Chelating properties and molecular weight distribution of soluble microbial products from an aerobic membrane bioreactor

This paper presents a detailed study on soluble microbial products (SMPs) in an aerobic membrane bioreactor (MBR) treating synthetic wastewater simulating municipal wastewater. The concentration of SMP in the reactor conformed to a cyclical pattern of accumulation and reduction in relation to SRT. The molecular weight (MW) distribution of accumulated SMP was determined to vary from <1 kD to >100 kD. Copper chelating properties of various SMP fractions in the MBR were compared before and after copper addition to the feed. The conditional stability constant (Log cK), complexation capacity (Cc), and SMP-ligand concentration (CL) were evaluated to determine the impact of copper on the chelating properties. The results indicated that accumulated SMP in the aerobic MBRs without copper addition are moderate chelators with Log cK values of 7.6–8.3 mol−1 for the moderate ligands and 6.3–6.8 mol−1 for the relatively weaker ligands. SMPs with MW of 1–10 kD were found to have the highest complexation capacity among all SMP fractions. The complexation capacity of accumulated SMP after feeding copper was 0.11 μmol/mg of SMP, almost half of its value prior to feeding copper. The reduction of Cc after feeding copper was a result of an increase in large molecular weight SMP (>100 kD).