If you live in one of four major U.S. cities chances are you’re letting the benefits of a ubiquitous natural resource go right down the drain — when it could be used to cut down your water bill. Research by a team of Drexel University environmental engineers indicates that it rains enough in Philadelphia, New York, Seattle and Chicago that if homeowners had a way to collect and store even just the rain falling on their roofs, they could flush their toilets often without having to use a drop of municipal water.
Toilet flushing is the biggest use of water in households in the United States and the United Kingdom, accounting for nearly one-third of potable water use. But there is no reason that clean, treated, municipal water needs to be used to flush a toilet — rainwater could do the job just as well.
“People have been catching and using rain water for ages, but it’s only been in the last 20-30 years that we have realized that this is something that could be done systematically in certain urban areas to ease all different kinds of stresses on watersheds; potable water treatment and distribution systems; and urban drainage infrastructure,” said Franco Montalto, P.E., PhD, an associate professor in Drexel’s College of Engineering, and director of its Sustainable Water Resource Engineering Lab, who led the research effort. “The study looks at four of the largest metropolitan areas in the country to see if it rains enough to make implementation feasible and, if everyone did it, what effect it would have on domestic water demand and stormwater runoff generation in those cities.”
The process of collecting and using roof runoff, which researchers call rainwater harvesting, has been working its way into vogue among urban planners and water managers over the last couple decades and has been implemented widely in California in the wake of its water crisis. This study, which started as the graduate thesis of Drexel alumnus Nathan Rostad, was recently published in the journal Resources, Conservation and Recycling, and is one of the first to crunch the numbers and sort out just how feasible, and beneficial, it would be as a way of offsetting potable water use for non-potable purposes while at the same time reducing generation of undesirable urban stormwater runoff.
“When the natural landscape is replaced by a building, rain can no longer infiltrate into the ground,” Montalto said. “It runs off, is captured in drains, where it can cause downstream flooding, carry pollutants that settle out of the air into local water bodies or — in the case of a city like Philadelphia or New York — cause the sewer to overflow, which leads to a discharge of untreated wastewater into local streams and rivers. So capturing rainwater can help to reduce the demands on the water treatment system and ensure that it will still function properly even during heavy rainfall events.”
Taking into consideration the cities’ annual rainfall patterns, residential population and roof areas, the team calculated that, with enough water storage capacity — a little more than a standard 1,000-gallon home storage tank — a three person family in a home with the city’s average roof size would have enough water to cover over 80 percent of its flushes throughout the year simply by diverting their downspouts to collect stormwater.
This would reduce overall household potable water demand by approximately 25 percent, which could mean slashing the municipal water bill for an average-sized home by as much as one-fourth. But even without installing a storage tank capable of holding a year’s worth of flushing water, a scaled-back version would still help chip away at the water bill.
“In general, greater potable water savings are estimated in cities with either larger roof areas or lower population density. However, such savings would be accompanied by smaller reductions in runoff,” the authors write. “Philadelphia and Seattle are the two cities where percent water savings would be greatest if residential neighborhoods were all equipped with rainwater harvesting systems.”
From a stormwater management perspective, an average residence with a 1,000-gallon rainwater harvesting system could reduce runoff by over 40 percent, according to the study. Obviously this would vary by residence — with the size of the water storage container and the water demand for toilet flushing — but as a whole, cities could see a significant reduction in the amount of stormwater their infrastructure would have to handle during each storm.
Among the cities studied, Philadelphia would see the largest percentage of runoff reduction if rainwater-harvesting systems were installed in residences citywide. This is because the average roof size in Philadelphia is the smallest of the cities surveyed, so there is less runoff to manage from a single roof. The researchers found that larger percentages in runoff reduction from a rainwater harvesting system can be the result of either small roof sizes or high population densities. But managing stormwater is a concern for all urban areas.
“Think of it this way. Before the building was on the site, the rain was intercepted by vegetation canopies, and/or infiltrated into natural soils. Either way, the rain ended up replenishing soil moisture, allowing the plants to grow, and recharging the local groundwater aquifer,” Montalto said. “The more buildings that go up, the more we need to consider how to manage the water that would have landed in the drainage area they’re displacing.”
Dutch road construction company KWS Infra, subsidiary of VolkelWessels group, has started work on an initiative that would see waste plastics used to construct an alternative to conventional tarmac roads.
Called Plastic Road, the design features a hollow space which the firm said can be used for cables, pipes and rainwater.
VolkerWessels said it’s a “lightweight design, a fraction of the construction time, virtually maintenance free and three times the expected lifespan”.
As part of the project, recycled plastic will be made into prefabricated road parts that can be installed in one piece. The structure will be designed to handle temperatures as low as -40 degrees and as high as 80 degrees Celsius.
The idea is still in the early stages with the company looking for partners to collaborate with on the pilot, which will see the design built and tested in a laboratory to “make sure it’s safe in wet and slippery conditions”.
How can water be better managed to ensure enough supply for a growing global population? Our panel of water experts have their say.
