Effect of temperature and pH value on the growth-rate constants of nitrifying bacteria in the activated-sludge process

The specific growth rates of nitrifying organisms were determined under non-equilibrium conditions in laboratory-scale activated sludge plants at three temperatures and six pH values. The highest growth rate for the first stage (to nitrite) was 0.61 d⁻¹; no nitrification was observed over a 7 week period at pH 6.0 at any temperature, indicating a μ value of <0.06 d⁻¹. Fewer data were obtained for the second stage (to nitrate); in all but one of ten comparisons the specific growth rate was lower (by an average of 16%) than for the first stage. The effects of temperature and pH value were not as expected probably because of greater effects resulting from the differences in composition and strength of the sewages used.It is shown that calculations of μ from simple measurements made on full-scale plants can give an early warning of harmful changes in the aeration tank well before the concentration of ammonia in the effluent increases.