In separations of solids from liquids by filtration, the motion of liquid is customarily normal to the filtering surface, and solids are left on the filter. In a variation called cross-flow filtration, liquid is pumped parallel to the filtering surface. By this device, thickening of flux-limiting filtercake is slowed, and the original stream is separated into a large volume of filtrate and a concentrated slurry of solids. Results reported here were obtained in cross-flow filtration used as an element in physical-chemical treatment of municipal sewage, mostly with the effluent from primary settling, but in some cases, with clarified activated sludge effluent as feed. Filtrate from passage through fabric tubes (1-inch fire-hose jackets) of primary effluent, treated with iron or aluminum salts, with powdered activated carbon (PAC), or with both hydrolyzable ions and PAC, was generally of quality superior in turbidity, organic carbon and other respects to the effluent from biological secondary treatment. Effects on product quality and flux of pressure, circulation velocity, additive concentration, water recovery, pH and other variables were investigated. Based on production rates obtained, estimates of treatment costs were made.

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