Coupling GAC to ultra-low-pressure filtration to modify the biofouling layer and bio-community: Flux enhancement and water quality improvement

An integrated gravity-driven membrane (GDM) and granular activated carbon (GAC) filtration process was operated for 193 days to investigate the influence of integrated GAC on the flux stabilization, permeate quality, composition of the biofouling layer and eukaryotic community compared with a GDM control system. The results revealed that the presence of GAC resulted in a strongly increased stable flux (6 L m(-2) h(-1), compared to 2 L m(-2) h(-1) in the GDM control). Furthermore, the GAC/GDM system exhibited a significant removal of dissolved organic compounds (DOC) (50-70%) due to the adsorption and biodegradation process. By contrast, the DOC in the permeate of the control was approximately 20-30% higher than in the raw water, which was attributed to the hydrolysis of high-MW compounds. Furthermore, the composition of the biofouling layer attached on the membrane surface was significantly influenced by the presence of GAC, leading to a reduction of accumulated organic compounds by 60%. Likewise, the extracellular polymeric substances (EPS) were obviously reduced in the GAC/GDM system, especially the tightly bound EPS in the biofilm and EPS deposited in membrane pores. Additionally, the GAC could function as a eukaryotic pre-incubator, resulting in a higher diversity of the eukaryotic community and improving predation in the biofouling layer. Furthermore, a more porous, heterogeneous and permeable biofouling layer was observed in GAC/GDM. The higher flux in GAC/GDM system was attributed to (i) lower organic foulants (especially EPS) accumulated in the biofouling layer and membrane pores and (ii) highly heterogeneous structures due to improved activity of eukaryotes in the biofouling layer. These results are essential to develop robust maintenance-free, cost-effective and energy-efficient drinking water treatments.