Relationship between microbial community and environmental conditions in a constructed wetland system treating greywater

Abstract

Knowledge of the diversity of microbial communities and their relationship with biogeochemical cycles is a key factor in understanding and improving the performance of constructed wetland (CW) systems, allowing improvements in design and operation methods to reach the desired effluent quality. Assemblies of microbial groups in CWs are driven by several factors, including the environmental conditions of the mesocosm and substrate availability. This study combined physicochemical analyses, high-throughput DNA sequencing, and statistical methods to gain an insight into the influence of operational conditions on the bacteria and archaea communities in a modified constructed wetland system treating raw-light greywater. The EvaTAC system is composed of an upflow evapotranspiration and treatment tank (CEvaT) which has an inbuilt anaerobic chamber (AnC), followed by a horizontal subsurface flow constructed wetland (HSSF-CW). Samples were collected after five years of system operation in both units: sludge from the AnC, coarse gravel from the CEvaT, and fine gravel from the CW. The results showed that the system operated predominantly in anaerobic conditions, with redox potential (Eh) increasing from the inlet (−364.1 mV) to the outlet (−240.4 mV) zone. The Eh was the environmental condition that most influenced the microbial community diversity and richness, and, as well as Eh, increased along the light greywater (LGW) flow. Conversely, the chemical oxygen demand (COD) decreased, thus suggesting a negative correlation among these factors. The clustering analyses of microbial community showed that, besides environmental conditions, the media filter and related depth could also be drivers of microbial community composition. The bottom layer of CEvaT was assigned to the Proteobacteria and Synergistetes phyla indicating the accomplishment of the first steps of anaerobic digestion. The next layer was characterised by the presence of Desulfovibrio, Syntrophobacter, and Methanobacterium, microorganisms responsible for the sulfidogenesis and methanogenesis processes. Finally, in the CW zone, the community composition was dominated by microorganisms involved in the methane oxidation, such as Methylosinus, Bradyrhizobium, and Candidatus Koribacter. This study explored the universe of bacteria and archaea communities, and is an initial step in elucidating microbial diversity conversions in CW systems treating greywater, which is important knowledge to steer the conversion pathway and establish the ideal environmental conditions for the development of desired microbial communities in these systems.