From Wastewater to Groundwater: Orange County's Recycling System

Every day, wastewater treatment plants in Orange County, California, produce millions of gallons of used water. That water has already been through a full cycle of sewage treatment. Decades ago, it would have simply been released into the Pacific Ocean. Southern California has never had enough local rain to support its population on its own. For most of the last hundred years, the region imported water from Northern California and the Colorado River. That imported water has grown less reliable during repeated droughts. So Orange County started looking closer to home for a new source. Today, much of the treated wastewater that once went to the ocean travels instead to the Groundwater Replenishment System, known as GWRS. The Orange County Water District built this facility. It purifies wastewater far beyond ordinary drinking water standards, then returns that water to the region's supply.

Water moves through three treatment steps at GWRS. The first step is microfiltration. Water passes through membranes with pores far smaller than a human hair. This step filters out solids, bacteria, and other tiny organisms. The second step is reverse osmosis, the same process used in some ocean desalination plants. Pressure forces water through a membrane so fine that it blocks dissolved salts, viruses, and most drug residues. Only water molecules pass through in large amounts. The third step is advanced oxidation. Ultraviolet light combines with hydrogen peroxide to break down any trace chemicals that survived the first two steps. By the time water leaves GWRS, it has been tested and purified beyond ordinary drinking water rules. Workers check water quality at several points along the way, not just once at the end.

Even so, GWRS does not send its purified water straight to anyone's tap. Instead, workers inject the water into the Orange County Groundwater Basin, an underground aquifer that already supplies drinking water to much of the county. Some of the water also helps form a barrier that keeps seawater from seeping into the basin along the coast. Once underground, the purified water blends with the basin's existing groundwater. It moves slowly through soil and rock for months or years. Eventually it gets pumped back out through ordinary water wells. Engineers call this method indirect potable reuse. The water reaches a natural buffer, the aquifer, before it becomes drinking water again, rather than moving straight from the treatment plant to the tap. That underground trip adds a layer of natural filtering. It also gives regulators and the public more confidence in the water's safety.

GWRS is not a small pilot project. The Orange County Water District runs it at a capacity of about 130 million gallons per day. That makes it the world's largest indirect potable reuse facility of its kind. It belongs in the same conversation as the nation's largest ocean desalination plant, in Carlsbad, California, which produces about 50 million gallons of drinking water per day from seawater. Both facilities exist for the same reason: Southern California's population and water demand have grown faster than imported river water and local rainfall can reliably meet, especially during drought years.

Recycling wastewater and desalinating seawater solve a similar problem in different ways, and each comes with its own cost. Turning seawater into fresh water through reverse osmosis typically uses roughly 11 to 15 kilowatt-hours (kWh) of energy for every 1,000 gallons produced. Ocean water is much saltier than treated wastewater, so it needs more pressure to force through the membrane. Recycling already-treated wastewater, as GWRS does, typically uses considerably less energy for a similar process, because treated wastewater holds far less salt. That energy gap is one reason many water districts in dry regions have expanded wastewater recycling even as they also weigh desalination, since recycling can often deliver more water for the same amount of electricity. Many districts do not treat recycling and desalination as rival choices. Instead, they plan for both, along with conservation, as parts of one strategy for keeping a reliable water supply during years when imported water and rainfall are not enough.

1. List the three treatment steps water passes through at GWRS, in order, and describe in one sentence what each step removes or does.
2. According to the passage, does purified water from GWRS travel directly to a household tap? Describe the route the water actually takes before it reaches a drinking water well.
3. Why do you think engineers and regulators send the purified water through a groundwater basin instead of piping it directly into the drinking water system? Use details from the passage to support your answer.
4. What is GWRS's production capacity, and how does the passage describe its scale compared to other facilities of its kind?
5. Compare GWRS (recycling treated wastewater) and the Carlsbad plant (desalinating seawater) as two approaches to expanding a region's water supply. Name one similarity and one difference between them, based on the passage.
6. Using the energy details in the passage, explain one reason a water district might prefer expanding wastewater recycling over building a new desalination plant.
7. Using context from the passage, explain in your own words what "indirect potable reuse" means.