Why are U.S. cities still irrigating with drinking water?
After the Millennium drought, Australian cities found other ways to irrigate parks, gardens, and landscapes, and so can we.
One of the most striking observations in urban ecology and hydrology is the sheer volume of water in, under, and downstream of U.S. cities, even in arid areas. The average American household uses 300 gallons (1136 liters) of water per day, and the lower the local rainfall, the higher the household water demand. So most cities import water from outside the city limits in extensive networks of pipes that inevitably leak. Many cities are sitting over a reservoir of water from leaking water and sewer pipes, as well as the excess water from over-irrigation of landscapes. Add to this huge volumes of stormwater runoff after rain events that can't soak into the ground because of sidewalks, asphalt, and other impervious surfaces, and you have enough water to quite literally flood cities on a regular basis. Downstream of cities, sewage treatment plants discharge effluent into streams, rivers, or directly into the ocean where it can't be recovered for reuse.
Given all this extra water, it's pretty inefficient to spend energy and resources transporting and treating imported water to drinking standards, and then, well, not actually drinking it. In this paper, researchers estimated that about 12% of primary energy consumption in the U.S. was used for water services in 2010. On top of that, the remote water sources for many cities are increasingly threatened by drought. Instead of transferring water from vulnerable ecosystems over hundreds of miles, it's only logical to make use of water that's already available in cities but is currently treated as waste.
In the 2000's, Australia experienced years of below average rainfall during the "Millennium drought." In response, cities implemented short-term watering restrictions, but they also accelerated longer-term efforts to develop alternative water sources for both indoor and outdoor uses. I've been looking at some of these efforts in and around Melbourne, and there's much to be learned here from taking advantage of non-potable sources to supply households with indoor and irrigation water.
My temporary home at the moment is Richmond, Victoria, an inner suburb of Melbourne just east of the CBD. All around my neighborhood are signs like the one below:
Rainwater capture from rooftops is common all over the city, where it's collected in tanks and used for landscape irrigation and/or indoor appliances like hot water heaters and toilets. As of 2013, more than 1/3 of Australian households with the potential to capture rooftop water had a rainwater tank, and the numbers are increasing. Substituting urban rainwater for potable drinking water has a number of benefits: it reduces the need to import water from remote sources, lowers energy consumption for treating and transporting drinking water, and reduces flooding and urban stream degradation from excess stormwater runoff.
Stormwater harvesting and passive irrigation
Once rainfall lands on the surface it becomes stormwater, which is diverted away from streets, sidewalks, and parking lots through storm drains, gutters, and pipes. Because cities are mostly covered in impervious roofs, concrete, and asphalt instead of water absorbing soil, they have a very "flashy" hydrology, meaning that they're much quicker to flood than rural areas. This makes stormwater a major nuisance for all modern cities. Back in Utah, I live in a desert city that's normally very dry in the summer, but an occasional heavy summer rainstorm can easily flood the streets.
To minimize flooding, cities built extensive infrastructure during the 20th century to get stormwater out of urban areas as quickly as possible. But given the droughts and water shortages of the 21st century, we're now recognizing that this water is a valuable resource that can be used as a substitute for potable water. Stormwater harvesting is increasingly popular in and around Melbourne, where it's captured and used in different ways, including watering street tree plantings and irrigating public gardens. There are even efforts underway to include stormwater in the drinking water supply, which could dramatically reduce the need to import water from drought-stricken natural watersheds.
The household water we send down the drain from sinks, showers, and even toilets is a significant possible resource for irrigation. This water does need to be treated to be applied to the landscape, but it doesn't need to be fully potable.
Water recycling is particularly successful in Israel, where I'm headed later this year, but there are interesting recycling projects in Australia as well. I had a chance to visit a real estate development underway as a public-private partnership between South East Water and Villawood Properties called Aquarevo, a 450 unit development located on land owned by South East Water. It includes a suite of cutting edge, and sometimes experimental, approaches for energy and water efficiency.
In addition to rainwater capture and smart meters, the development includes an advanced water recycling system with a small, dedicated water treatment facility that sends treated sewage effluent back to households for indoor and outdoor use through purple pipes. This could be the future of water recycling: it's difficult and costly to retrofit cities with recycled sewage pipes from large, centralized sewage treatment plants. But small, decentralized, energy efficient wastewater systems make recycled water a feasible, and abundant, water source.
South East Water has a stated goal of reducing household water consumption by 70% through its combination of rainwater capture, water recycling, and smart irrigation systems. Given all the imported and wastewater flowing through and under cities, this seems like a feasible goal, and one that U.S. cities could take on themselves.