Development of specific membrane bioreactors for membrane fouling control during wastewater treatment for reuse
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In recent years, membrane fouling has become a critical issue of membrane bioreactor (MBR) in wastewater treatment. To resolve this obstacle, introducing biomass carriers or flocculants into submerged MBR (SMBR) has become one of the effective technologies for membrane fouling control. This study aims to provide an in-depth analysis on membrane fouling behaviour in SMBRs with sponge and/or the patented green bioflocculant by considering the properties of activated sludge and cake layer. A new functional media (sponge modified plastic carrier) was also developed to enhance the performance of integrated moving bed biofilm reactor-membrane bioreactor (MBBR-MBR) systems. The results suggested that sponge addition in a SMBR (SSMBR) or bioflocculant addition in a SMBR (MBR-G) reduced cake layer formation and limited pore blocking, thus effectively minimizing membrane fouling. Better sludge characteristics were obtained in both of the SSMBR and the MBR-G due to less soluble microbial products (SMP), lower biomass growth and sludge viscosity, higher protein to polysaccharide ratio in extracellular polymeric substances, higher zeta potential, greater relative hydrophobicity, larger floc size and better flocculation ability. The presence of sponge or bioflocculant in the SMBR also eliminated extracellular polymeric substances (EPS), SMP and/or biopolymer clusters (BPC) on membrane surface. Consequently, cake layer (Rc) and pore blocking resistance (Rp) were decreased in the SSMBR and the MBR-G. A modified resistance-in-series model proposed for the SMBR with and without bioflocculant could quantitatively demonstrate the impacts of sludge characteristics on membrane fouling. In the SSMBR, a longer hydraulic retention time (HRT) of 6.67 h permitted more considerably fouling reduction comparing to shorter HRTs (5.33 and 4.00 h). Moreover, lower Rp and Rc at the prolonged HRT were mainly ascribed to the elevated protein to polysaccharide ratio in SMP (SMPp/SMPc) of mixed liquor, together with the declined EPS and BPC in cake layer. SMP was not the primary membrane foulant when the SSMBRs were operated at different HRTs. Bioflocculant addition at the optimum HRT of 6.67 h further mitigated fouling in the SSMBR by improving activated sludge and cake layer characteristics. The integrated MBBR-MBR with the sponge modified plastic carriers showed better removal of DOC, NH₄₋N, T-N and PO₄₋P than the MBBR-MBR with plastic carriers only. Furthermore, the sponge modified plastic carriers also eliminated SMP of mixed liquor, and reduced SMP and BPC on membrane surface, which ameliorated membrane fouling, Rp and Rc as compared to the plastic carriers.
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