The concept of microbial barriers in water supply and wastewater treatment involves a variety of processes other than disinfection. Many of these treatment processes have evolved from the enhancement of some factors associated with natural self-purification (sedimentation, aeration, nutrient limitations, water pH, etc.). Utilizing the multiple barrier concept in treatment of wastes before discharge to receiving waters and the further refinement possible by natural self-purification processes in the stream are beneficial in optimizing the quality of this raw water resource prior to further processing into drinking water supplies. A flow diagram based on typical percent removals in various treatment processes illustrates how water contaminated with fecal wastes can be improved in quality to protect downstream users from microbial hazards.