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Co-director of Berkeley Water Center

This story is part of what happened next, our complete guide to the future. Read more predictions about the future of water.

When the ancient Romans built their first aqueduct, they came up with an enduring idea: Modern cities need to build infrastructure to pump water through, through and around mountains. Most cities today transport this water through large treatment plants and then distribute it to our homes through a labyrinth of underground pipelines. When we run out of it, we transport waste water to treatment plants, which release large amounts of liquid into our rivers, and treated sewage usually accounts for the majority of the flow.

The water infrastructure on which modern cities depend is not cheap. City leaders in North America, Europe, and Australia are raising funds to refurbish an aging water supply system in the mid-20th century; U.S. utilities spend nearly $110 billion on water and sewage services each year. At the same time, their counterparts in developing countries are considering plans to replicate the infrastructure of the world’s wealthy cities. In the long run, they may not be able to maintain these infrastructures.

It is worth considering that there may be a better way. Rather than rebuilding the ancient Roman water supply system, we still have the opportunity to create sustainable infrastructure that is more resilient, less costly, and has less impact on the environment.

In order to transition to the next generation of water supply systems, cities need to focus on rural communities that have never invested in complex water supply infrastructure. Around the world, millions of people still draw water from local wells or rainwater collection ponds. Millions of people use septic tanks and leaching sites to treat and dispose of their wastewater. These simple techniques are not used in cities because roofs and wells cannot provide enough water for multi-storey apartment buildings, and when too many people try to dispose of their waste in the same place, the septic tank leaching site Will fail.

However, with a little creativity, the modern "off-grid" system used in rural communities can be practical in our suburbs. The first step is to reduce our water footprint. First, off-the-shelf water-saving appliances can significantly reduce the domestic water consumption of a typical household. Coupled with equipment that recycles water from washing machines and showers, the overall water footprint of a household may be reduced by 75%. This reduction may enable new housing development projects in all regions of the world except the driest regions to be constructed without incurring the costs and environmental damage associated with imported water and centralized sewage treatment plants.

In the long run, domestic water supply systems can also serve densely populated city centers. Existing technology can recycle almost all the water used in our homes (pdf). With a slight modification, a household unit consisting of a refrigerator-sized treatment device and a similarly sized water storage tank can create a way to reuse the same water over and over again. The salt and nutrients in the wastewater that cannot be converted into carbon dioxide and water during the treatment process will be dried and left on the side of the road for recycling instead of expensive sewer networks. Any water lost from the system can be replenished with water from human waste.

In cities where the existing urban water supply system can still meet our needs, it may be difficult to justify this radical transformation. But for places lacking modern water supply infrastructure, the next water supply system may be different for the first time in two thousand years.

This story is part of what happened next, our complete guide to the future. Read more predictions about the future of water.

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