A thesis submitted in partial fulfillment of the requirement for the degree of Master of Science, School of Environment, Resources & Development

Thesis (M. Sc.) - Asian Institute of Technology, 1994

It is known that A/O process can produce well-settling sludge. But sludge bulking can also occur in these processes due to some causes such as composition of wastewater, operating conditions, and, configurations of the systems. From this experiment, it has been observed that 0.2 mIJL of acetic acid can cause zoogloea bulking in A/O processes which had only two anaerobic tanks but the process that had three anaerobic tanks can control this kind of bulking. It also has been observed that strictly anaerobic initial contact zone (ICZ) can suppress filamentous bulking more effective than anoxic one. Furthermore, some of unsuitable operating conditions , i.e. too low organic loading, too high return sludge ratio, and too long influent hydraulic retention time can deteriorate sludge settleability and phosphorus removal efficiency but improve the removal efficiency of nitrogen. Additionally, it has been found that concept of contact stabilization activated sludge, i.e. reaeration of return sludge, is difficult to be applied with A/O process because it can introduce small amount of dissolved oxygen into ICZ so filamentous bacteria can use this 00 as electron acceptor in their metabolic mechanisms. Floes in units which had reaeration were very small siz.e and not compact.