Unplanned indirect potable reuse is the unintentional addition of wastewater (treated or not) to a water supply that is subsequently used (usually by downstream communities) as a water source, with additional treatment prior to delivery. As noted earlier, many communities already unintentionally practice such unplanned indirect potable reuse.

Direct potable water reuse is the immediate addition of reclaimed wastewater to the water distribution system. This practice has not been adopted by, or approved for, any water system in the United States.

With planned or unplanned indirect potable reuse, the storage provided between treatment and consumption allows time for mixing, dilution, and natural physical, chemical, and biological processes to purify the water. In contrast, with direct potable reuse, the water is reused with no intervening environmental buffer.

With planned indirect potable reuse and direct potable reuse, the wastewater is treated to a much higher degree than it would be were it being discharged directly to a surface water without specific plans for reuse. The wastewater generally is first treated as it would be in a conventional municipal wastewater treatment plant, then subjected to various advanced treatment processes.

Conventional wastewater treatment begins with preliminary screening and grit removal to separate sands, solids, and rags that would settle in channels and interfere with treatment processes (Henry and Heinke, 1989). Primary treatment follows this preliminary screening and usually involves gravity sedimentation. Primary treatment removes slightly more than one-half of the suspended solids and about one-third of the biochemical oxygen demand (BOD) from decomposable organic matter, as well as some nutrients, pathogenic organisms, trace elements, and potentially toxic organic compounds.

Secondary treatment usually involves a biological process. Microorganisms in suspension (in the "activated sludge" process), attached to media (in a "trickling filter" or one of its variations), or in ponds or other processes are used to remove biodegradable organic material. Part of the organic material is oxidized by the microorganisms to produce carbon dioxide and other end products, and the remaining organic material provides the energy and materials needed to support the microorganism community. Secondary treatment processes can remove up to 95 percent of the BOD and suspended solids entering the process, as well as significant amounts of heavy metals and certain organic compounds (Water Pollution Control Federation, 1989). Conventional wastewater treatment usually ends with secondary treatment, except in special cases where tertiary treatment is needed to provide additional removal of contaminants such as microbial pathogens, particulates, or nutrients.

Advanced treatment processes beyond tertiary treatment are neces-

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