Abstract
Ammonia in the form of dissolved salts can he removed from industrial waste water by successive biological nitrification (oxidation to nitrate) and denitrification (reduction of nitrate to nitrogen gas). Nitrification of ammonium sulphate solutions was studied in a hatch activated sludge system, at low substrate concentrations (less than 60 g/m3 ammonia-nitrogen) to investigate substrate limitation and at high concentrations (up to 5000 g/m3 ) to investigate substrate inhibition. The kinetics of the system were considered in terms of a "specific substrate removal rate", analogous to microbial specific growth rate, to allow for the effects of non-viable but biochemically active bacterial cells in the sludge and the probable variation of bacterial yield coefficient with specific growth rate. The effects on this parameter of substrate concentration were summarised as mathematical models by applying a pattern-search curve-fitting procedure to the experimental data relating substrate concentration, product concentration and time obtained from batch runs at 25°C covering a range of initial substrate concentrations, mostly at a controlled pH of 7. At low concentrations, the effects of substrate concentration on the specific substrate removal rate were described better by a model based on the discontinuous function first proposed by Blackman than by one based on the Monod relation. At high concentrations, stagewise analysis of the hatch concentration-time data suggested that nitrification was subject to hysteresis, in that the specific substrate removal rate depended on the initial batch substrate concentration as well as the transient substrate concentration. A model based on the Haldane equation for enzyme inhibition gave a reasonable description of the effects of initial substrate concentration on the specific substrate removal rate. A correlation including three empirical parameters was found to describe the variation of specific substrate removal rate with transient substrate concentration for individual batch runs. When inocula from dilute stock sludge culture were introduced into concentrated substrate solutions, an acclimatisation phase was observed in which the specific substrate removal rate fell rapidly to the inhibited level. Inocula which were subjected to preliminary acclimatisation before inoculation did not show this effect. In additional work at low substrate concentrations, the optimum pH for nitrification at 25°C was found to be 8. 5, and empirical expressions were found to describe the relation between pH and specific substrate removal rate. The contribution to ammonia removal by chemical decomposition of ammonium nitrite (the van Slyke reaction) was found not to be significant in the conditions prevailing in microbiological nitrification.