Cities are leading the way in the circular economy transition. But what does it take to become a truly circular city? This series of articles explore the key services that cities need to adapt in order to become fully circular. These include energy, construction, water, food and mobility. This article delves in the challenges associated with designing circularity into water systems.

Why are water systems important for circular cities?

Without water, cities cannot function. Water keeps us hydrated and allows us to cook and clean but it also enables the manufacturing of every day goods, the growing of food, and the heating and cooling of buildings. To put into context how much water cities use, Beijing’s annual water consumption in 2013 reached 950 billion gallons – the equivalent of 19 billion bath tubs.

Yet, many cities around the world, such as Mexico City, São Paulo in Brazil and Sana’a in Yemen are beginning to experience severe water shortages due to a combination of rapid population growth, polluted water sources and climate change. These challenges are unlikely to go away and are in fact predicted to get much worse. The UN warns that half the world population will face water scarcity by 2030. Moreover, cities expend huge amounts of energy (and money) to maintain the water system. For example, 30%-40% of a municipality’s energy bill is dedicated solely to pubic drinking water.

A number of cities are now turning to the principles of the circular economy to ensure sustainable urban water systems are developed to cope with such challenges. The transition to a circular economy presents a significant opportunity to combine new ways of thinking with advanced technologies to create a step change in the efficiency of urban water systems.

There are two key deliberations when designing a circular city water system. Firstly, how can cities circulate and reuse water as much as possible before it escapes the system and secondly, where will the energy come from to power this circulation?

Closing the Loops

Traditional water systems have been designed in the fashion that many other societal services have been designed – linear. A typical city will extract water from large sources nearby such as lakes, river or aquifers – or it will be imported from elsewhere. Water is then transported around the city, losing up to 40% through leaky pipes. The water is then used both in industry and domestically and then treated and dumped into a source that transports the water away from the city – such as rivers or oceans.

This was an acceptable model when the demand from cities was small enough not to extract more water from the fresh sources than the rate at which the sources could be replenished. However, many global cities have experienced unprecedented population growth and climate change leading to the continued drawdown of these sources to the point where they are in real danger of disappearing.

So how can the linear water systems be redesigned to prevent this from happening? Four main solutions spring to mind. Firstly, cities can complement traditional water sources with alternative renewable sources of water such as rainwater. Tokyo, for example, has made it mandatory for any new building to build underground rain water tanks where more than 1,000 buildings, including the Skytree (Japan’s tallest building), are now harvesting rain and feeding it into the water system. A project in Morocco has successfully managed to capture water from fog, otherwise known as cloud fishing, whereby a series of mesh nets capture tiny water droplets and transport them to large water tanks. The project is currently producing 6,300 liters of water per day. This method, however, is unlikely to provide water to the masses however and is very dependent on climatic conditions.

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Cloud Fishing in Morocco. Dar Si Hmad.

Many coastal cities are looking toward desalination as a solution. This is a viable solution in the Middle East, where energy prices are very low and finance is available, however in other parts of the world desalination remains prohibitively expensive.

Secondly, cities can reuse wastewater rather than throwing it away. A number of cities, such as San Diego and Sacramento, are seriously exploring the concept of Direct Potable Reuse (DPR) which cleans wastewater from the sewage system so thoroughly that it can be used for potable water. A major advantage of DPR is that the reused water is available much closer to the location compared to traditional sources and therefore significantly reduces the energy bill associated with transporting it.

Spurred on by the constant threat of drought, Wichita Falls, Texas, installed one of the world’s first DPR systems, which produces two million gallons of drinking water a day from wastewater. However, such solutions require a high level of education to the public to change attitudes towards reusing wastewater. To this day, regulatory frameworks and local opposition are major barriers to widespread deployment of DPR systems. In 2008, the Australian city of Toowoomba overwhelmingly rejected DPR, even though they were experiencing a devastating drought.

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The DPR System at Wichita Falls. Water Online.

Thirdly, cities can improve water efficiency by reducing leakage from water systems. Currently, seven billion gallons of water are lost through leaky pipes per day in the US alone. The World Bank estimates that leaky pipes are costing the global economy approximately $14 billion a year. Solving this problem involves putting in sturdy pipes and reacting fast to any leaks.

Tokyo, for example, has one of the most efficient water systems in the world due to the City’s attitude towards immediately detecting and repairing leaks, saving around 19 billion gallons of water per year. It has employed a whole suite of technologies to achieve this, such as electronic leak detectors and an automatic detection computer system, as well as more basic solutions such as ensuring people install leak proof taps and fittings.

Finally, the public can be encouraged to consume less. Some cities have chosen the stick approach, penalizing people by charging them directly for the amount of water they consume, banning certain activities such as washing cars or strictly mandating that builders install super efficient water systems. Other cities have found it more useful to incentivize people by providing rebates on replacement fixtures to save water or giving away water saving products for free.

Powering Urban Water Supplies

If cities are to stay true to the principles of the circular economy, their water systems must be powered by renewable energy. Does this mean simply installing a bunch of solar panels on all the water treatment works? Far from it.

There are a myriad of different ways that energy can be harvested from the water system itself. In hilly cities, for example, electricity can be generated from the distribution systems, whereby micro turbines can be installed directly next to pressure reduction vales. Heat exchangers can also be attached to sewage and greywater pipes – which are one of the main sources of heat leakage from buildings.

Alternatively, gas, electricity and heat can be produced from the biosolids extracted from DPR sites and wastewater treatment sites. Using technologies such as hydrolysis or gasification, sewage treatment plants can cover up to 60% of their energy demands. One of the most cutting edge energy recovery technologies called EcoVolt is being deployed by Cambrian Innovation which uses a novel bioelectric process to simultaneously treat water and generate biogas energy.

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Generating Biogas from Wastewater. Denver Off the Wagon.

Finally, renewables such as solar and wind are expected to play a vital role in powering the remaining parts of the water system and are sometimes used as a safety net from fluctuating energy prices. A wastewater treatment plant in California is powered almost entirely by solar power.

Remaining Challenges to Address

In a world of rapid population growth and increasing climate change, the transition to circular urban water systems is essential. A wide range of water conservation and energy generation technologies exist with which to facilitate such a transition.

However, one has to keep in mind that cities exist within broader catchment areas that include farmland and other urban areas outside city boundaries – therefore there needs to be conscious effort to take a holistic joined-up approach to the entire water catchment.

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