ALTERNATING AEROBIC-ANOXIC PROCESS FOR NITROGEN REMOVAL - PROCESS EVALUATION

The completely mixed alternating aerobic-anoxic (AAA) activated sludge process for nitrogen removal was investigated to observe it under nor mal operating conditions, evaluate its dynamic responses, and investig ate the feasibility of real-time sensors (dissolved oxygen [DO], pH, a nd oxidation-reduction potential [ORP]) as process control parameters. Total nitrogen reduction of 72% to 83% could be achieved in the AAA p rocess with an aerobic fraction of 50% and mean cell residence time be tween 7 and 20 days. The steady-state process evaluation suggested ene rgy savings from lesser aeration time and enhancement in oxygen transf er efficiency. As compared with the sludge from the control aerobic re actor, a slight deterioration in sludge settleability in terms of zone settling velocity was noticed. Feasibility study on the use of pH or ORP as a real-time process control parameter for the AAA process indic ates that either parameter may be used. Several control points on the pH profile were identified and defined. Particularly significant are t he points that define the end of nitrification in the aerobic cycle an d the end of anoxic respiration in the anoxic cycle. Application of th ese points to control the duration of aerobic and anoxic cycles is dis cussed.