The alternating aerobic-anoxic (AAA) process offers many advantages for wastewater treatment and sludge digestion. Laboratory scale experiments were conducted to investigate the feasibility of using pH as a realtime control parameter for this process. Several well defined control points on the pH curve were identified, e.g., ammonia valley and nitrate apex. These points were directly related to the alternating nitrification and denitrification reactions in the process, and were found to be consistent with those previously identified using ORP. Although both pH and ORP can be used as real-time control parameters, pH provides several additional advantages over ORP. Based on these findings, real-time control strategies (fixed pH setpoints and dynamic pH control) were developed and tested. Results from laboratory scale AAA sludge digestion using fixed pH setpoints (6-8) indicated that 36% MLVSS reduction, 39% TN reduction and almost 48% saving in aeration energy was achieved after 300 hrs of digestion.

ABSTRACf: 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 feasibilityof real-time sensors (dissolved oxygen [00], 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. Water Environ. Res .. 68, 83 (1996).

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A dynamic mathematical model was developed for the alternating aerobic-anoxic (AAA) completely mixed activated sludge process for the removal of carbonaceous substrate, nitrification, and denitrification. The model is constructed on the Activated Sludge Model No. I (ASM 1) of the International Association on Water Pollution Research Control with some revisions including pH and alkalinity. Experimental data obtained from a bench-scale AAA reactor under different operational conditions were used for parameter estimation and model validation. Model parameters were found to agree well with those used by the ASM 1. Process performance at steady-state and dynamic conditions can be well predicted by the AAA model. Because nitrite is not treated as a state variable in the ASM I, a modified model incorporating nitrite as a state variable has been developed to account for nitrite accumulation. Water Environ. Res., 68, 94 (I 996).

A laboratory study was conducted to investigate the characteristics of an aerobic upflow sludge blanket reactor (AUSB) treating synthetic wastewater at a high organic loading. The response of the system was evaluated at different operating conditions, such as pressurization, extent of flow recirculation, and hydraulic retention time. Aeration shear to the biomass in the reactor was totally eliminated by employing oxygenation under pressurization in an external chamber. As a result, the biomass concentration in the reactor could be maintained at 7-10 g VSS l(-1). The AUSB system was able to handle efficiently a volumetric loading of as high as 3.8 kg TOC m(-3) d(-1), which was possibly due to high reactor biomass level and higher bioactivity as indicated by higher sludge specific oxygen uptake rates. However, the treatment capacity of the AUSB was somewhat limited by the substrate and oxygen transport in the sludge bed due to low mixing intensity. The effluent TOC of the AUSB system was relatively higher due to its operation at a high organic loading. The AUSB system performed well under a short term loading shock, and the bacterial activity in the oxygenation chamber was demonstrated to contribute to its improved substrate removal capacity under the shock loading.