The removal of microorganisms by membrane systems was investigated using single element membranes and five species of microorganisms in a plant setting at East St. Louis, Missouri. Single element membranes included a cellulose acetate ultrafilter (UF), a polysulfone microfilter (MF), a cellulose acetate (CA) nanofilter (NF), and two composite thin-film (CTF) nanofilters. Microorganism challenge studies were conducted using raw, alum coagulated-settled, and finished plant water. Model microorganisms consisted of MS2 (~0.025 um) for virus rejection, PRD1 (~0.1 um) phage and Clostridium perfringens (strain 26) spores (~1-5 um) for bacteria simulation, and Cryptosporidium parvum oocysts (~4-6 um) and Giardia lamblia cysts (~8-14 um) for cyst rejection. Sixty-eight observations of microorganism rejection were gathered over one year of operation in eight separate challenge events where microorganisms were spiked separately and as a mixture. The University of Central Florida in cooperation with the American Water Works Service Company, Inc., conducted the project at the Illinois-American East St. Louis surface water treatment facility. The water treatment plant intake is located on the Mississippi River several miles downstream of the confluence of the Mississippi and Missouri Rivers, a source that has varied water quality throughout the year, but is generally regarded as a high turbidity, moderate organic source water. Includes 25 references, table, figures.