Membrane fouling reduction by backpulsing and surface modification

Abstract

A combined method of backpulsing and membrane surface modification was employed for the reduction of membrane fouling. A novel photoinduced grafting method was used to render polypropylene (PP) membranes hydrophilic with neutral or positively or negatively charged surfaces by grafting monomers of poly(ethylene glycol 200) monomethacrylate (PEG200MA), dimethyl aminoethyl methacrylate (DMAEMA), or acrylic acid (AA), respectively. Both unmodified and modified PP membranes, as well as commercial cellulose acetate (CA) membranes, were evaluated in a crossflow microfiltration system with and without backpulsing in the presence of Escherichia coli bacterial suspensions. Without backpulsing, the resulting permeate volume is nearly the same for the different membranes. With backpulsing, however, considerable improvement was obtained by surface modification, especially for low feed concentrations and for short filtration times. For example, the permeate volume for backpulsed filtration of 0.14 g/l E. coli for 1 h using the unmodified PP membrane is 1.7 times that without backpulsing, and it is even higher for the other membranes. The permeate volume with backpulsing is highest for the neutral, hydrophilically modified PP membrane and for the commercial CA membrane, approximately 2.6 times that of the unmodified PP membrane without backpulsing. Moreover, the recovered pure buffer flux after backwashing the fouled membranes for an extended period is approximately twice as high for these neutral hydrophilic membranes as it is for the unmodified hydrophobic PP membrane. However, the recovered flux after a long backwash of the membranes fouled with backpulsing is 20–40% lower than that of the membrane fouled without backpulsing, apparently due to greater adhesive internal fouling when the membrane surface is frequently exposed by rapid backpulsing.