Background

Submerged membrane bioreactor (MBR), since its introduction in the late 1980s as a cost-effective alternative to the side-stream MBR, has been successfully used for treatment of different kinds of wastewater. Membrane fouling- an inevitable consequence of interactions between membranes and the mixed liquor, however, impedes widespread use of the submerged MBR technology. Industrial wastewater in particular, being a complex and highly variable mixture of many polluting substances, may impose aggravated pore blocking or gelatinous cake-layer fouling, depending on the type and arrangement of the modules and the associated operating parameters.

In this study, performance of hollow-fiber modules containing spacer was assessed. The aim of introducing spacer was to avoid merging of fibers by reducing the intrusion of sludge into the module as much as possible. The little amount of intruded sludge may then be backwashed through the bottom end while the sludge deposited on the surface may be effectively cleaned by air-scouring. It was expected that, in this way, efficient utilization of cleaning solution and air may be made for backwashing and surface- cleaning, respectively. Spacer filled spiral-wound module has a long history of successful application in different sectors of water and wastewater treatment. On the other hand, previous studies have focused on different operational considerations and design aspects of submerged hollow fiber membrane modules. However, our study is the first instance where spacer has been incorporated within a hollow-fiber module with the specific aim of fouling mitigation in case of wastewater applications.

Performance

1. Under similar conditions, while the usual hollow-fiber bundles exhibited fatal cake-layer fouling within a day or so, the modules with spacer sustained stable performance for a month.
2. Among the explored modules, a hybrid module (fiber packing density =61.5 %, surface area=1.07 m2) obtained by winding a rigid spacer (thickness=1 mm, opening=7 mm x 7 mm) on the surface of a module originally containing a thin spacer (opening=1 mm x 1 mm) exhibited the optimum compactness so as to minimize intrusion of sludge while simultaneously allowing wash-out of the small amount of sludge trapped within it.
3. Periodic in situ chemical backwashing with a small dose (500 mg Cl/L, 100 ml/m2, twice/week) and intermittent surface-cleaning with a specially designed aeration device (1 min/30 min @1 L air/min) enabled stable operation for a prolonged period under the selected average flux (7.64x10-6 m/s) and MLSS concentrations (up to 25 g/L).