Catherine Trifiletti, multimedia intern for the U.S. National Science Foundation (NSF) contributed this article to Live Science's Expert Voices: Op-Ed & Insights.
Every single day, the average American family of four uses four hundred gallons of water, according to the U.S. Environmental Protection Agency (EPA). That's water that has to make its way back to municipal treatment plants, consuming money and energy along the way.
At the University of Miami, a residence hall apartment of four has now treated their water for two years, helping revolutionize the underlying water treatment technology to make it economically leaner and environmentally greener.
Re-using dorm wastewater
The idea was dreamed up by Jim Englehardt, professor of environmental engineering at the University of Miami, and brought to life with support from NSF. Englehardt wanted to create a closed-loop water re-use system to treat wastewater and recycle it back to be used — all in one place.
Using the Miami on-campus apartment as his guinea pig, Englehardt gave the net-zero water system of his dreams the old college try. And it's working.
So far, the water has been treated and recycled for laundry, dishwashing and showering. The students have used city water for drinking, although the dorm water has been independently verified as safe for consumption, and Englehardt drinks it himself.
Here's how the system works: Wastewater from the four bedroom, four bath dormitory apartment (with kitchen and laundry) goes first to a buried septic tank, where solids settle and decompose slowly. Liquids then flow to the buried membrane bioreactor (MBR), where the water is aerated to support aerobic microbiological decomposition of the organics. Purified wastewater is removed from the MBR by vacuum pump through a membrane filter (with pores about 1/50 the size of a typical bacterium) to a buried holding tank. In addition, three buried cisterns collect rainwater.
The purified wastewater, together with 15 percent rainwater, then enters a tank where aluminum electrodes are corroded by a tiny electric current, producing a gelatinous aluminum hydroxide coagulant in the water that traps impurities. Water containing that aluminum mineral coagulant passes through a "floc tank" to further attract impurities, and then passes to a second vacuum membrane filtration unit with even smaller pores (about 1/4 the size of a typical virus). Next, clean water is drawn by vacuum pump to a large tank, where it spends about two days circulating with hydrogen peroxide and traveling past ultraviolet lamps. The combination of those natural oxidizers produces an even stronger oxidant (hydroxyl radical), which converts any remaining carbon compounds and microbes to carbon dioxide, while the oxidants decompose to oxygen and water.
Because minerals are only partially removed, 15 percent of the treated potable water is disposed, to provide a sink for excess minerals that would otherwise accumulate. The finished mineral water is stored in a tank, with chlorine residual to prevent re-growth of microbes, before being pumped to the apartment for use. At the taps and showers, treated water passes through two activated carbon filters, as a polishing barrier.
Bypassing the waste of wastewater treatment
The problem with letting water go the traditional municipal treatment system route is that hot water energy goes down the drain — far more energy than is used to treat and transport the water, Englehardt said.
Specifically, household water heaters use electric or gas or another energy source to heat the hot water drawn at the tap. When the water goes down the drain to the sewage treatment plant, it carries that energy with it, to be disposed to the environment (ocean, river or aquifer). In a net-zero water system, that energy remains in the water, about 85 percent of which returns to the tap. Hence, for many uses the water needs no further heating, and when needed, re-heating of the warm water requires less energy.
And, roughly eighty percent of the energy used in municipal water management is for transporting water back and forth from central treatment plants; only twenty percent of the energy used goes into treating the water itself, according to a report from the Electric Power Research Institute (EPRI). And for all that, it's not even better water.
The water treated at the dorm "more closely [resembles] water purified naturally in the environment," than that treated at municipal plants, Englehardt said, and because hot-water energy is retained in the system, the process is highly carbon-negative (it saves more energy than it uses).
He also said that most wastewater that leaves treatment plants is already very close to meeting drinking water standards. Even raw sewage is typically more than 99.9 percent freshwater, according to the Florida Water Environment Association. Often, in south Florida, the water management system dumps the treated waste into the ocean, re-contaminating it instead of treating it further, continuing the cycle of waste.
Closing the water-cycle loop
A closed-loop water system seems like the obvious choice, but the project still faced profound challenges.
The first challenges were economic. Englehardt and his team's initial estimation of $100,000 for retrofitting the system in the test apartment came in far below the actual price. Reworking plans to get the price down to $500,000 and securing additional funds tacked on a year and a half to the completion date. [Everyday Tech From Space: Water Recyclers Make Pee Potable]
In addition to the economic and engineering challenges of an on-site, single-home system, Englehardt foresees issues with regulatory permitting as the technology is so new that it's not yet supported by necessary data.
The technology is ready for larger-scale implementation now, according to Englehardt. It is a very attractive and economical option particularly for drought-stricken and arid areas where water is a commodity.
The team estimates that the system would be most economical for a community of 100 to 100,000 homes. Moreover, the system destroys pharmaceuticals and other chemicals that pass through conventional treatment plants and can cause hormone disruption in fish and wildlife.
In Cloudcroft, N.M., for example, the first closed-loop system was implemented in 2011 out of sheer necessity. At 9,000 feet above sea-level, the small mountain community could not fully depend on a larger centralized water source. The small population of 1,000 residents didn't need much convincing to put the innovative water system into action after one dry summer where they were forced to truck 20,000 gallons of water to town — each day — in order to sustain demand in their peak tourist season.
Even in Miami, despite 40 to 60 inches of recorded rainfall each year, the issue of clean water and its availability remains. If implemented in south Florida, this system could stockpile a half-billion gallons of water from the Everglades for reuse in a sustainable loop that benefits both the natural environment and its human inhabitants.
The other challenge? The "yuck" factor. This system faces a stigma from people who don't respond well to the idea that they are drinking second-hand water. Researchers, like Englehardt hope that this can be diminished through education and community outreach. [Would You Drink Recycled Sewage? Why It Grosses Us Out ]
So, reimagining how we use and treat water could lead to the standardization of this practice, lowering costs and any inhibitions about embracing it. This can get people off the grid and on to a more efficient, environmentally, and economically friendly way to use this natural resource.
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