| Submerged membrane bioreactor (MBR) processes have been successfully used in many wastewater treatment plants to provide excellent solid-liquid separation efficiency. However, their widespread application is hindered by excessive membrane fouling. For better understanding of fouling mechanisms and development of fouling control strategies, a series of laboratory- and pilot-scale submerged MBR tests were conducted. The critical flux was measured using the stepwise flux method, while the activated sludge was characterized by measuring the colloidal particle concentration, mixed liquor suspended solids (MLSS) concentration, temperature, time to filtration (TTF), diluted sludge volume index (DSVI), particle size distribution, and zeta potential. The colloidal particle concentration was represented by the colloidal total organic carbon (TOC), which is the TOC difference between the filtrate after passing through a 1.5 mum pore size filter and the permeate collected from membrane modules with a pore size of 0.04 mum. In addition, the activated sludge was divided into three components: MLSS particles, colloids, and solutes, to account for their different roles in fouling.; The results revealed that the fouling mechanisms were different at different aeration conditions. With no aeration, fouling was mainly controlled by the MLSS particles and their interactions with colloids. With aeration, however, the deposition of colloidal particles on the membrane surfaces gradually became predominant with increasing aeration intensity. Further, fouling was affected not only by sludge composition and aeration conditions, but also by their interactions. The results also demonstrated that, despite different sludge characteristics tested, the critical flux was almost solely related to colloidal TOC. Therefore, it is suggested that the colloidal TOC be used as a filterability index for MBR processes. Furthermore, the colloidal TOC can be greatly reduced by the addition of ferric chloride, alum, or a patented organic polymer, resulting in lower fouling rates. Finally, a long-term irreversible fouling model was developed in which MLSS concentration, air flow rate, temperature, permeate COD, and flux were identified as the key process parameters affecting biofouling. |
