Polyelectrolytes are used in water treatment systems for a variety of applications including coagulation, flocculation, ballasted sedimentation, filtration, dissolved air flotation, and sludge conditioning. There are multiple advantages to using polyelectrolytes including more efficient removal of particles, organics, and color; reduced sludge formation; sludge conditioning; and reduced reliance on metal salts. This project was developed to determine the degree to which organic polyelectrolytes react with other chemicals used in water treatment applications. The objectives of this project were to: evaluate current patterns of chemical use for drinking water production; conduct bench-scale testing to evaluate polyelectrolyte interactions and the impacts of these interactions on process performance, water quality, and operations; assess the degree to which exposure to ultraviolet light impacts polyelectrolyte reactions with oxidants; and assess the effect of precoating filter media with various polyelectrolytes on filtration efficiency and filter backwashing effectiveness. The research approach included an Internet survey, development of direct and indirect characterization methods to assess polyelectrolyte activity, bench-scale testing of polyelectrolyte interactions with other chemicals, and pilot-scale testing. Screening tools included organic carbon oxidation, absorption spectra, and reactions with oxidants. Performance measures for polymer effectiveness included tests of particle size, filtration rate, and residual turbidity. Disinfection by-product formation potentials were also used to compare the relative degree of oxidation that occurs for individual polyelectrolytes. Pilot-scale tests were conducted to evaluate the effects of precoating filter media with polyelectrolytes on filter run times and filter ripening. A fluorescently labeled version of one of the polyelectrolytes was used to determine the fate of the polyelectrolytes through water treatment systems. Originally published by AwwaRF for its subscribers in 2004.