An inhibition study showed that toxic compounds caused two responses when present at low concentration. One is stimulation of the biomass by simply serving as an energy source; this caused an increase in the total respiration rate. The other is inhibition of the reaction. AR14 was more toxic than AO7 for biofilm from reactors fed with a primary substrate. However, AO7 demonstrated inhibition for biofilm from reactors fed with AR14 and primary substrate, and AR14 could serve as a carbon source for the same film.