On December 20, 2015, a mountain of urban refuse collapsed in Shenzhen, China, killing at least 69 people and destroying dozens of buildings. The disaster brought to life the towers of waste depicted in the 2008 dystopian children's movie WALL-E, which portrayed the horrible yet real idea that our trash could pile up uncontrollably, squeezing us out of our habitat. A powerful way to transform an existing city into a sustainable one—a city that preserves the earth rather than ruining it—is to reduce all the waste streams and then use what remains as a resource. Waste from one process becomes raw material for another.

Many people continue to migrate to urban centers worldwide, which puts cities in a prime position to solve global resource problems. Mayors are taking more responsibility for designing solutions simply because they have to, especially in countries where national enthusiasm for tackling environmental issues has cooled off. International climate agreements forged in Paris in December 2015 also acknowledged a central role for cities. More than 1,000 mayors flocked to the French capital during the talks to share their pledges to reduce emissions. Changing building codes and investing in energy efficiency are just two starting points that many city leaders said they could initiate much more quickly than national governments.

It makes sense for cities to step up. Some of them—New York City, Mexico City, Beijing—house more people than entire countries do. And urban landscapes are where the challenges of managing our lives come crashing together in concentrated form. Cities can lead because they can quickly scale up solutions and because they are living laboratories for improving quality of life without using up the earth's resources, polluting its air and water, and harming human health in the process.

Machines dig through rubble in Shenzhen, China, after a mountain of refuse collapsed, burying dozens of buildings. Credit: Getty Images

Cities are rife with wasted energy, wasted carbon dioxide, wasted food, wasted water, wasted space and wasted time. Reducing each waste stream and managing it as a resource—rather than a cost—can solve multiple problems simultaneously, creating a more sustainable future for billions of people.

Pollution as Solution

Lessons about waste abound in history. John Snow, a London doctor, deduced that terrible cholera outbreaks struck London in 1848 and 1854 because public water wells were contaminated by sewage. Building sewers was an obvious solution, but political leaders rejected Snow's findings because his ideas did not fit prevailing ideologies and because the actions were deemed too expensive. Similar rejection is offered for today's climate scientists, who tell us that our waste is killing us, though in a much slower and less direct fashion, and that fixing the problem will require significant investments in new infrastructure. Snow was later vindicated as a hero (perhaps the same fate awaits our present-day scientists) after new leaders created ambitious public works projects to cram 1,200 miles of sewers into a crowded city of three million people, ending the cholera problem. The work also created the lovely river embankments that still stand as a key piece of London's urban environs and along which many people stroll.

Today just flushing the waste away is not enough, however. After we reduce it, we should close the loop and use the remainder again. First, limit waste, then put it to work.

This new thinking begins by redefining our concept of pollution. Raj Bhattarai, a well-known engineer at the municipal water utility in Austin, Tex., taught me a new definition for pollution: resources out of place. Substances are harmful if they are in the wrong place: our bodies, the air, the water. But in the right place, they are useful. For example, instead of our sending solid waste to a landfill and paying the bill, it can be incinerated to generate electricity. And the sewage for a million-person community can be mined for millions of dollars of gold and other precious metals annually for use in local manufacturing.

This idea fits with the larger concept of the so-called circular economy—where society's different actions and processes feed into one another beneficially. Simply put, waste is what you have when you run out of imagination.

Less is More

One obvious place to start reducing waste is leaky water pipes. A staggering 10 to 40 percent of a city's water is typically lost in pipes. And because the municipality has cleaned that water and powered pumps to move it, the leaks throw away energy, too.

Energy consumption itself is incredibly wasteful. More than half the energy a city consumes is released as waste heat from smokestacks, tailpipes, and the backs of heaters, air conditioners and appliances. Making all that equipment more efficient reduces how much energy we need to produce, distribute and clean up.

Refuse is another waste stream to consolidate. The U.S. generates more than four pounds of solid waste per person every day. Despite efforts to compost, recycle or incinerate some of it, a little more than half is still dumped in landfills. Reducing packaging is one way to lessen this volume while also generating other benefits. Big retailers such as Walmart, for example, have found that reducing packaging results in fewer trucks needed for shipping and more shelf space to display goods.

Wasted food is its own heart-wrenching issue. Despite famine and food scarcity in many places globally, Americans throw away 25 to 50 percent of their edible food. Food requires vast amounts of energy, land and water to grow, produce, store, prepare, cook and dispose—so wasted food leaves a significant imprint. Initiatives that have popped up in the U.S., such as the I Value Food campaign, and in the U.K. are a start toward solving this vital issue.

