control system would make the application of this relatively simple technology feasible for a large number of existing oxidation ditch plants. This would be of particular benefit to small municipalities that may lack highly skilled operating personnel and the techni- cal support from vendors and service technicians that are available in larger communities.

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