Effects of nutrients conditions and solids retention time (SRT) on performance and membrane fouling of aerobic membrane bioreactors (MBRs)

Abstract

This thesis investigated the effect of chemical oxygen demand (COD) to nitrogen ratio (COD:N) in feed on the biological performance of aerobic membrane bioreactor (MBR). Meanwhile, the effects of nutrients condition (COD:N ratios) and solids retention time (SRT) (7, 12 and 20 days) on sludge properties and their role in membrane fouling were systematically studied using well-controlled aerobic membrane bioreactor receiving a synthetic high strength industrial wastewater containing glucose. The results showed an increased COD:N ratio from 100:5 to 100:2.5 and 100:1.8 had limited impact on COD removal efficiency and further led to a significant improvement in membrane performance, a reduced sludge yield, and improved effluent quality in terms of residual nutrients. The results suggest that an increased COD:N ratio will benefit the industrial wastewater treatment using membrane bioreactors by reducing membrane fouling and sludge yield, saving chemical costs, and reducing secondary hand pollution by nutrients. Membrane performance was improved with an increase in the COD:N ratio (e.g. reduced N dosage). Surface analysis of sludge by X-ray photoelectron spectroscopy (XPS) suggests that significant differences in the surface concentrations of elements C, O and N were observed under different COD:N ratios, implying significant differences in extracellular polymeric substances (EPS) composition. A unique characteristic peak at 1735 cm -1 was observed under nitrogen limitation conditions by using Fourier transform-infrared spectroscopy (FTIR). Total EPS, proteins and the ratio of proteins to carbohydrates in EPS decreased with an increase in COD:N ratio, while carbohydrates in EPS increased with an increase in COD:N ratio. There were no significant differences in the total soluble microbial products (SMPs) but the ratio of proteins to carbohydrates in SMPs decreased with an increase in COD:N ratios. Sludge cake formation was the dominant mechanism of membrane fouling. Membrane performance was improved with an increase in SRT. Surface analysis of sludge by X-ray photoelectron spectroscopy (XPS) suggests that significant differences in the surface concentration of element C and N were observed under different SRTs, implying significant differences in EPS composition. A larger amount of total EPS was found at the lowest SRT (7 days) tested but the ratio of proteins to carbohydrates in EPS increased with an increase in SRT. Similarly, the quantity of SMPs decreased with an increase in SRT but the ratio of proteins to carbohydrates in SMPs increased with an increase in SRT. The quantity of total EPS, total SMPs, and proteins to carbohydrates ratios positively correlated to membrane fouling rates. Sludge cake formation is the dominant mechanisms of membrane fouling.