Putting Waste to Work

Once cities reduce waste streams, they should use waste from one urban process as a resource for another. This arrangement is rare, but compelling projects are rising. Modern waste-to-energy systems, such as one in Zurich, burn trash cleanly, and some, including one in Palm Beach, Fla., recover more than 95 percent of the metals in the gritty ash that is left by the combustion. Rural villages, such as Jühnde in Germany, create enough biogas from cattle and pig manure to heat or power a large portion of their homes. My research group at the University of Texas at Austin has demonstrated that a cement plant in New Braunfels, Tex., can burn fuel pellets made of unrecyclable plastics rather than coal, avoiding carbon dioxide emissions and impacts from coal mining.

Even trash that is put in landfills can provide some value. Cities can collect methane that rises as the waste decomposes, which is an obvious improvement over flaring (burning off) the gas or simply letting the methane waft up into the atmosphere, where it traps much more heat than the equivalent amount of carbon dioxide. Power generators can convert the collected gas into electricity. Vancouver's landfills extract the methane and burn it to heat nearby greenhouses that grow tomatoes.

Vancouver burns methane collected at landfills to produce heat that warms tomato greenhouses run by Village Farms. Credit: Village Farms

Even then, landfills are still leaky. That inspired Vancouver, which has pledged to become the greenest city on earth, to give residents separate bins for trash and organic matter (food scraps, yard clippings and tree trimmings). Officials expect citizens to use them properly and deploy city inspectors to check that waste haulers are dumping refuse that is separated correctly. The city produces methane from the organic waste while generating solids known as amendments that can make soil more fertile. These solutions solve multiple problems at once—saving money for energy that would otherwise have been purchased, reducing the need for expensive landfilling and avoiding unnecessary use and damage of land—while improving agriculture.

Austin does something similar with its wastewater sludge, passing it through anaerobic digesters to make biogas it sells or uses on-site for generating heat. It converts the remaining solids into a popular soil amendment known as Dillo Dirt (a reference to the armadillo, one of its local creatures). The city earns money by selling the Dillo Dirt, offsetting some of the cost of treating wastewater. Although composting is a growing and popular trend among residents—and one worth pursuing—doing it poorly can actually lead to more methane emissions. For Austin, it makes more sense for residents to put food scraps down the drain and through a grinder so that the city's industrial-scale harvesters at the wastewater plant can do the work of the composter but with greater efficiency.

Waste heat is another big opportunity. Harvesting it is difficult because low temperatures are hard to convert into electricity. NASA developed thermoelectric generators to do this on its spacecraft, but the technology is expensive and inefficient. Nevertheless, advanced materials that can more effectively convert heat to electricity are coming. A place to start is the hot wastewater that goes down the drain when we wash our clothes, dishes or bodies. Sandvika, a suburb of Oslo, has massive heat exchangers along city waste pipes that extract heat to warm dozens of nearby buildings or defrost sidewalks and roadways. By turning on heat pumps in the summer, it can use some of the heat to cool those same buildings. Vancouver liked the idea so much that it repeated the concept, using wastewater to heat hundreds of buildings and the Olympic Village.

Taking that idea further is the Kalundborg Symbiosis in Denmark, a leading example of closed-loop thinking. The industrial park has seven companies plus municipal facilities—centered on electric, water, wastewater and solid-waste facilities—that are interconnected such that the waste from one is an input for another. Pipes, wires and ducts move steam, gas, electricity, water and wastes back and forth to improve overall efficiency and reduce total wastes, including CO2 emissions. For example, wastewater from the oil refinery flows to the power plant, where it is used to clean and stabilize fly ash from coal combustion. The refinery also sends waste steam to Novo Nordisk, which puts the heat to work for growing about half the world's supply of insulin with bacteria and yeast [see box below]. The entire park looks like a living, industrial organism. And it has demonstrated economic growth with flatlined or reduced emissions.

Credit: Harry Campbell; Source: Symbiosis Center Denmark www.symbiosecenter.dk

Data-Driven Decisions

Can the Kalundborg Symbiosis model be replicated on a larger scale, for cities worldwide? Yes, but only if we make cities smart. An industrial park is flexible because it has only a few tenants and decision makers, but a city has many individuals and organizations making independent decisions about energy, water and waste every day. Integrating them requires a cultural shift toward cooperation, boosted by advances in smart technologies. “Smart cities” will depend on ubiquitous sensing and cheap computing, compounded by machine learning and artificial intelligence. This combination can identify inefficiencies and optimize operations, reducing wastes and costs while operating all kinds of equipment automatically.

Thankfully, making cities smart is an alluring objective for planners who want to accommodate higher densities of people without diminishing quality of life. For example, in India, where population and public health problems are severe, Prime Minister Narendra Modi has announced his intention to convert 100 small and medium-sized municipalities into smart cities as a possible solution.

