Impact of hydraulic retention time on organic and nutrient removal in a membrane coupled sequencing batch reactor

Although solids retention time (SRT) is the key parameter in wastewater treatment design and operation, this study determined the effect of hydraulic retention time (HRT) on biological nutrient removal in a membrane coupled sequencing batch reactor (MSBR) at the fixed SRT of 10 days. During more than 200 days of operation, the HRT of the MSBR were decreased from 24 to 12 and to 6 h while the volumetric exchange ratio in each operating cycle was fixed at 50%. The decrease of HRT led to a proportional increase in biomass concentration at the fixed SRT. The system demonstrated excellent removal of organic matter with the highest COD removal efficiency (97%) achieved at the shortest HRT of 6 h. As HRT was reduced from 24 to 12 h, the total nitrogen removal efficiency improved from 68 ± 5% to 80 ± 4%, but there was no further improvement when HRT decreased to 6 h. Coincidently, similar and higher abundance of nitrifying bacteria was observed in the MSBR operated at the HRTs of 6 and 12 h than that at the HRT of 24 h. The total phosphorus removal efficiencies were 62 ± 15%, 77 ± 4% and 85 ± 3% at the HRTs of 24, 12 and 6 h, respectively. The maximum P release rates for activated sludge at the HRTs of 24, 12 and 6 h were 3.7 ± 0.5, 6.4 ± 0.2 and 8.7 ± 0.1 mg P/h, respectively, while the maximum P uptake rates were 3.2 ± 0.1, 8.6 ± 0.2 and 15.2 ± 0.2 mg P/h, respectively. Contradictory to the theory that effluent water quality is solely SRT dependent, the results suggest that it is also affected by HRT and resultant biomass concentration possibly due to factors such as change in hydrolysis of particulate organic matter, the unique microenvironment and transition between anaerobic and aerobic metabolism at high biomass concentrations in MSBR operation.