A hypothesis on Microthrix parvicella proliferation in biological nutrient removal activated sludge systems with selector tanks.

The objective of this study was to evaluate the ability of Microthrix parvicella for long-chain fatty acids uptake under anaerobic, anoxic, and aerobic conditions as well as its ability to utilize volatile fatty acids and long-chain fatty acids under anoxic and aerobic conditions. According to the results, a hypothesis on the competition between floc-forming microorganisms and M. parvicella for long-chain fatty acids uptake under aerobic, anoxic, and anaerobic conditions was formulated. According to this hypothesis, M. parvicella exhibits similar long-chain fatty acids uptake capacity with floc-forming microorganisms even at relatively high floc loading values that are very often imposed at selector tanks. Following this hypothesis, the failure of selector tanks to provide for an effective M. parvicella control is evidenced. Based on the experimental results, the ability of M. parvicella to utilize long-chain fatty acids with rates comparable to those of floc formers, even in anoxic conditions, in conjunction with its lower acetate utilization rates, provides a good explanation regarding its preference to slowly biodegradable organic carbon compounds.