The effect of solids retention times on the characterization of extracellular polymeric substances and soluble microbial products in a submerged membrane bioreactor

•More carbohydrates and proteins in shorter SRT were due to higher biomass activity.•Macromolecules and small molecules were the main fraction of EPS and SMP.•Other organic moieties were exuded by microbes into the solution.•The shorter SRT had more small molecules and less macromolecules.•The shorter SRT had different OH bonds in hydroxyl functional groups.

In this study, the effect of solids retention times (SRTs) on extracellular polymeric substances (EPS) and soluble microbial products (SMPs) were investigated in a membrane bioreactor (MBR) at SRTs of 10, 5 and 3 days. The results showed that more carbohydrates and proteins were accumulated at short SRT, which can due to the higher biomass activity in the reactor. The molecular weight (MW) distribution analysis suggested that macromolecules (MW > 30 kDa) and small molecules (MW < 1 kDa) were the dominant fraction of EPS and SMP, respectively. The reactor at shorter SRT had more small molecules and less macromolecules of carbohydrates. The MW distribution of total organic carbon (TOC) suggested that other organic moieties were exuded by microbes into the solution. The shorter SRT had more undefined microbial by-product-like substances and different OH bonds in hydroxyl functional groups.