The main obstacle for wider use of membrane bioreactors (MBRs) for wastewater treatment is membrane fouling (i.e., deterioration of membrane permeability),which increases operating costs. For more efficient control of membrane fouling in MBRs, an understanding of the mechanisms of membrane fouling is important. However, there is a lack of information on membrane fouling in MBRs, especially information on features of components that are responsible for the fouling. We conducted a pilot-scale experiment using real municipal wastewater with three identical MBRs under different operating conditions. The results obtained in this study suggested that the food-microorganisms ratio (F/M) and membrane filtration flux were the important operating parameters that significantly influenced membrane fouling in MBRs. Neither concentrations of dissolved organic matter in the reactors nor viscosity of mixed liquor, which have been thought to have influences on fouling in MBRs, showed clear relationships with membrane fouling in this study. Organic substances that had caused the membrane fouling were desorbed from fouled membranes of the MBRs at the termination of the operation and were subjected to Fourier transform infrared (FTIR) and 13C nuclear magnetic resonance (NMR) analyses. These analyses revealed that the nature of the membrane foulant changes depending on F/M. It was shown that high F/M would make the foulant more proteinaceous. Carbohydrates were dominant in membrane foulants in this study, while features of humic substances were not apparent.

No abstract

Control of membrane fouling is important for more efficient use of membranes in water treatment. Control of physically irreversible fouling, which is defined as fouling that requires chemical cleaning to be cancelled, is particularly important for reduction of operation cost in a membrane process. In this study, a long-term filtration experiment using three different types of MF and UF membranes was carried out at an existing water purification plant, and the evolution of physically irreversible fouling was investigated. The experimental results demonstrated that the extent of physically irreversible fouling differed significantly depending on the membrane type. Cleaning of the fouled membranes with various chemical reagents demonstrated that organic matter was mainly responsible for physically irreversible fouling. Organic matter that had caused physically irreversible fouling in the long-term operation was desorbed from the fouled membranes and was subjected to Fourier transform infrared and 13C nuclear magnetic resonance analyses. These analyses revealed that carbohydrates were dominant in the membrane foulant regardless of the type of membrane. Based on measurements of molecular weight distribution of organic matter in the feedwater and the permeates from the membranes, a two-step fouling mechanism is proposed to explain the dominance of carbohydrates in the foulant: hydrophobic (humic-like) components with small molecular weight are first adsorbed on the membrane and, consequently, narrow the size of micro-pores of membranes, and then hydrophilic (carbohydrate-like) compounds with larger molecular weight plug the narrowed pores or the hydrophilic compounds are adsorbed onto the membrane surface conditioned by the hydrophobic components.

The reaction of Lal-,Ca,Mn03 (x = 0, 0.1, 0.2) with ZrOz-8 mol% YzO3 (YSZ) has been investigated at temperatures ranging from 1300" to 1425°C in air. Substitution of Ca for La in LaMn03 depresses the reactivity with YSZ. A layer of La2Zrz07 is formed at the Lal-,Ca,MnO,/YSZ interface after an induction period, and its formation is accelerated when the Lal-,Ca,Mn03 phase is porous. The reaction proceeds by unidirectional diffusion of La, Mn, and/or Ca ions, mainly Mn ions, into YSZ. The diffusion coefficients of La and Mn ions in YSZ, which are estimated using a LaMn03/ single-crystal YSZ couple, are much lower than that of oxygen ion. From the experimental data, a reaction mechanism is proposed. [

No abstract