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The use of oxidation-reduction potential (orp) as a process a process control parameter in wastewater treatment systems Wareham, David Geraint


This research explored the use of Oxidation-Reduction Potential to control two lab-scale sequencing batch reactor (SBR) wastewater treatment processes. The treatment schemes investigated were the aerobic-anoxic digestion of activated sludge (AASD) and the excess biological phosphorus (Bio-P) removal process. Evaluation of each process consisted of a consideration of the reactor performances coupled with the control stability achieved using two different operating strategies. The first strategy was known as "Fixed-Time Control" (FT), since it represents the "classical" management approach; control is based on conditions externally "fixed" by an operator. For the AASD set of experiments, the "fixed" variable was the ratio of air-on to air-off (3 hours each). For the Bio-P experiments, it was the time of addition of acetate to the reactor (1 hour 25 minutes into the non-aerated sequence). The second strategy was known as "Real-Time Control" (RT), since it represents an optimization technique whereby control conditions are continuously evaluated as time progresses. The Real-Time aspect of control is derived from the fact that ORP measurements evaluate the reactor conditions on-line, by invoking a bacterial vision of the process scheme. For the AASD experiments, this evaluation took the form of proportioning the ratio of air-on to air-off, based upon the bacterial "need" for sufficient time to reduce the nitrates completely to nitrogen gas (denitrification). Sufficient time is determined by the distinctive breakpoint (correlated to nitrate disappearance) occurring in the ORP-time profile. The first experiment (AASD#1) , therefore, had an air-on/airoff ratio of 3 hours air-on/nitrate-breakpoint-determined airoff. The second experiment (AASD#2) had the length of aeration time determined by a match to the previous length of time for denitrification, as determined by the breakpoint. In the Bio-P experiments, the ORP breakpoint was used to "trigger" the addition of acetate to the reactor, thus ensuring the maximum amount of carbon was available for storage by Bio-P organisms. Comparisons between the two reactors revealed that for the AASD strategies, the Real-Time reactor had essentially the same solids degradation as the Fixed-Time reactor (14% - 21%), depending upon the strategy considered, the type of solids (TSS or VSS) and the method of mass balancing used. The RT reactor was observed to obtain marginally better nitrogen removal (up to 6 % in some cases) over the FT reactor. Evaluation of the ORP parameter as a "response indicator", by subjecting the AASD reactors to unsteady process input conditions, revealed that the Real-Time reactor more readily accommodated disturbances to the system. Neither reactor in the Bio-P experiment was particularly successful in consistently removing phosphorus. A potentially useful screening protocol was developed for evaluating reactor performances, based upon the time-of-occurrence of the nitrate breakpoint, assessed against whether it hindered or aided the purpose of acetate addition to a Bio-P SBR.

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