Out of sight, out of mind: we’ve been flushing away human waste ever since sewers were invented, using copious amounts of fresh water to expel it from our homes and cities as fast as the pipes can carry it. Modern urban water systems are widely regarded as one of the greatest achievements of the past century. They provide us with clean drinking water, channel our wastewater to treatment plants and divert rainwater away from built-up areas. “As a result, we enjoy dry and hygienic living conditions, two of the mainstays of public health in densely populated urban areas,” says Max Maurer, Professor of Urban Water Systems at ETH Zurich and Eawag, the Swiss Federal Institute of Aquatic Science and Technology, which is part of the ETH Domain.
To achieve this, industrialised countries have built a vast amount of infrastructure – some 230 billion Swiss francs’ worth in Switzerland alone. Laid end to end, Switzerland’s 200,000-odd kilometres of water and sewerage pipes would encircle the globe five times. The extensive network of underground sewers carries wastewater to nearly 800 centralised treatment plants.
This approach to water infrastructure has proved its worth in industrialised countries – and for decades, it was also regarded as a benchmark for the rest of the world. “But the truth is that conventional urban water systems are no longer sustainable,” says Maurer.
From waste to resource
Kai Udert, Professor at the Institute of Environmental Engineering at ETH Zurich and Senior Scientist at Eawag, is equally sceptical about conventional water infrastructure. “We use drinking water to dilute faeces, urine and slightly dirty water from bathrooms and kitchens and move them through the sewerage system – that’s patently absurd!” he says.
Udert, an expert in process engineering, sees sewage not as foul-smelling waste that needs to be disposed of, but rather as a valuable resource that should be properly exploited. His explanation of why we need to take a new approach is simple. “Wastewater is one of the last linear waste streams,” he says. “We dispose of everything in the same way, regardless of whether it’s clean or dirty. That’s inefficient, and it creates all sorts of problems that people have been trying to fix for years.” For example, conventional systems squander not only huge amounts of water and energy but also valuable nutrients which, if not fed back into the cycle, end up damaging the environment.