There are three basic process configurations used in commercial applications of cross-flow ultrafiltration and microfiltration: single-pass, batch, and feed-and-bleed. The single-pass configuration is used in some water purification systems. In the single-pass configuration, the feed is pumped through the membrane unit and the retentate is collected or fed to a subsequent processing step. In the classical batch filtration process, the entire retentate stream is recycled back to a single large feed tank, allowing the permeate removal per pass to be dramatically smaller than that in a single-pass system. The feed-and-bleed configuration recycles only part of the retentate, with the remainder of the retentate continuously collected or fed to a subsequent processing step. In diafiltration, water is added to the retentate during the filtration, with the membrane-permeating species removed from the feed as this excess fluid is filtered through the membrane.