Dynamic mechanisms of biocide-mediated biofouling control in two-stage RO systems in wastewater reclamation: efficacy and microbial adaptation†

Abstract

This study investigates the role of biocides in mitigating biofouling during the onsite operation of a two stage reverse osmosis (RO) system for chemical engineering wastewater reclamation. A laboratory scale fouling mimic system was employed to simulate filtration performance under different dosages of a commercial biocide methylisothiazolinone (MIT). The introduction of MIT (5 mg L⁻¹) to the feedwater of the first and second stage RO processes resulted in a limited alleviation of flux decline (8–14% flux improvement) in the early stages of filtration (<6 days), and the effectiveness diminished as filtration progresses. The higher dosage of MIT (5 mg L⁻¹) for both the first and second stage RO led to greater inhibition on bacterial growth, microbial organic matter secretion and reduced signaling molecule production on the membrane surface compared with those for lower dosage of biocide (2 mg L⁻¹). High-throughput sequencing revealed that the biocide reduced bacterial diversity in the early filtration stages and suppressed the enrichment of biofilm forming bacteria (e.g., Proteobacteria Amb-16S-1323 and Limnobacter) on the membrane surface of both RO stages. However, salt and biocide tolerant resistant bacteria (e.g., Patescibacteria, Methyloversatilis and Microbacteriaceae) progressively adapted to the conditions, becoming dominant in the fouling layer and contributing to the formation of dense biofilms. Fouling layers treated with low-dose biocide exhibited higher bacterial diversity compared to those treated with a higher dose, which could cause its low biofouling control efficiency. Correlation analysis indicated that flux decline was positively correlated with biofilm-related parameters, such as bacterial counts, organic matter content, and quorum sensing molecules present in the fouling layers of both stages of RO, suggesting that biofouling played crucial roles in flux decline. The feedwater bacterial count was the most critical parameter contributing to the flux decline of both stages of RO, suggesting the significance of controlling bacterial load to dual stage RO systems. The findings of this study could provide scientific and technical basis for dealing with biofouling in two stage RO systems for wastewater reclamation.