Comparison of NaOH and NaOCl on-line chemical enhanced backwash on membrane fouling of high flux membrane bioreactor treating sewage

Abstract

Different physio-chemical and biological methods have been applied to reduce membrane fouling and to maintain the flux of membrane bioreactor (MBR). Periodic chemically enhanced backwash (CEB) has been recently developed and displayed good performance to recover the membrane permeability. However, the comparative effect of on-line CEB (using NaOCl and NaOH) on fouling mitigation and the effluent quality of MBR was poorly known. This study aimed to evaluate the changes in membrane resistance, membrane fouling and the effluent quality with and without CEB in MBR to reveal the effect of different NaOCl and NaOH concentrations. Lab-scale MBR treating sewage at high flux were operated for 8 min continuously at a flux of 20 L/(m2.h) followed by a backwash duration of 30 s. In reference MBR permeate (without chemical) was used as a backwash solution. The study found that MBR with CEB has higher operational time and lower fouling rates than reference MBR. Overall, backwash with NaOH increased the run-time by 29% – 45% compared to the control MBR, and backwash with NaOCl increased run-time from 34% to 61% compared to the control MBR. NaOCl was significantly more effective by 7% – 28% compared to NaOH in fouling reduction at different concentrations. CEB has no significant (p > 0.05) effect on the removal efficiencies of different pollutants. The chemical oxygen demand removal efficiencies varied between 82 and 84%. The removals of total kjeldahl nitrogen (50 – 54%) and total phosphorus (45 – 52%) were lower due to the short solids and hydraulic retention time. This study highlights the potential of periodic CEB and concentrations of chemicals on MBR fouling and provides insights into potential approaches for mitigating this issue in MBR systems.

Keywords

• Chemically enhanced backwash
• Membrane bioreactor
• Fouling
• Sodium hypochlorite
• Sodium hydroxide