Investigations on fouling and cleaning of membranes used for direct membrane filtration of municipal wastewater

The concept of direct membrane filtration (DMF) is relatively new for treating municipal wastewater. In contrast to a conventional membrane bioreactor, DMF decouples filtration and biological treatment. This presentation is based on a pilot scale DMF study conducted at the municipal wastewater treatment plant in Svedala (Sweden), combined with lab-scale experiments and analysis to optimise the pilot performance. To reduce fouling, membrane operation at pilot scale included membrane relaxation and aeriation induced crossflow. Chemical cleaning was conducted by soaking and/or backwashing with chemicals to restore membrane performance as per manufacturer recommendations. Acid cleaning was performed with citric acid and hydrochloric acid followed by an alkaline cleaning with hydrogen peroxide and sodium hydroxide. Operation at pilot scale with an initial permeability of 29 L/m2/h/bar resulted in rapid fouling of the membrane. During the pilot trails, it was observed that the recommended cleaning method was insufficient to restore the membrane performance completely; a recovery of 64% was observed after acid and alkaline backwashing. Therefore, membrane cleaning was investigated on lab-scale to improve the cleaning procedure. In this study, different chemical reagents and relaxation protocols were tested. To determine cleaning performance along with potential foulants, Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) coupled with Energy-dispersive X-ray spectroscopy (EDS) were used. Based on the findings, a new relaxation and chemical cleaning protocol was developed with pure alkaline cleaning as a key step to ensure high flux recovery rates over long cycles. Overall, this study revealed that a systematic approach towards membrane cleaning can improve cleaning efficiency and thus reduces the impact of fouling in DMF operation.