“Sewer mining” – efficient water recycling coming to a community near you
It sounds yucky at best, but mining sewage is growing in popularity, especially in Sydney, Australia, where a decade of drought forced some creative thinking about how to get, use and manage water.
In 2004, when reservoir levels around Sydney hit record lows during the Big Dry, Sydney Water, the municipal water provider, tightened water-use restrictions to stretch the city’s drinking water supplies. One of its customers, the Pennant Hills Golf Club, founded in 1923 and boasting a championship course, got anxious that the curbs on water use would cause its prized greens to turn to ugly browns.
The club decided to take an unusual step: it requested permission to tap into the sewer line that ran through the golf course, and then treat and use that wastewater to irrigate its 23 hectares (57 acres) of greens.
The Pennant Hills project was one of Sydney’s earliest. Today at least four sewer-mining schemes are operating in the New South Wales capital, with eight more in the works, including at Macquarie University and the Sydney Airport.
Just as the name implies, sewer mining involves tapping into a wastewater collection system, siphoning some of the sewage off to a treatment facility, and then reusing the reclaimed water onsite for landscape irrigation, toilet flushing or other uses not requiring water pure enough to drink.
Areas of Israel, California and other water-short parts of the world have practiced wastewater reuse for decades. What’s unique about sewer mining, however, is that it’s smaller in scale and decentralized. Instead of collecting a whole city’s wastewater and sending it to a large centralized treatment plant and then piping the reclaimed water great distances for re-use, with sewer mining the acquisition, treatment and re-use of the sewage typically all happens in the same location.
When well planned and designed, sewer mining can yield a variety of benefits. It can relieve overtaxed wastewater systems, trim water and wastewater infrastructure costs, reduce energy and chemical use, and save drinking water for activities that really need drinking-quality water. These water-savings, in turn, can help keep more water in rivers, lakes and streams – which is especially crucial during droughts and summer months, when river flows are low and water demands are high.
“Sydney Water initially couldn’t get its head around the concept (of sewer mining),” said Kurt Dahl, managing director of Permeate Partners, the consultancy that designed the Pennant Hills system.
It was a new concept to me, too. I toured the club’s water reclamation system during a trip to Sydney. The more I saw and listened, the more it made sense.
The sewer pipe running through the golf course carries wastewater from about one thousand homes to the coastal town of Manly, where it receives primary (very basic) treatment and then gets dumped into the sea. So the project was mining wastewater that not only would go unused but would add pollutants to the ocean. And as long as the golf club siphoned off flow during peak hours of toilet flushing and showering – the morning and evening – it wouldn’t interfere with the pressure and flow rate needed to get the remaining sewage to Manly.
The sewer-mining scheme has cut Pennant Hills’ potable water use by 92 percent, which earned the club an award from Sydney Water. Due to the club’s use of treated wastewater onsite, Sydney Water no longer needs to supply it with some 70 megaliters (18.5 million gallons) per year of drinking water.
In addition, the nitrogen in the sewage has virtually eliminated the need to fertilize the golf course: small amounts of nitrogen get added every time the greens are irrigated. The fertilizer savings are somewhat offset, though, by the need to add gypsum to the soil to counteract the extra sodium in the reclaimed water.
Overall, the system has proven to be a cost-effective way to drought-proof the links and reduce stresses on Sydney’s water supply. And the golfers, apparently, are pleased.
“Old-time club members say this is the best the golf course has looked in thirty years,” Dahl said.
What makes sewer mining feasible – and the reason it is likely to catch on and spread – are the advancements in membrane technologies over the last decade. At the heart of the Pennant Hills treatment process, as well as many other sewer-mining projects, is a membrane bioreactor (MBR). The raw sewage first goes through biological treatment by microorganisms. The resulting product is then drawn through a membrane: while the water makes it through, the membrane’s microscopic pores block the solids and pathogens. This dark “mixed liquor,” or activated sludge, then returns to the first chamber of the bioreactor process, while the water either moves on to its intended use or gets further treatment, typically ultraviolet or chlorine disinfection.
The membrane bioreactor process has been around for thirty years, but both the cost of the membranes as well as their energy requirements have decreased substantially. As a result, MBRs are now cost-competitive with conventional wastewater treatment, even while producing higher-quality water.
According to Dahl of Permeate Partners, MBRs now treat more than 3 billion liters (792 million gallons) of water a day, and installed capacity is growing rapidly.
Dockside Green, a 15-acre mixed residential-commercial site in Victoria, British Columbia, and one of the first planned communities to earn LEED Platinum certification, uses an MBR system to treat all of the community’s sewage. The reclaimed water is used to flush toilets, irrigate the landscape and add flow to a local creek. According to Chris Allen, regional manager with General Electric’s Water and Process Technologies division, water conservation measures — including the installation of dual-flush toilets, water-efficient fixtures, and gray water systems — have added further benefit, reducing indoor water use by 65 percent.
Solaire, a 293-unit apartment complex in the Battery Park section of New York City, has the first U.S. onsite water reclamation system built right inside a residential apartment building. According to Allen, it recycles about 25,000 gallons per day to the building’s cooling towers, toilets and landscapes.
One of the world’s largest sewer mining projects is at Cauley Creek, a high-end community in Fulton County, Georgia, northeast of Atlanta. As with Sydney’s Pennant Hills Golf Club, Cauley Creek residents became concerned that drought conditions would lead to restrictions on water withdrawals from the Chattahoochee River. Commissioned in April 2002, the community’s MBR system can now reclaim 5 million gallons of wastewater per day, which get used by area schools, churches, homes and a golf course. In addition to allowing more water to remain in the Chattahoochee, the system produces reclaimed water of high enough quality to release any excess back to the river. In keeping with the community’s rural character, the treatment plant sits quietly inside a classic red barn, complete with a weather vane.
As we more fully adapt to water’s limits, we will choose landscapes that don’t require irrigation at all and recycle more water to grow food rather than thirsty lawns. But creative ways to take the “waste” out of wastewater will be crucial to closing the gap between our growing water demands and Earth’s finite supply.
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