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.