Using effective membrane cleaning schemes to maintain designed permeability is one of the key factors that determine the long-term productivity of a microfiltration (MF)/ultrafiltration (UF) membrane plant. This paper presents the results from a membrane performance testing program conducted by the Minneapolis Water Works for the selection of membrane supplier for its new 70-mgd UF membrane plant. During this six-month testing program, two membrane manufactures utilized different chemical cleaning concepts to maintain membrane performance. One manufacturer used the conventional cleaning-in-place (CIP) approach, in which the membranes were chemically cleaned when the operating transmembrane pressure reached the maximum allowable level, while the other manufacturer adopted a new "chemical enhanced backwash" (CEB) cleaning regime that applied frequent, low-dosed, periodical chemical cleaning. Although membrane fouling is a very complicated process and can be affected by many operating conditions, the testing results indicated that the CEB procedure with a 16- hour low-dose cleaning cycle could effectively control the fouling and the performance was much more stable than the other system that used the conventional CIP approach. This result is consistant with findings from other research that demonstrated that interrupting fouling accumulation at an early stage could prevent the formation of an aggregated massive fouling layer that could be very difficult to remove even by chemical cleaning. A membrane autopsy study was conducted at the end of the testing program. Various surface analysis techniques were used to identify the composition of the fouling material. The microscopy images suggested that most of the majority of the fouling material was powdered activated carbon, which was used in the pretreatment processes for taste and odor control. Includes tables, figures.