Just (re-)add water: How circular systems can enhance urban water cycles
Communities are increasingly turning to water reuse as a tool to bolster water supply reliability in the face of numerous uncertainties. Droughts result in curtailments of surface water allocations that serve as drinking water lifelines to many cities. Just look at this year’s "day zero" warnings from Cape Town, or when California’s governor made a 2015 executive order requiring a 25 percent water use reduction by cities and towns.
Communities that rely on groundwater may find their aquifers in a state of overdraft without adequate management or replenishment. In a world filled with uncertainty, many agencies are looking at water in every part of the urban water cycle as a resource that can be put to better use before discarding it.
It’s all One Water
The urban water cycle starts with raw water, moves through drinking water treatment then delivery to homes and businesses. Once used, sewers convey it to wastewater treatment plants, where it is treated to public health standards for discharge to rivers, land application, the ocean, or treated further for recycled water. Some water is purified through advanced treatment processes and used to augment water supplies.
Purified water injected into aquifers to augment water supply also can halt groundwater subsidence or seawater intrusion, providing multiple benefits. Wastewater treatment facilities can produce energy via co-generation to supply some energy needs and offset their carbon footprint. Communities may consider alternative water supplies, such as seawater desalination, stormwater capture, or water reuse to increase water supply reliability and resiliency to prepare for the future.
What is water reuse?
Water reuse involves additional treatment of wastewater for other uses. Non-potable reuse, also known as recycled or reclaimed water, is suitable for uses such as landscape or agricultural irrigation, industrial cooling or toilet flushing. Potable reuse involves advanced treatment processes to produce purified water for addition to groundwater aquifers or reservoirs to supplement a community’s water supply.
Drivers for water reuse in the urban setting
Communities have been employing non-potable reuse for decades, mostly for landscape irrigation. One limitation for non-potable reuse is that the demand is seasonal, with increased need during the hot, dry summer months. While offsetting potable water use during the summer, this seasonally dependent demand pattern does not maximize the amount of water that could be reused for other purposes throughout the year.
Additionally, most non-potable water systems must be built with a separate, purple pipe distribution system. Purple piping are lines designated for recycled water, which is treated to a level suitable for irrigation and industrial use, but not for drinking. This results in a binary pipe network with high capital costs and operations and maintenance requirements separate from the potable water system. Long pipelines are expensive, and cross-connection prevention requirements add complexity to non-potable reuse systems. These limitations and challenges have led to new trends and innovations for water reuse in an urban setting.
Because of limitations and challenges associated with non-potable reuse, many communities are considering potable reuse as a way to maximize a precious resource before they discharge it to a river or the ocean. Advanced treatment processes such as reverse osmosis and advanced oxidation processes have been demonstrated in California for almost two decades to provide purified water for injection to groundwater, as a tool to stem the tide of seawater intrusion and to augment drinking water supplies.
The project would help increase the sustainability of the district’s groundwater supply, upon which it currently relies for 100 percent of its water supply, reduce the degree of overdraft conditions in the district’s groundwater basin, protect against and aid in preventing further seawater intrusion of the groundwater basin, and promote beneficial reuse by reducing discharge of treated wastewater to the Monterey Bay National Marine Sanctuary. Brown and Caldwell is working with the district as program manager as the project moves through technical feasibility and environmental review.
In parched Southern California, where most of their water supply is imported, potable reuse allows communities to strengthen water supply reliability and lessen dependence on imported supplies. Some states are developing regulations to allow purified water to be introduced immediately upstream of a drinking water treatment plant, otherwise known as direct potable reuse (DPR). As advances in treatment technologies continue, there is greater possibility for incorporating potable reuse as a viable water supply alternative.
San Francisco leads the way in decentralized, non-potable water reuse systems in the U.S. The headquarters of the San Francisco Public Utilities Commission uses the Living Machine to treat all of its wastewater onsite, producing recycled water to supply water for 100 percent of the buildings’ urinals and toilets. This onsite system recycles about 5,000 gallons per weekday and reduces total water use by about 65 percent. Decentralized systems are becoming more popular as an option to keep resources local, earn LEED credits for new buildings and developments, and control water reliability at a small scale.
The California Urban Water Agencies (CUWA) is developing a white paper on Decision-Making for Water Reuse Using a Distributed Systems Approach. A distributed systems approach considers regionally optimized supplies, from decentralized to centralized reuse.
Decisions could hinge on greenhouse gas production, the regulatory environment, and operational requirements and responsibility. This is an area where both possibilities and the universe of considerations associated with decentralized systems will continue to be explored.
Utilities, trade organizations and equipment providers are pushing to innovate urban reuse. Communities may seek expanded applications for water reuse and push for regulatory approval of additional uses, as is the case in Colorado to add more official non-potable uses such as urinal and toilet flushing. Much current research is focused on optimizing treatment processes used in potable reuse applications, evaluating performance of alternative treatment processes, and anticipating needs associated with direct potable reuse. A One Water perspective often spurs innovation by challenging planners and engineers to look beyond traditional institutional silos of the past and recognize value in water, no matter where it resides in the urban water cycle.
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