From Wastewater to Drinking Water
Across the globe, 2 out of 10 people do not have access to safe drinking water, and in the U.S., many states face water shortages and droughts. Meanwhile, reports Robert Glennon in Unquenchable: America’s Water Crisis and What to Do About It, Americans use 24 gallons of water each day to flush their toilets—approximately 5.8 billion gallons. What a waste! As the global population continues to grow and climate change results in more water crises, where will we find enough water to meet our needs?
In the U.S., we spend billions of dollars treating water to drinking water quality when we use only 10% of it for drinking and cooking, then flush most of the rest down the toilet or drain. So the growing use of recycled wastewater for irrigation, landscaping, industry and toilet flushing, is a good way to conserve our fresh water resources. Recycled water is also used to replenish sensitive ecosystems where wildlife, fish and plants are left vulnerable when water is diverted for urban or rural needs. In coastal areas, recycled water helps recharge groundwater aquifers to prevent the intrusion of saltwater, which occurs when groundwater has been over pumped.
The use of recycled water for drinking, however, is less common, largely because many people are repelled by the thought of water that’s been in our toilets going to our taps. But a few countries like Singapore, Australia and Namibia, and states such as California, Virginia and New Mexico are already drinking recycled water, demonstrating that purified wastewater can be safe and clean, and help ease water shortages.
The term “toilet to tap,” used to drum up opposition to drinking recycled water, is misleading because recycled water that ends up in drinking water undergoes extensive and thorough purification. In addition, it is usually added to groundwater or surface water for further cleansing before being sent to a drinking water supply where it is again treated. In fact, it has been shown to have fewer contaminants than existing treated water supplies.
There are a number of technologies used to recycle water, depending on how pure it needs to be and what it will be used for. Here’s how it’s done at the Point Loma Wastewater Treatment plant in San Diego—the city is currently studying the feasibility of recycling water for drinking.
Sewage first goes through advanced primary treatment in which water is separated from large particles, then enters sedimentation tanks where chemicals are used to make primary sludge settle to the bottom and scum rise to the top. Once the water is separated out, 80% of the solids have been removed, and the wastewater is clean enough to be discharged to the ocean. (Though wastewater is a potentially valuable resource, most wastewater produced along our coasts ends up in the ocean.)
In secondary treatment, bacteria are added to the wastewater to ingest organic solids, producing secondary sludge that settles to the bottom.
Tertiary treatment filters the water to remove whatever solids remain, disinfects it with chlorine, and removes the salt. In California, tertiary-treated water is called “recycled water” and can be used for irrigation or industry.
For Indirect Potable Reuse (IPR)—recycled water that eventually becomes drinking water—tertiary-treated water undergoes advanced water technology, then spends time in groundwater or surface water, such as a reservoir, before being sent to drinking water supplies. Advanced water technology first involves microfiltration that strains out any remaining solids.
Next, reverse osmosis, which applies pressure to water on one side of a membrane allowing pure water to pass through, eliminates viruses, bacteria, protozoa, and pharmaceuticals. The water is then disinfected by ultra violet light (UV) or ozone and hydrogen peroxide. Finally it is added to groundwater or surface water reservoirs where it stays for an average of 6 months to be further purified by natural processes. (This is done mainly to assuage public anxiety about drinking recycled water.) Once drawn from the groundwater or reservoir, the recycled water goes through the standard water purification process all drinking water undergoes to meet U.S. Environmental Protection Agency standards.
In fact San Diego is already drinking recycled water because it imports 85% of its water from Northern California and the Colorado River, into which upstream communities like Las Vegas discharge wastewater that is later treated for drinking purposes. Because of recent restrictions on Northern California water and drought on the Colorado River, San Diego, which recycles sewage water for irrigation, invested $11.8 million into an IPR study. The demo project at the North City Water Reclamation Plant will end in 2013. During this time, its Advanced Water Purification Facility is producing 1 million gallons of purified water each day, though no water is being sent to the reservoir.
IPR is more economical for San Diego than recycling more sewage for irrigation would be because recycled irrigation water must be conveyed through special purple pipes to separate it from potable water; expanding the purple pipe infrastructure would cost more than IPR. Recycled water is also less expensive than desalinating seawater. In Orange County, for example, IPR costs $800-$850 to produce enough recycled water for 2 families of 4 for a year. Desalinating an equal amount of seawater would require $1,200-$1,800 because of the amount of energy needed.
To deal with its growing population and salt intrusion into the groundwater, the Orange County Water District in California opened its $480 million state-of-the-art water reclamation facility, the largest in the U.S., in January 2008. It costs $29 million a year to operate. After advanced water treatment, half the recycled water is injected into the aquifer to create a barrier against saltwater intrusion. The other half goes to a percolation pond for further filtration by the soils, and then after about 6 months, ends up in drinking water well intakes. By this year, it’s expected to produce 85 million gallons a day.
Singapore, with no natural aquifers and a small landmass, has struggled to provide a sustainable water supply for its residents for decades.
In 2003, it opened the first plants to produce NEWater, recycled drinking water purified by advanced membrane techniques including microfiltration, reverse osmosis and UV disinfection. After treatment, the water is added to the reservoirs. NEWater, which has passed more than 65,000 scientific tests and surpasses World Health Organization drinking water standards, is clean enough to be used for the electronics industry and to be bottled as drinking water. It is expected to produce 2.5% of Singapore’s total daily consumption this year.
Namibia, the most arid country in southern Africa, has been drinking recycled water since 1969. The water reclamation plants produce 35% of the water for Windhoek, the capital city. To date, there have been no negative health impacts connected with the consumption of recycled water.
In 2001, a $55 million water recycling project for water-stressed Los Angeles was scuttled by the public’s revulsion at the thought of drinking recycled water and the term “toilet to tap” was born. Are the public’s fears grounded?
A recent science advisory panel report examined the potential human health implications of “chemicals of emerging concern” (CECs) such as pharmaceuticals, pesticides, and industrial chemicals, in recycled water. The scientists reviewed epidemiological and other studies of recycled water from the last 40 years. While some early studies reported the presence of chlorine disinfection byproducts, the panel noted that treatment methods at that time were less sophisticated. Current methods have been refined and disinfection byproducts have decreased. More recent studies of recycled water found no adverse health effects in populations using recycled water. Though the scientists acknowledged that the effects of long-term exposure (over generations) to CECs and to substances that have not yet been detected are unknown, they concluded that there was “robust evidence that recycled water represents a source of safe drinking water.”
Hopefully public opinion is starting to turn. Dr. Shane Snyder, Professor of Environmental Engineering at the University of Arizona and a member of the science advisory panel, is now studying public perception of recycled water and is finding that “if they trust the utility, the majority of people understand that recycling water is unavoidable.”
The truth is that all water is being recycled over and over—no water on earth is truly pristine. Snyder concludes, “We’re going to drink recycled water one way or another, whether it comes from downstream flow or groundwater. I strongly believe we should to do it through engineered systems where we can actively control the process.”