Cleaning Water to Drink Again: The Science of Potable Reuse

About 97 percent of Earth's water is salty ocean, and less than 1 percent is fresh water that people can easily reach. As cities grow and droughts stretch longer, some communities have turned to a supply they already control: their own used water. Potable reuse is the practice of cleaning treated wastewater until it is clean enough to drink. The science behind reuse rests on a straightforward principle: the safety of drinking water depends on how thoroughly it has been treated and tested, rather than on where the water was before.

Water leaving an ordinary wastewater plant is clean enough to return to a river, yet nowhere near clean enough to drink. Potable reuse adds several advanced steps, called barriers, and each barrier removes something different. The first barrier is usually microfiltration, which pushes the water through membranes with pores far too small for bacteria and suspended particles to slip through. The second barrier is reverse osmosis. Here the water is forced under high pressure against an even tighter membrane that strains out dissolved salts, many dissolved chemicals, and viruses. The third barrier combines ultraviolet light with an added oxidizer, a step called advanced oxidation, which breaks apart trace compounds that survive the earlier stages. Layering several barriers means that if one stage misses something, the next is likely to catch it. By the end, the water has passed through more treatment than most ordinary tap water ever receives.

Engineers describe two ways to return this purified water to people. In indirect potable reuse, the water is first sent into an environmental buffer, such as a groundwater aquifer or a surface reservoir. There it blends with other water and sits for a time before being pumped out and treated once more as a normal drinking-water source. In direct potable reuse, the purified water goes into the drinking-water system without that natural pause. The buffer in the indirect method gives operators extra time to test the water and respond if a reading looks wrong, so most large projects built so far use the indirect approach.

The largest example operates in Southern California. Orange County's Groundwater Replenishment System is the world's largest indirect potable reuse facility, producing about 130 MGD of purified water. That water is injected into the ground, where it refills an aquifer that supplies much of north and central Orange County. The injected water does a second job as well: it forms a barrier that helps hold back seawater, which would otherwise seep into the freshwater wells near the coast.

A community facing a long drought has few easy options. It can build a new reservoir, but many rivers are already stretched thin. It can desalinate seawater, yet pushing salty ocean water through reverse osmosis is energy-hungry, taking roughly 11 to 15 kWh of electricity for every 1,000 gallons, compared with about 1 to 2 kWh for conventional treatment and delivery. Reuse begins with water that is far less salty than the ocean, so purifying it usually takes less energy than seawater desalination. Just as important, a community's used water keeps flowing even during a drought, so reuse offers a supply that does not dry up when rain does not fall.

Despite the heavy treatment, some residents are uneasy about drinking water that began as sewage, a reaction often labeled the "toilet to tap" worry. Utilities answer this in two ways. They open their plants to public tours so people can see the barriers at work, and they publish the results of continuous testing. Their argument is that thorough treatment and testing, rather than a water's origin, determine whether it is safe to drink. Potable reuse also eases two environmental pressures at the same time. It returns less treated wastewater to oceans and rivers, and it withdraws less water from stressed rivers and aquifers, because the community keeps reusing the water it already has rather than always taking more.

1. According to the passage, what is potable reuse?
2. Name the three advanced-treatment barriers the passage describes, in order.
3. What is the difference between indirect and direct potable reuse as the passage defines them?
4. Why might a community facing a long drought choose potable reuse instead of building a new reservoir or a seawater desalination plant? Use details from the passage.
5. The passage says reuse water passes through more treatment than most ordinary tap water. Explain why supporters argue this makes it safe, and why some residents are still uneasy.
6. Explain how potable reuse eases two different pressures on the environment, according to the passage.