IMMERSED MEMBRANE FILTRATION FOR THE PRODUCTION OF DRINKING-WATER - COMBINATION WITH PAC FOR NOM AND SOCS REMOVAL

The use of microfiltration (MF) or ultrafiltration (UF) processes is e xpanding rapidly as an alternative to conventional clarification and f iltration processes to meet increasingly stringent regulations related to the treated water quality and in particular to pathogens such as G iardia and Cryptosporidium. The membranes used in these processes do n ot remove color, natural organic matter (NOM) or synthetic organic che micals (SOCs). MF and UF must be combined with other conventional tech nologies such as activated carbon adsorption and coagulation to overco me some of these limitations. In this context recent advances in immer sed membranes configurations offer an opportunity to develop new combi ned treatment processes. In most of these systems, shell-less hollow f iber membranes are directly immersed in the reaction vessel receiving the water to be purified and operated under slight negative pressure ( whereas they are traditionally housed in pressure vessels in conventio nal MF and UF systems). The membranes represent a positive barrier bet ween a ''reaction zone'' where adsorption, biodegradation or coagulati on can be carried out and the water treated. The system offers a great flexibility of operation by allowing operation at high suspended soli ds concentrations and to adapt the type and age of the suspension to t he required treatment. The association of immersed membranes and powde red activated carbon provides the following benefits: (1) better physi cal removal of NOM and SOCs through optimal use of PAC, (2) biological removal of the biodegradable fraction of NOM, (3) reduced sludge volu mes, and (4) absolute containment of the PAC within the system indepen dent of process conditions. The very high concentration of PAC (10-20 g/L) carried in the reactor offers the required buffer capacity. The p resent research program, carried out at Compagnie Generale des Eaux Re search Laboratory (in Maisons-Laffitte, France), has examined the use of immersed membranes for significantly enhancing SOCs and NOM removal at bench and pilot scales. An onsite pilot-study was conducted in Gra nville (Normandie) over one year to evaluate the immersed membrane pro cess on river water. The process was first evaluated as a polishing mi crofiltration/PAC slurry reactor, after conventional coagulation/flocc ulation and sedimentation. In a second phase, the process was directly used on raw water with in-line coagulation; in that case, the coagula ted matter and the PAC were allowed to co-exist in the membrane tank. The results of these studies show that the combination of immersed mem branes with PAC and/or coagulant provides (1) excellent water quality through significant enhancement of particulate matters as well as NOM and SOCs removal, (2) excellent response to variations of the feed wat er quality and (3) suitability for plant upgrading by conversion of ex isting clarifiers or sand filters into membrane reactors.