Evaluation of membrane fouling in a microalgal-bacterial membrane photobioreactor: Effects of SRT.

As a novel system, the microalgal-bacterial membrane photobioreactor (MPBR) has better performance than the conventional MBRs in membrane fouling control. Nevertheless, how the operating conditions affect its fouling performance is rarely reported. In this study, a microalgal-bacterial MPBR was set and continuously operated to treat synthetic wastewater. Effects of solids retention time (SRT, 10, 20, and 30 d) on the membrane fouling were investigated. The results showed that the relationship between membrane fouling and SRT was nonlinear and the fastest membrane fouling was observed at SRT 20 d. The predominant fouling mechanism was gel layer formation. X-ray photoelectron spectroscopy results showed a significant difference in the surface composition of the microalgal-bacterial consortia at different SRTs. The biological flocs at SRT of 20 d had the largest floc size, moderate filament abundance, and the highest content of bound EPS and SMP. The highest membrane fouling at SRT 20 d was mainly attributed to the highest concentration of EPS and SMP. Environmental stresses and fierce competition between microalgae and bacteria are considered to be the underlying reasons for the elevated production of EPS and SMP. In brief, optimizing the SRT value to control the balanced growth of microalgae and bacteria and keep them at an appropriate ratio is critical for delaying membrane fouling in microalgal-bacterial MPBR.