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San Francisco’s decentralised approach to water recycling

  • San Francisco’s decentralised approach to water recycling

Water to flush toilets does not need to be drinking water. A key strategy for sustainable water resource management is matching water quality to its intended use, and that opens many possibilities when it comes to water recycling, which can be cleaned to different standards according to its intended use.

Centralized water reuse has long been used in different countries and is expanding as a result of water scarcity, including water for non-potable purposes such as landscaping or street cleaning by municipalities, crop irrigation, and indirect potable reuse. Orange County in California has been using the Groundwater Replenishing System for indirect potable water reuse since 2008, and other communities in Southern California are following in its footsteps with projects such as Pure Water Oceanside and Pure Water San Diego. The state is also in the process of regulating direct potable reuse.

An emerging trend discussed in a recent article in Yale Environment is the extreme decentralization of water recycling through distributed systems. It involves water reuse at the scale of a neighbourhood or a university campus, or at an even smaller scale of commercial or residential buildings, through on-site recycling plants. San Francisco is leading the way in implementing on-site water recycling systems in new buildings, where since 2015 new buildings of more than 100,000 square feet (about 9,300 square metres) must have on-site water reuse systems.

So far on-site recycling systems for grey water – from sinks, showers and laundry – and black water – from toilets and kitchen sinks – have been implemented in several buildings, and many more are being planned in the city. The headquarters of the San Francisco Public Utilities Commission has a blackwater recycling system, known as the Living Machine, that reduces the building's imported potable water by 40 per cent, using wetlands built into the sidewalks to treat wastewater that is then reused for toilet flushing. Recycling grey water can also save substantial amounts of water, and using recycled water for showers could further reduce water demand; the safety of this practice is currently being studied and is not allowed yet in San Francisco.

The recycling systems in San Francisco’s buildings still need outside water for potable uses, but in the future water may be reused on-site for potable uses as well. Having a circular system with both potable and non-potable circuits, a building could become completely self-sufficient or water neutral, but it is still a few years away according to experts. The technology exists to collect and treat all water used in a building to potable standards, but the U.S. regulations do not allow it yet. The building and operating costs of such systems are also a potential barrier, though obviously there would be savings in water and sewer costs. Energy use and its cost is also a consideration. While it takes energy to transport water over long distances, energy is also required to pump water through a building. Finally, experts cite the yuck factor as another barrier to the wider adoption of water reuse, whether it be through centralised or on-site systems.

Decentralised water and wastewater systems reduce the demand for potable water and the need for centralised sewerage and treatment, and seem to have a bright future in the toolbox to increase the reliability of our water supply in the face of climate challenges.

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