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 expanding rapidly as an alternative to conventional clarification and filtration processes to meet increasingly stringent regulations related to the treated water quality and in particular to pathogens such as Giardia and Cryptosporidium. The membranes used in these processes do not remove color, natural organic matter (NOM) or synthetic organic chemicals (SOCs). MF and UF must be combined with other conventional technologies such as activated carbon adsorption and coagulation to overcome some of these limitations. In this context recent advances in immersed membranes configurations offer an opportunity to develop new combined treatment processes. In most of these systems, shell-less hollow fiber 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 conventional MF and UF systems). The membranes represent a positive barrier between a "reaction zone" where adsorption, biodegradation or coagulation can be carried out and the water treated. The system offers a great flexibility of operation by allowing operation at high suspended solids concentrations and to adapt the type and age of the suspension to the required treatment. The association of immersed membranes and powdered activated carbon provides the following benefits: (1) better physical removal of NOM and SOCs through optimal use of PAC, (2) biological removal of the biodegradable fraction of NOM, (3) reduced sludge volumes, and (4) absolute containment of the PAC within the system independent of process conditions. The very high concentration of PAC (10-20 g/L) carried in the reactor offers the required buffer capacity. The present research program, carried out at Compagnie Generale des Eaux Research 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 Granville (Normandie) over one year to evaluate the immersed membrane process on river water. The process was first evaluated as a polishing microfiltration/PAC slurry reactor, after conventional coagulation/flocculation and sedimentation. In a second phase, the process was directly used on raw water with in-line coagulation; in that case, the coagulated matter and the PAC were allowed to co-exist in the membrane tank. The results of these studies show that the combination of immersed membranes 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 water quality and (3) suitability for plant upgrading by conversion of existing clarifiers or sand filters into membrane reactors.