The objectives of this study were to provide information on the removal of physical, chemical and biological constituents in filter backwash water by microfiltration under "in-use" conditions. The specific aims for the project are to evaluate the reduction and/or removal of natural populations of total plate count bacteria, turbidity, particles, total organic carbon, total trihalomethanes and haloacetic acids. Additionally, a separate study was conducted to determine the ability of the microfiltration system to remove protozoan cysts, viruses, coliform and heterotrophic bacteria. Includes 11 references, tables, figures.