The “smart” moniker itself is an accusation that most cities are dumb. That accusation sticks because municipalities rife with waste seem to be operating blind. The U.S. National Science Foundation has just launched a major research initiative called Smart & Connected Communities to help cities make better use of data. That name, by the way, indicates that intelligence is not enough—interconnections among systems and people matter, too.

Smart cities rely heavily on big data gathered from widespread sensor networks and advanced algorithms to quickly gain insights, draw conclusions and make decisions on those data. Connected networks then communicate those analyses to equipment all across the city. Smart meters for closely tracking electricity, natural gas and water use by time of day, household and industrial appliance are an obvious place to start. Real-time traffic sensors, air-quality monitors and leak detectors are also at hand. The Pecan Street consortium in Austin is collecting data from hundreds of homes to learn how access to such data streams might help consumers change their behaviors in ways that reduce consumption while saving costs. Cities such as Phoenix and military bases such as Fort Carson in Colorado have pledged to become self-sufficient users of energy and water and net-zero producers of waste. Achieving those ambitious goals will require a lot of interconnected data.

Better transportation may give urbanites their first glimpse of a smart city's benefits by cutting wasted time. Reducing the footprint of transportation means cleaning up the fuels, making the vehicles more efficient, reducing trip distances and duration, increasing vehicle occupancy and cutting back on the number of trips. If people live close to their work, they can walk or bike or use mass transit. Studies show that building protected bicycle lanes leads to dramatically increased ridership, and because bicycles require so little space, compared with cars, they can reduce congestion on the roads.

A driverless city will also free up wasted space and time associated with parking. With shared or autonomous cars in constant motion instead of private cars that are parked at home and work, the number of parking spaces can be restricted dramatically, opening up wasted space and easing congestion further. Researchers at the Center for Transportation Research at the University of Texas at Austin used sophisticated models to determine that shared, autonomous vehicles would lessen the number of cars needed in a city by an order of magnitude and would cut emissions, despite causing a slight increase in total miles traveled because the vehicles would stay in motion. Instead of wasting their time driving, commuters can rest, read e-mails, place phone calls or conduct other business. That work can create economic value—and trim a person's office hours so he or she can get home earlier for dinner.

Pipes in Kalundborg, Denmark, carry waste steam from the DONG Energy power plant to companies that use it for manufacturing. Credit: Kalundborg Symbiosis

Making our infrastructure smarter is certainly the key to solving basic problems such as leaky water pipes. Identifying leaks should be easy if meters are distributed throughout a water system to track flows and readily pinpoint the amount and location of those leaks. Researchers in Birmingham, England, developed a system with tiny pressure sensors that use a small amount of power to frequently check for and detect leaks in water networks, a big improvement over the old technique of waiting for someone to call and complain that water is shooting like a geyser out of the road. And someday we might send smart robots down the pipes to repair the problems.

High-performance sensors will also let us find and predict natural gas leaks before accidents happen. Gas leaks are not only bad for the environment and a waste of resources but dangerous, as we see in headline-grabbing explosions in urban areas with aging infrastructure.

It is hard to know where smart, waste-conscious cities may arise. I imagine a likely candidate will be a Midwestern town with a million people or more that needs to reinvent itself because its economy was gutted decades ago. Indianapolis comes to mind, in part because it needs to rebuild water, wastewater and sewer systems based on bad decisions a century ago. The city has been investing in its downtown and is on the rise. Pittsburgh is leveraging its existing assets—a vibrant urban core, city pride, forward-looking leadership from Mayor William Peduto, the strength of Carnegie Mellon University and other hotbeds of innovation—to go from being defined by its smokestacks to being defined by its brainpower. Indeed, Uber launched its autonomous-vehicle service there. Columbus, Ohio, which is the state capital and home to a major university, is another place to look for cutting-edge experiments in becoming smart. The U.S. Department of Transportation recently awarded Columbus a $40-million grant to reinvent its approach to mobility.

Getting from Here to There

Turning profligate cities into places that reduce waste and reuse what is left will not be easy. Integrated R&D investments from the federal government have to be combined with practical policies from all levels of government. Unfortunately, R&D funding is in recent decline, and in the U.S., it may drop further under the Trump administration.

Investment has to be socially savvy as well. Studies show that R&D for smart cities has focused more on technology than what the citizenry needs. Done the wrong way, the benefits of a smart city might accrue to those who already have Internet connectivity and access to advanced technologies, which would only widen the technology gap on top of other socioeconomic divides.

Municipalities also need to help residents become smarter citizens because each individual makes resource decisions every time he or she buys a product or flips a switch. Access to education and data will be paramount. Connecting those citizens also requires collaboration and neighborly interactions: parks, playgrounds, shared spaces, schools, and religious and community centers–all of which were central tenets of centuries-old designs for thriving cities. The more modern and smart our cities become, the more we might need these old-world elements to keep us together.