INFLUENCE OF DISSOLVED-OXYGEN CONCENTRATION ON NITRITE ACCUMULATION IN A BIOFILM AIRLIFT SUSPENSION REACTOR

The biofilm airlift suspension (BAS) reactor can treat wastewater at a high volumetric loading rate combined with a low sludge loading. Two BAS reactors were operated, with an ammonium load of 5 kg N/(m(3) d), in order to study the influence of biomass and oxygen concentration on the nitrification process. After start-up the nitrifying biomass in t he reactors gradually increased up to 30 g VSS/L. Due to this increase d biomass concentration the gas-liquid mass transfer coefficient was n egatively influenced. The resulting gradual decrease in dissolved oxyg en concentration (over a 2-month period) was associated with a concomi tantly nitrite build-up. Short term experiments showed a similar relat ion between dissolved oxygen concentration (DO) and nitrite accumulati on. it was possible to obtain full ammonium conversion with approximat ely 50% nitrate and 50% nitrite in the effluent. The facts that (i) ni trite build up occurred only when DO dropped, (ii) the nitrite formati on was stable over long periods, and (iii) fully depending on DO level s in short term experiments, red to the conclusion that it was not aff ected by microbial adaptations but associated with intrinsic character istics of the microbial growth system. A simple biofilm model based on the often reported difference of oxygen affinity between ammonium and nitrite oxydizers was capable of adequately describing the phenomena. Measurements of biomass density and concentration are critical for th e interpretation of the results, but highly sensitive to sampling proc edures. Therefore we have developed an independent method, based on th e residence time of Dextran Blue, to check the experimental methods. T here was a good agreement between procedures. The relation between bio mass concentration, oxygen mass transfer rate and nitrification in a B AS reactor is discussed. (C) 1997 John Wiley & Sons, Inc.