Permeate microbiome reflects the biofilm microbial community in a gravity-driven woven-fiber microfiltration (WFMF) system for wastewater treatment

United Nations Sustainable Development Goal 6.3 aims to half the proportion of untreated wastewater and increase recycling and safe water reuse. Therefore, developing robust technologies to achieve these goals, specifically in low- to middle-income countries, is of concern. One such technology, gravity-driven woven-fiber microfiltration (WFMF) has been shown to be a reliable, low-cost, and versatile water treatment process. This study investigated a gravity-driven WFMF system for treating secondary wastewater. Given the significant role of the microbial community in biological treatment processes, this investigation focused on the inter-relationship between the microbiomes of the influent, biofilm, and permeate, while further examining the system's performance regarding microbial activity and permeate water quality. The WFMF system reached a quasi-steady state after approximately 10 days. The microbiome analysis, specifically beta diversity analysis, showed that the biofilm and permeate had similar microbiomes, proving a direct impact of the biofilm's microbial community on the permeate water quality. It also showed that the microbial community composition changed within the system. Alpha diversity analysis indicated that biofilm had higher richness and alpha diversity indices (e.g., OTUs and Shannon) than the influent. The system reduced the concentrations of fecal indicators Escherichia coli and Enterococcus spp. and the relative abundance of putative pathogens such as Legionella spp. Furthermore, it reduced microbial activity, as measured by intact cell counts and intracellular ATP, by 48.7% and 58.0%, respectively. Biofilm structure, investigated by various imaging techniques, including scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM) and optical coherence tomography (OCT), depicted a heterogeneous distribution over the membrane surface. The findings of this study underscore the role of the biofilm on the permeate's microbiome and, consequently, its impact on the permeate's biological stability and suitability for discharge and reuse. Thoroughly understanding microbial dynamics has implications for performance optimization, field implementation, and permeate discharge or reuse.