SBIR-STTR Award

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 possible to maintain mixing with the brush rotor aerators off, thereby creating anoxic conditions in the ditch. 71be potential advantages of d& process modification include enhanced nutrient removal, @entous sludge b@g control, and energy savings. 'Me @ss is @ on a stmgy @ maximizes the use of influent soluble BOD (SBOD) under arioxic conditions. Under anoxic conditions, only nonfilamentous bacteria use nitrate in place of oxygen, and thus the process "selects" against filamentous organisms, while at the same time reducing total nitrogen and redu ''g aeration energy requirements. Preliminary empirical observations at Grand Coulee have been encouraging; however, the key to realwng the full potential of this process @d making widespread application practical and cost-effecuve is the development of an automatic control system. The focus of this proposal will be to conduct the basic Tesearch necessary to investigate the feasibility of using oxidation-reduction potential (ORP) as an on-line process .. variable in the automated control of the anoc ng system. The automated control system would make the application of this relatively simple technology feasible for a large number of existing oxidation ditch plants. @ would be of particular benefit to small municipalities gm may lack highly skilled operating personnel and the technical support from vendors and service technicians that are available in larger communities.

During the Phase I investigation at a full-scale oxidation ditch plant at Grand Coulee, Washington, anoxic/aerobic operating cycles were tested. Anoxic conditions were created by operating a mixer in the ditch while the aerators were off. The study showed that the occurrence of nitrate depletion during the anoxic period could be identified by continuous readings from an Oxidation-Reduction Potential (ORP) probe placed in the ditch. Observed benefits were improved control of sludge bulking, energy savings, and higher nitrogen removal. A mathematical model was developed that could simulate anoxic/aerobic operation.Many combinations of anoxic/aerobic periods are possible, and additional ones will be tested during Phase II studies to evaluate the effects of nitrogen removal, sludge volume index, and effluent ammonia. The mathematical model will be further calibrated to provide a tool to assess and select different operating strategies .Phase II work will focus on developing an automated control system using ORP measurement to detect nitrate depletion. Control algorithms will be developed that can select for the length of the aeration time as well as the length of the aerator off time. The control system and full-scale operating strategies will be tested and demonstrated in two different full-scale oxidation ditch facilities.