Influence of COD:N ratio on sludge properties and their role in membrane fouling of a submerged membrane bioreactor

• An increase in COD:N ratio led to improved membrane performance.
• Surface concentrations of elements O and N and C bonds are different under different COD:N ratios.
• Bound EPS/SMP composition is more important than the quantity in controlling membrane fouling.
• The presence of a small quantity of filamentous microorganisms led to improved membrane performance.
• A characteristic peak of FTIR at 1735 cm−1 was observed for sludge under N deficiency.

The effect of COD:N ratio on sludge properties and their role in membrane fouling were examined using a well-controlled aerobic membrane bioreactor receiving a synthetic high strength wastewater containing glucose. Membrane performance was improved with an increase in the COD/N ratio (100:5–100:1.8) (i.e. reduced N dosage). Surface analysis of sludge by X-ray photoelectron spectroscopy (XPS) indicates significant differences in surface concentrations of elements C, O and N that were observed under different COD/N ratios, implying changes in the composition of extracellular polymeric substances (EPS). Fourier transform-infrared spectroscopy (FTIR) revealed a unique characteristic peak (CO bonds) at 1735 cm−1 under nitrogen limitation conditions. Total EPS decreased with an increase in COD/N ratio, corresponding to a decrease in the proteins (PN) to carbohydrates (CH) ratio in EPS. There were no significant differences in the total soluble microbial products (SMPs) but the ratio of PN/CH in SMPs decreased with an increase in COD/N ratios. The results suggest that EPS and SMP composition and the presence of a small quantity of filamentous microorganisms played an important role in controlling membrane fouling.

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