Production of EPS-flocculants from glycerol-rich synthetic wastewater: The influence of inoculum and process parameters

Synthetic flocculants are applied in various fields to induce the agglomeration of dispersed particles. However, their environmental impact raises concern. Microbial extracellular polymeric substances (EPS) offer a promising green alternative to synthetic flocculants, due to their charged functional groups, high molecular weight and biodegradability. In this study, we investigated the potential of EPS produced by mixed microbial cultures (MMC) during wastewater treatment as bio-flocculant. Two sets of experiments ( Exp 1 and Exp 2 ) were conducted to assess the effects of inoculum sludge (municipal and industrial), changes in the COD:N ratio (from 20 to 60 to 100), and in solids retention time (SRT) (from 6 to 1.5 days) on rapid start-up of lab-scale membrane bioreactors (MBRs) and EPS overproduction. Overproduction of EPS by the MMC was achieved by applying nitrogen limitation (COD:N 100) with short SRTs (3 to 1.5 days) from both inocula. Under these conditions, the production yield reached up to 0.2-0.25 g COD EPS /g COD converted . Quantitative analysis coupled with fluorescent staining highlighted polysaccharides as the dominant EPS component (58-75%), with an average molecular weight between 2.2-2.4 MDa. The flocculation efficiency of the EPS was evaluated against kaolinite, reaching 81-87.4% using EPS produced at 3 days SRT, and stabilizing at 65.9-73.9% with EPS produced at 1.5 days SRT. This study provides a strong basis for developing a reproducible process for synthesis of polysaccharide-rich EPS-flocculants from wastewater using MMC in MBRs. We demonstrated that coupling nitrogen limitation with short SRT is crucial for achieving stable EPS overproduction with a fast start-up.