Impacts of sludge retention time on the performance of an algal-bacterial bioreactor

The impacts of sludge retention time (SRT) on the performance of algal-bacterial reactors were evaluated. Membrane reactors A and B were applied at SRTs of 10 days and 20 days, respectively. A low hydraulic retention time, eight hours, was intentionally applied for showing the effect of SRT. Results showed that biomass concentration did not exceed 2 g/L. Without external oxygen supply, the chemical oxygen demand (COD) removal efficiencies were approximately 60% and 50% at the SRTs of 20 days and 10 days, respectively. Ammonium removal efficiencies were approximately 50% in the two reactors. Slight aeration increased the COD and ammonium removal efficiencies to approximately 90% and 100% respectively in the reactor B, whereas the improvements in the reactor A were less. Ammonium oxidation bacteria (AOB) were effectively washed out by applying the SRT of 10 days, whereas much more AOB were detected in the reactor B. The presence of phosphate accumulation organisms in continuously illuminated reactors was proved by high throughput sequencing. Strikingly, the reactor B showed higher phosphate removal efficiency at the conditions that more AOB, which competed for ammonium against algae, were detected in the reactor B. Therefore, improving phosphate removal by washing out AOB via SRT control is not feasible. Furthermore, the SRT also had significant impacts on algal ecology. Microscopic examination showed that Oscillatoria sp., the most active algae, dominated in the reactors, whereas the low SRT washed out Chlorella sp., Diatoms sp., Closterium sp. Generally, experimental results suggested that SRT of 20 days showed better reactor performance.