It is demonstrated with two parallel single reactor intermittently aerated nitrification denitrification systems fed municipal wastewater as influent, that Microthrix parvicella bulking can be stimulated and cured by manipulating the ammonia concentration in the aerobic period (by inhibiting the nitrifiers) to high and low values respectively. The proliferation or not of M. parvicella is hypothesized to be due to their requirement for ammonia as a nitrogen source for growth. In terms of this hypothesis, if nitrification is rapid and complete, ammonia is not freely available and will limit M. parvicella growth. If nitrification is not complete for whatever reason, ammonia is available for the growth of the slow growing M. parvicella, enabling their proliferation to cause a bulking sludge. This hypothesis does not overturn or replace the anoxic-aerobic (AA, or low Food/Microorganism, F/M, ratio) filament bulking hypothesis of Casey et al. (Water SA 25(4) (1999) 425) but appears to be additional to it. Future research will focus on determining how elements of both hypotheses superimpose on the conditions in BNR systems, to produce an AA filament bulking sludge or not.
