Case histories are presented for two BNR plants that have experienced long term Microthrix parvicella induced foaming and poor settling. Neither plant was able to reduce SRT or to install classifying selectors to wash out the filament. Trials were held from 2006 through 2009 to evaluate the impact of adding a polyaluminum chloride based compound on SVI. In most cases, within a two to four week period of adding the compound, the SVI declined from values of 200-400 mL/g to values of 100-200 mL/g. This performance was achieved at doses of 0.7 to 1.5 kg Al/ 1000 kg MLSS-day. This was lower than reported in European treatment plants. The low SVI values were maintained over the winter periods at both plants when low temperatures favored the growth of M. parvicella.