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 normal operating conditions, evaluate its dynamic responses, and investigate the feasibility of real-time sensors (dissolved oxygen [DO], pH, and oxidation-reduction potential [ORP]) as process control parameters. Total nitrogen reduction of 72% to 83% could be achieved in the AAA process with an aerobic fraction of 50% and mean cell residence time between 7 and 20 days. The steady-state process evaluation suggested energy savings from lesser aeration time and enhancement in oxygen transfer efficiency. As compared with the sludge from the control aerobic reactor, 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 indicates that either parameter may be used. Several control points on the pH profile were identified and defined. Particularly significant are the points that define the end of nitrification in the aerobic cycle and the end of anoxic respiration in the anoxic cycle. Application of these points to control the duration of aerobic and anoxic cycles is discussed.