Indirect Potable Reuse
For more than 50 years, California has been a pioneer in water recycling. Advances in technology and new philosophies about preventing the waste of water have combined to make water recycling an increasingly important part of water resources planning. The next challenge is to expand the existing uses of recycled water to encompass potable reuse (drinking, cooking and bathing). Direct potable reuse - where the product water is released into a municipal distribution system immediately after treatment - is practiced only in Windhoeck, Namibia, at this time and is probably far in the future in the United States. However, indirect potable reuse is practiced more widely and becoming more accepted.
What It IsWith indirect potable reuse, a highly-treated recycled water is returned to the natural environment (ground water reservoir, storage reservoir or stream) and mixes with other waters for an extended period of time. Then, the blended water is diverted to a water treatment plant for sedimentation, filtration and disinfection before it is distributed. The mixing and travel time through the natural environment provide several benefits:
- sufficient time to assure that the treatment system has performed as designed with no failures
- opportunity for additional treatment through natural processes such as sunlight and filtration through soil
- increased public confidence that the water source is safe.
Unplanned indirect potable reuse is occurring in virtually every major river system in the United States today.
The Technology UsedMembrane treatment is the most advanced technology for removal of the tiniest particles - including small ions such as sodium and chloride - from the recycled water. The most common membrane process employed is reverse osmosis (RO). Under relatively high pressure, water is forced across the semi-permeable RO membranes in special vessels to produce nearly pure water. Impurities are collected in a separate brine stream for disposal.
The Denver Water Board, with assistance from the U.S. Environmental Protection Agency, conducted an intensive study of potable reuse, using a 1-million-gallon-per-day pilot plant for five years. Several combinations of treatment processes were tested, and potable water was produced and analyzed for nearly all known contaminants. In addition, feeding studies were performed on rats and mice. Over several generations, rats and mice were given recycled water concentrates, while similar control groups were given water concentrates from the snowmelt from the highest peaks of the Rocky Mountains. No significant health differences were found between the two groups.
For more than 20 years, the Upper Occoquan Sewage Authority (UOSA) Regional Water Reclamation Plant has been discharging to the Occoquan Reservoir, a principal water supply source for approximately 1 million people in northern Virginia. Because of the plant's reliable, state-of-the-art performance and the high-quality water produced, regulatory authorities have endorsed UOSA plant expansion over the years to increase the safe yield of the reservoir. UOSA recycled water now is an integral part of the water supply plans for the Washington metropolitan area. Other major projects with proven track records are in Los Angeles County and Orange County, Calif., and in El Paso, Texas. After decades of research, pilot studies and demonstration, the City of San Diego is designing an indirect potable reuse project.