Long chain fatty acids removal in selector tanks: Evidence for insufficient Microthrix parvicella control

Microthrix parvicella is the dominant filamentous microorganism prevailed in biological nutrient removal (BNR) activated sludge systems experiencing temporarily or even permanently filamentous bulking and foaming problems. Reports regarding the ability of M. parvicella to uptake and utilize long chain fatty acids (LCFA) are rather controversial. In the context of this study a series of carbon uptake batch experiments with oleate as the external substrate under aerobic, anoxic and anaerobic conditions were performed in order to investigate the biosorption capacity of activated sludge samples experiencing serious filamentous bulking and foaming problems due to M. parvicella proliferation. According to the results LCFA removal efficiency is reversely proportional to the applied organic loading. Removal efficiencies in the order of 70-90% were observed under aerobic conditions for floc loadings lower than 100 mgCOD/gSS, whereas lower removal efficiencies were experienced (30-40%) for significantly higher floc loadings. Similar correlation of removal efficiencies with floc loading was evidenced under anaerobic and anoxic conditions (50% LCFA removal for floc loadings lower than 100 mgCOD/gSS). Based on these findings it can be stated that under the conditions prevailing in anaerobic, anoxic and aerobic selector tanks, only a portion of LCFA is removed, and therefore the remaining LCFA under the completely mixed conditions prevailing at the aeration zone of a bioreactor would establish low enough floc loadings, thus stimulating M. parvicella growth.