A HYPOTHESIS FOR THE CAUSES AND CONTROL OF ANOXIC-AEROBIC (AA) FILAMENT BULKING IN NUTRIENT REMOVAL ACTIVATED-SLUDGE SYSTEMS

From laboratory research and a literature review of the biochemical pa thways of aerobic-facultative heterotrophic organisms, an hypothesis i s proposed for the proliferation of anoxic-aerobic (AA) filamentous or ganisms in nitrification-denitrification (ND) and nitrification-denitr ification biological excess phosphorus removal (NDBEPR) systems. In ac tivated sludge, under anode conditions floc-forming organisms execute the denitrification of nitrate (NO3-) through each of the denitrifrcat ion intermediates to dinitrogen (N-2), in the process of which the int ermediate nitric oxide (NO) is accumulated intracellularly. Intracellu lar NO is inhibitory to the utilization of oxygen in the subsequent ae robic zone. In contrast, the filamentous organisms execute only part o f the denitrifrcation pathway, i.e. the reduction of NO3- to NO2-; the y do not accumulate NO and hence are not inhibited in the subsequent a erobic zone. Thus in anoxic-aerobic systems, floc-formers are placed a t a disadvantage in the aerobic zone giving an advantage to the filame nts in the competition for substrate. Experimental evidence to support this hypothesis is presented and a tentative proposal of a strategy f or control of AA filament proliferation is described and tested experi mentally.