Adaptability of microbial community in a membrane bioreactor treating produced water to varying salinities

Abstract:

Produced water (PW) is challenging to treat due to the presence of a wide variety of contaminants such as hydrocarbons, heavy metals, and minerals, at high concentrations. Despite these challenges, some bioreactor studies have shown that PW is amenable to biological treatment. These bioreactors were inoculated with sludge from municipal wastewater treatment plants and incrementally acclimated to high salinity of PW. However, very little is known about the microbial communities within functioning bioreactors or how they adapt to changes in salinity.

To address this knowledge gap, we operated and monitored an aerated, 130 L PW-fed membrane bioreactor (MBR) for a year. During the acclimation phase, the MBR was fed with a gradually varying mixture of dechlorinated tap water and PW in order to increase the salinity until the MBR was fed by only PW (Salinity- 29 g/L). Beyond this phase, salt was gradually added to the feed to increase the salinity to 100 g/L to test bioreactor performance and microbial community dynamics at varying salinity levels. Microbial communities were profiled via 16S rRNA amplicon sequencing and further explored by cultivation-dependent techniques. In parallel, physicochemical analyses like dissolved organic carbon, ion chromatography and excitation-emission-matrices of both the feed and permeate were performed.

Despite the huge range of salinities tested in this study, there was little change in reactor performance as measured in terms of dissolved organic carbon removal throughout the study period. Beyond the initial acclimation phase, 5 major Amplicon Sequence Variants (ASVs) belonging to the genera Iodidimonas, Roseovarius, Methylophaga, unclassified Rhodobacteraceae and Rehaibacterium constituted greater than 70% of the sequence tags at all salinity levels (29-100 g/L). Of these major ASVs, we were successful in obtaining isolates of Iodidimonas, Roseovarius and unclassified Rhodobacteraceae. We are further characterizing these isolates through whole genome sequencing and biochemical tests to shed better light on their role in treating PW. Thus, we have demonstrated that contaminant-degrading microbes in a membrane bioreactor can adapt to varying degrees of salinity making biological treatment a promising approach to treat PW.