Calculate the water available: We need a better accounting of our “water balance sheet”. In many places, we don’t have any idea how current and near-term future demand matches up with the available surface and groundwater supplies. The WRI’s Aqueduct tool has a water supply/demand indicator – called “baseline water stress” – that gives a good preliminary read on whether local water use is sustainable or not. Betsy Otto, global director – water programme, World Resources Institute, Washington DC, US, @wriaqueduct
Link global water use: Although the Swiss are quite efficient at using water within our country, we have a huge water footprint because of all the food and goods we import, often from very water stressed parts of the world. Globalisation means there is a global water economy at play. Government regulation or taxation could nudge behaviours onto a more sustainable path. Sean Furey, water and sanitation specialist, Skat, St Gallen, Switzerland, @thewatercyclist
Think across sectors: Currently, those who work on “water services” think almost exclusively in terms of access, and those who work on “water resources” think in terms of sectors and water usage. I think the water service people (myself included) need to think harder about where the water for increasing coverage is going to come from, and how we can best implement sanitation services that protect water resources. Sophie Trémolet, director, Trémolet Consulting, London, United Kingdom, @stremolet
Treat water resources better: For a long time we treated water as limitless, and the incentive structures in cities and rural areas pushed people towards unsustainable practices. Water distribution being highly subsidised by governments doesn’t help create awareness about its actual value. We must make measurable efforts to change water-use habits in a global scale. Carlos Hurtado Aguilar, manager – sustainable development of water resources, FEMSA Foundation, Monterrey, Mexico
Develop water monitoring and regulation: Governments can provide both regulatory sideboards – such as requirements for full cost recovery on water tariffs – and incentives – such as cost-share on water reuse and rainwater harvesting systems. For developing countries (and many developed countries) this may feel like a daunting task, but governments do this sort of thing for education, energy, and other sectors. It’s high time to do the same for water. Betsy Otto
Establish accountability mechanisms: To secure a safe water supply for the poorest people, service providers should get into trouble when they fail to provide the services the poorest need. There should be cross-subsidies between the rich and the poor but most importantly cross-subsidies that work in reverse should be eliminated. With the money saved, direct subsidies can be given to the poor. We should also encourage the poorest people to be more self-reliant (e.g. encourage rainwater harvesting practices) and to demand good quality services as customers. Sophie Trémolet
Construct better water points: I’ve been looking at water point data in various countries and the number of boreholes and wells that are reported dry or seasonal only is shocking. In places like Sierra Leone, Liberia and Tanzania, more than 15 to 20% of water points fail in the first year after construction. That’s why we are working with Wateraid and Unicef to improve water well drilling practices. Poor communities often have to contribute a great deal for a new water point, so it clearly isn’t right when they are left with a dud. Sean Furey
Invest in simple, efficient irrigation technology: Some means of beating water scarcity in agriculture – for example, farming close to rivers – are cheap but unsustainable. This could of course be prevented if there is an effort to invest in simple but efficient technologies for irrigation. This would break the vicious cycle where water scarcity leads to the invasion of marginal lands near rivers, which in turn undermines the ability of the river system to replenish its water resources, leading to further scarcity. Greenwell Matchaya, researcher and economist, International Water Management Institute, Pretoria, South Africa, @IWMI_
Promote rainwater harvesting: We need to challenge the way that rainwater harvesting is thought of. Everyone knows about it, but its use and implementation is piecemeal and I don’t see any big agencies or donors pushing it forward. Can we have a ‘reinvent rainwater harvesting’ challenge? Sean Furey
Secure sufficient financing. To guarantee future populations have reliable access to water and sanitation, the top priority is securing the money to ensure that systems are built and adequately maintained over the years. Sophie Trémolet
Work with communities: The sustainability of water interventions is essential if we want communities to actually have better opportunities for development in the future. Helping community leaders take ownership of their water solutions and transferring that to their neighbours is one of the best ways to ensure projects remain a part of people’s lives. Carlos Hurtado Aguilar
Invest in staff skills and capacity: To get good water data requires skilled hydrometric staff. It isn’t sexy and it is often the first budget line to be cut when departments are squeezed but it’s essential. I worked in Liberia (before the Ebola outbreak) last year and one of the major challenges for managing water resources we found is that there are hardly any measurements of anything and it’s difficult to guarantee the quality of the information that does exist. Sean Furey
Apply smart strategies: WRI’s global analysis is finding that future water stress is driven far more by demand than supply. Even in areas that will experience big hydrologic impacts from climate change, unmanaged demand will be a bigger impact. Ironically, that is cause for some optimism. If we apply the smart strategies that we already know work in the urban, rural and agricultural contexts, we can reduce future conflict and secure more water for equitable development and growth. Betsy Otto
Christensen & Co Architects recently completed the first phase of Lund’s City Hall, a building that when complete, will be the greenest City Hall in all of Sweden. Designed with a pleated W-shaped plan, the building’s curtain walls bring in natural light and offer views out towards the landscape. The City Hall is located at Lund’s historic city center and will host 269,000 square feet of office space, conference facilities, a public ground floor, and accessible green roof.
Equipped with solar panels and earth water cooling, Lund City Hall only uses a fraction of the energy normally consumed by typical government buildings. Its dynamic, W-shaped facade was key in reducing the building’s energy footprint, and each facade is carefully oriented to optimize passive technologies and solar heat gain. The north-facing facades are completely glazed to maximize natural lighting indoors, while the south-facing facades are covered with dynamic solar-control panels that respond to daylight conditions. The facades facing the city center are designed with classical features to match region’s historic character.
The accessible green roof also plays a big role in making Lund City Hall energy efficient. In addition to sequestering carbon, the green roof helps regulate internal temperatures, store rainwater, and provide natural habitat for local flora and fauna. The green roof also acts as an extension between the neighboring new park and the adjacent historic city center.
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