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California’s Water Resources Control Board is scheduled to vote tomorrow on whether to adopt permanent restrictions against wasting water, as drought worries once again creep into focus.

is an assistant professor of civil and environmental engineering at ���ϲ�����’s College of Engineering and Computer Science. Zeng, who researches public health implications of water reuse, answers three questions about the ways in California and Cape Town – a city facing a massive drought that has led to threats of water running out by this summer – can provide long-term solutions for the large population areas.

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The planned replenishment of groundwater basins with recycled water has been practiced in California for more than 50 years. How has it changed, or how does it need to be updated with the present drought concerns?

Zeng says:

“California is already ahead of the game, but gaining public acceptance of reuse projects is still key.

“The first project in the U.S. that employed direct injection of recycled water into a potable aquifer started in Orange County in 1976. In 2008, the Orange County Water District re-developed their Groundwater Replenishment System to expand the utility’s potable reuse capacity up to 70 million gallons per day. To address the increased water need, the CA State Water Board has set a mandate of increasing the use of recycled water by 200,000 acre-feet per year (AFY) by 2020 and an additional 300,000 AFY by 2030. In fact, groundwater replenishment for potable reuse has expanded to 8 approved projects, mostly in Southern California, with more than a dozen projects planned throughout the State.

“Furthermore, the California State Water Board has recently completed a comprehensive study to evaluate the feasibility of developing uniform water recycling criteria for direct potable reuse projects. That being said.”

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What role could wastewater recycling play in addressing the ongoing water crisis in Cape Town, South Africa?

Zeng says:

“A deliberate and phased approach is needed to ensure public health protection and continued consumer confidence in the public water supply. A few U.S. communities have turned to direct potable reuse (DPR) as an emergency drinking water source during a drought but discontinued DPR when the emergency ended.

“Cape Town may consider piloting such DPR projects for emergency water supplies but only after a careful evaluation of technical challenges and potential public health risks.

“A recent example is with Wichita Falls, TX during the 2010-2015 drought. For about 12 months between 2014 and 2015, wastewater effluents in Wichita Falls were further treated with microfiltration and reverse osmosis and diverted to the city’s drinking water treatment plant for treatment to meet the standards.

“The very first wastewater recycling treatment plant was actually commissioned in the City of Windhoek, Namibia in 1968, and the treatment capacity has been upgraded through the years to its current capacity of 5.5 million gallons per day. There was some initial public opposition to the Windhoek project, but over time, the opposition has faded, and no public opposition to the project has emerged in recent years. Cape Town would likely benefit from the experience of the Windhoek project if they decide to undertake large-scale potable reuse projects in future.”

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Explain the process of treating wastewater. What has to happen to make it drinkable again?

Zeng says:

“The most commonly implemented treatment process for wastewater recycling is MF-RO-UV/AOP or the so-called Full Advanced Treatment (FAT). To ensure adequate removal of chemical and microbiological pollutants, the effluents from municipal wastewater (sewage) treatment plants are purified by the multi-barrier advanced treatment trains.

“Existing advanced treatment trains for potable reuse typically consist of microfiltration (MF) and reverse osmosis (RO) as broad-screen physical removal processes, followed by a UV/hydrogen peroxide advanced oxidation process (AOP), a broad-screen chemical oxidation process to further destroy contaminants. Recycled water (or product water) from advanced treatment is typically routed to augment groundwater basins or surface water reservoirs that are used as drinking water supplies (called the “indirect” potable reuse; IPR).

“In recent years, there has been increasing interest in blending recycled water directly with the influents of drinking water treatment plants, or even into the distribution systems.”

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