Real‐Time Control of Aerobic‐Anoxic Sludge Digestion Using ORP

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Abstract
This research documents the use of oxidation‐reduction potential (ORP) to control two lab‐scale aerobic‐anoxic sludge digesters. The reactors were operated in sequencing batch reactor fashion, and one reactor had the ratio of air‐on/air‐off arbitrarily “fixed” at 3 h for each portion of the cycle. The other reactor operated in a real‐time manner with 3 h of air‐on and the air‐off time determined by computer detection of the nitrate breakpoint in the orp‐time profile. Reactor performances were evaluated from a chemical (mass balance) removal perspective [total and volatile suspended solids (TSS and VSS), nitrogen, and phosphorus] coupled with a consideration of the control stability achieved when the reactors were subjected to spikes of sodium nitrate, ammonium chloride, and hydrogen peroxide. The reactors showed comparable chemical removals, however the real‐time control reactor displayed a greater ability to accommodate the disturbances investigated.