CAUSES, CONTROL, AND MISCONCEPTIONS OF FILAMENTOUS BACTERIAL BULKING IN THE AERATION TANK OF THE SWTP

Abstract

Filamentous growth in the aeration tank of the Secondary Water Treatment Plant (SWTP) can easily be detected by calculating the Sludge Volume Index 30 (SVI30). Once detected, 12.5% sodium hypochlorite @ 15 L/hour effectively reduced 71% of filamentous bulking after 12 days. It was recommended not to use sodium hypochlorite for more than 10-12 days because it will kill microorganisms present in the aeration tank of the SWTP. However, sometimes filamentous bulking is not the cause of high SVI30, and the increased value of SVI30 can be misleading due to the nitrification and denitrification processes in the aeration tank and the clarifiers. It was concluded after the 33 days trial that despite the fact SVI30 was high, no filamentous bacterial growth was seen under the compound microscope and SEM as well. High nitrate values (29 ppm) were found in the aeration tank and clarifiers, causing solids to float on the surface due to nitrification and denitrification. Nitrogen (N₂) gas pushed and assisted the solids to float up on the surface of the clarifier. At this stage, SVI30 of 136 mL/g was observed. Reduced airflow through the air blowers helped to reduce the nitrification process in the aeration tank of the SWTP. Almost no literature is available on the misconception that increased SVI30 is always due to filamentous bulking. It was found due to the denitrification and nitrification processes in the aeration tank.