The release of water effluents rich in nutrients such as nitrogen and phosphorus without adequate treatment represents environmental and human health concerns. Growing concerns about these impacts have resulted in increasingly stringent water quality regulations that encouraged the adoption of advanced treatment processes. Microalgae-based advanced wastewater treatment has gained momentum owing to its well-known advantages for advanced wastewater treatment, including the recovering of nutrients for the production of fertilizers, biofuels and fine chemical from microalgal biomass. Nevertheless, the progressive membrane fouling and permeate flux declining hamper the large-scale commercialization of membrane photobioreactors (MPBRs) in the wastewater sector. In order to get a further understanding of the fouling mechanisms and antifouling control strategies, this study investigated the effect of the hydraulic retention time on the fouling development, and the effect of different physical fouling control strategies in the fouling mitigation. A synthetic secondary effluent was continuously fed to three MPBRs operated at different HRTs (12, 24 and 36 hours). Different fouling behaviors were found as the HRT changed, which was confirmed by continuously monitoring the transmembrane pressure (TMP) and by measurements in the biomass and its algal organic matter (AOM) properties. Lowering the HRT resulted in higher fouling rates due to changes in the biomass and AOM properties. Higher HRTs led to lower fouling rates and to a lower organic rejection across the membrane. The retention of small-MW organics in SMPBR12h was found to exacerbate the fouling resistance, whereas the accumulation of large-MW biopolymers enhanced the rejection of organics, despite of not imparting significant resistance in SMPBR24h. In order to assess the impact of different physical fouling control strategies, namely relaxation, backwash and air scouring, OCT in-situ monitoring was employed in MPBR12h to provide real-time information of the fouling layer properties (thickness and relative roughness) and its interaction with the membrane surface. Different fouling mechanisms were observed under different fouling control strategies. MPBRRLX and MPBRBW presented similar fouling rates despite of the lower permeate productivities of the latter. The lowest fouling rates were observed in MPBRSC, where stronger interactions between the membrane and small-MW organics and particles was observed.
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