Effectiveness of the membrane bioreactor in the biodegradation of high molecular weight compounds

This paper presents biological and physical performance data from a pilot-scale membrane bioreactor system fed with synthetic wastewater containing high molecular weight compounds. At steady state, high effluent quality was obtained and maintained for about 350 days. The membrane was effective in retaining heterotrophic microorganisms and MS-2 viruses, eliminating the need for effluent disinfection. The flux through the membrane decreased rapidly with increasing total suspended solids (TSS) concentrations between 2 and 15 g/L. The flux increased linearly with the transmembrane pressure (TMP), before reaching a maximum. A linear relationship between the maximum flux and the cross flow velocity (CFV) for different TSS concentrations was observed. The ability of the bioreactor culture to degrade high molecular weight compounds in the wastewater, and the mechanisms involved in this process were examined in detail by performing respirometry experiments. Casein was used as the model compound and different initial substrate to biomass ratios (So/Xo) were tested. In experiments with high So/Xo, several observations were made: (1) the oxidation of casein occurred in two distinct steps, (2) the yield coefficient is smaller than 0.15 mg VSS/mg COD consumed, and (3) nitrification does not occur. Under low So/Xo conditions, using 14C-casein, the following observations were made: (1) complete hydrolysis of casein occurs very fast (less than 100 h) and (2) oxygen uptake and nitrogen measurements suggest nitrification takes place.