Wastewater as a resource
Pooh, yuck! Admittedly, sewage is a somewhat smelly subject. That may explain why it took so long to discover its value as a resource, or, to be historically correct: to rediscover it. Among others, the people in ancient Rome, who were far ahead of their time in terms of sewers anyway, used to produce dung from excrements and utilized urea for tanning and dyeing purposes. Afterwards, the subject of sewage literally and largely seeped into the soil for a long time. It regained importance only in the wake of industrialization and rapid urban growth in the 18th and 19th centuries because the smelly rivulets running through the streets were not only revolting but also pestholes. Consequently, in the subsequent decades up until today, endless labyrinths of pipelines were buried underneath the cities of the world. In the “basement” of the European Union alone, sewage pipes are said to be meandering along 2.5 million kilometers (1.5 million miles), enough to wrap around the Earth 62 times. The value of the pipeline system amounts to some 2.5 quadrillion euros.
Pipes, though, are only part of the solution, especially if they end in a river or elsewhere in the natural environment. In South America, for instance, only a little less than ten percent of the sewage in bigger cities is purified in treatment plants. Out of sight, out of mind is the motto that applies to the rest. Yuck! A classic sewage treatment plant to purify the smelly disposal cargo at the end of the pipe is indispensable to protecting the environment, albeit, the residues of the treatment process, the so-called sewage sludge, often remains unused. It’s simply incinerated and valuable resources along with it: raw materials and potential for energy generation.
Researchers around the world are working on tapping into those resources. They include the Fraunhofer Institute, where engineers and scientists have already tested a variety of technologies including electrochemical water treatment that can be used to produce sulfates that can serve as agricultural fertilizers, for example. Metals such as aluminum, iron, manganese, or nickel can be extracted and recycled in this way – a solution that’s equally ecological and economical in view of constantly increasing raw material prices.
Another method is high-load digestion that’s implemented on sewage treatment plants. It converts the sludge not only into biogas as a renewable carbon and energy source but also delivers sludge water and digestion residues as further usable substance streams. Metals, fertilizers, energy – sewage has a lot to offer, not least: water. EU Environmental Commissioner Virginijus Sinkevičius from Lithuania supports that notion: “[…] the use of reclaimed water in the agriculture sector can play an important part in addressing water stress and drought […].”
Early separation and treatment
Because sewage is so beneficial, Kai Udert, a professor at the Institute of Environmental Engineering at ETH Zurich, raises the question of why wastewater is one of the last linear waste streams even in leading industrial nations. “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,” he criticizes. In an article published on the ETH website, he and his colleagues call for a paradigm shift in wastewater management. One of their proposals is substance separation at the source, which facilitates the treatment as well as the recovery of valuable substances contained in sewage while reducing the consumption of freshwater.
“We use drinking water to dilute feces, urine and slightly dirty water from bathrooms and kitchens and move them through the sewerage system – that’s patently absurd!”Kai Udert, a professor at the Institute of Environmental Engineering at ETH Zurich
On the other hand, small, highly efficient, decentralized sewage treatment plants are supposed to purify wastewater more flexibly and as close as possible to the source in order to save long transportation routes. In addition, Professor Kai Udert points out that such decentralized small-scale treatment plants might also be a solution for purifying and using the sewage of the 2.3 billion people that are not yet connected to any sewer systems. According to ETH, such small-scale plants could be combined with a biogas reactor where in a rubber balloon methane gas from an anaerobic digestion process of fecal sludge could be recovered and used for cooking, for example.
Here toilets will soon be flushed with flushing water
Examples of substance separation at the source can be found at “Jenfelder Au,” a district on the outskirts of Hamburg. There the “Hamburg Wasser” utility is already separating graywater from blackwater that is conveyed to a biogas plant in the neighborhood via special vacuum toilets. The graywater is treated separately in a two-stage process. First, the sewage is treated biologically. In a so-called packed bed bioreactor, sludge is formed in which microorganisms remove pollutants. Subsequently, the water enters a special filtering system with an ultra-filtration membrane in which any residual micropollutants are removed. The remainder is water that is not drinking water but meets all the requirements for sanitary or service water and could be used without hesitation to fill toilet tanks or to irrigate green areas, for example.
No leakage yet?
There’s another aspect supporting a reorganization of the existing water system: Many pipes have leaks. As a result, one in four liters of drinking water in all of Europe seeps into the soil. In some regions of Italy, a country that has been particularly burdened by droughts in recent years, it’s up to 80 percent. The World Bank assesses the resulting economic losses at 14 billion dollars worldwide. Although the water isn’t lost because it reenters the circular system via the groundwater, it’s currently lacking: on fields, for example. Posing an even greater problem are leaks in sewer lines through which metals, medicine residues and other, partly toxic substances that should be recycled, enter the soil and consequently the groundwater at some point in time. In the United States, that amounts to 860 billion liters (227 billion gallons) per year. That’s enough for every American to fill a bathtub once a week, although no one would voluntarily take a bath in that sludge …
A look at Spain shows the magnitude of the need to act as well: The country that, like Italy, is threatened by droughts is planning to invest 23 billion euros in the coming years to repair dilapidated existing infrastructure and to implement new plants and systems such as a large-scale treatment plant for service water. That’s another case in point for the growing importance of water as a resource.