Characterization of soluble microbial products in 10 large-scale membrane bioreactors for municipal wastewater treatment in China

In order to fill the knowledge gaps between lab/pilot-scale and full-scale operations and contribute basic information to long-term and stable operation of large-scale membrane bioreactors (MBRs), a systematic investigation was first conducted focused on soluble microbial products (SMPs) in 10 large-scale MBRs (capacity over 10,000 m3/d) for municipal wastewater treatment distributed in 4 different areas of China from fall to winter. The majority of these MBR plants had been operated stably for at least 1 year. Fundamental properties of SMPs were investigated, including composition, molecular weight distribution, hydrophobicity, fluorescent characteristics and fouling potential. The results showed that the concentration of SMPs ranged roughly from 5 to 25 mg TOC/L, with the major component being polysaccharides (ca. 3–18 mg/L) followed by humic substances (ca. 2–10 mg/L); while the protein concentration was relatively low (<5 mg/L). The SMPs presented a broad molecular weight distribution from smaller than 1 kDa to over 100 kDa. About half of the SMPs were hydrophilic substances mainly contributed by polysaccharides; humic substances were concentrated in hydrophobic fractions while proteins showed a relatively wide distribution. The fluorescent properties were found to be affected appreciably by the influent quality. The batch fouling tests indicated that the initial fouling rate correlated significantly with SMPs concentrations, which was particularly the case for hydrophilic and large-molecular-weight fractions. These findings may contribute to better understanding of membrane fouling in engineering conditions and assist in long-term and stable operation of full-scale MBRs.