Ultrafiltration of wastewater: Effects of particles, mode of operation, and backwash effectiveness

The effects that wastewater quality and mode of operation have on the perfo rmance of an asymmetric, hollow fiber, polysulfone, ultrafiltration (UF) me mbrane with a molecular weight cutoff of 100,000 Daltons were investigated. Performance was assessed through monitoring membrane flux, transmembrane p ressure, effluent biochemical oxygen demand, and operational cost of the ex perimental system while treating filtered secondary, secondary, and filtere d primary effluents. Fluxes achieved for filtered secondary (129-173 l/m(2) h), secondary (101-158 l/m(2) h), and filtered primary (20-41 l/m(2) h) ef fluents were compared to those obtained at three other locations where simi lar UF systems were operated. A conceptual model of the impact of an insuff icient backwash and of operating the UF system at constant flux on membrane performance is presented to explain the differences in fluxes. Employing p re-membrane granular filtration to remove a portion of the problematic part icles in secondary effluent prior to UF led to optimal operational conditio ns. The costs associated with the operation of pre-membrane granular filtra tion were offset by the increase in production achieved. Although the use o f recirculation could increase maintainable flux when treating a concentrat ed feed (e.g., filtered primary effluent), the associated costs were high. Improved UF performance was found to result from allowing flux to decline n aturally, rather than using a constant flux mode of operation. The effluent s produced when altered secondary and secondary effluents were the feeds wo uld be equivalent to an oxidized, coagulated, clarified, and filtered waste water as per Title 22 California Wastewater Reclamation Criteria. (C) 2000 Elsevier Science Ltd. All rights reserved.