EFFECTS OF COD/N RATIOS ON TREATMENT PERFORMANCE AND FOULING PROPENSITY IN A MEMBRANE BIOREACTOR (MBR)

Abstract

Membrane bioreactor (MBR) is considered as a good integration of conventional activated sludge system and advanced membrane separation, thus enabling the independent control of sludge retention time and hydraulic retention time and retaining a high concentration of sludge biomass within the bioreactor. In present study laboratory scale suspended growth membrane bioreactor (SG-MBR) was developed to investigate the effect of influent COD/N ratio on this system in terms of treatment performance and membrane fouling propensity. This system was operated at two conditions, in first condition COD/N ratio of 20 was maintained and in second condition it was 10. Hollow fiber membrane module (Mitsubishi Rayon) was submerged in bioreactor of 14 L working volume. The MBR was operated intermittently, 10 minutes on and 2 minutes off cyclic mode, at a flux of 8.76 L/m .h and the air supply was maintained at 7 L/min. The organic loading rate (OLR) was maintained 3 kg/m3/d for two conditions while nitrogen loading rate (NLR) was changed, in first condition it was 0.15 kg/m3/d and in second condition it was 0.30 kg/m3/d at 8 hours HRT. Results showed that above 95.0% removal efficiencies of organic matter were achieved; indicating COD removal was irrespective of CODIN ratio. The average removal efficiencies of ammonium nitrogen (NH4+-N), total nitrogen (TN) and total phosphorus (TP), with a COD/N ratio of 20 were the highest at 90.5%, 74.0%, and 38.0% respectively. NH4+-N removal in condition 2 was 89.3%, while TN removal was adversely affected and it was observed to be 35% only. TP removal in condition 2 dropped to 14%. With changing condition sludge characteristics were changed significantly especially particle size and distribution. Fouling behavior was also examined in two conditions in terms of fouling resistances and it was found that total hydraulic resistance was more in condition 2. So to achieve better nutrients removal and lower cake layer fouling this suspended growth system is good to operate at NLR 0.15 kg/m3/d (COIYN ratio 20). Higher NLR suppresses the denitrification process.