Sewage treatment plants clean our wastewater, but in doing so they also waste industrially usable nutrients such as nitrogen and phosphorus. In order to prevent this from happening in the future, researchers have now worked on various upgrades for existing sewage treatment plants. In the long term, they should enable a sustainable circular economy that protects resources, the climate and ecosystems. The various procedures have now successfully completed the first test phase.
Over nine billion cubic meters of wastewater are treated in German sewage treatment plants every year. In addition to organic contaminants, the systems also filter out nutrients such as nitrogen and phosphorus. This is intended to prevent them from entering water bodies in large quantities, worsening the water quality there and, in the worst case, even costing the lives of aquatic animals.
After the sewage treatment plants have filtered out the nitrogen compounds contained in the wastewater, they are converted into molecular nitrogen with high energy consumption, which escapes into the atmosphere as a gas. Phosphorus, in turn, is usually precipitated in the form of iron or aluminum phosphates and disposed of with the sewage sludge.
Test facility times seven
According to Marius Mohr from the Fraunhofer Institute for Interfacial Engineering and Biochemical Engineering and his team, this approach is a huge waste. Both nitrogen and phosphorus can be of great benefit to industry and agriculture. For example, natural phosphate reserves for the production of fertilizers are becoming increasingly scarce, which is why, in the best case scenario, not a single gram of phosphorus should simply be “thrown away”.
In order to change that, Mohr and his colleagues tested various novel processes for sewage treatment plants as part of a three-year project. For this purpose, the researchers operated a total of seven demonstration plants on the site of the Erbach an der Donau sewage treatment plant. The various approaches were designed from the outset as independent modules that are intended to serve as a kind of upgrade to existing sewage treatment plants in the future.
Waste becomes fertilizer
One way to convert sewage treatment plants into a climate-friendly circular economy is to make use of the so-called sewage sludge digestion, which already takes place in sewage treatment plants. Organic substances from wastewater are fermented to produce biogas as a renewable energy source. After digestion, the sludge is then dewatered, creating so-called sludge water, which is rich in phosphorus and nitrogen. “Since substances can be recovered better the more concentrated they are, this is exactly where we start,” explains Mohr.
He and his team transfer the muddy water to a special plant in which the phosphorus it contains is used to produce so-called struvite – a fertilizer for agriculture. Nitrogen can also be recovered from the muddy water and processed into fertilizer. As it turned out at the test site, both this and other processes work efficiently and also improve the climate balance of the sewage treatment plants. Among other things, their use reduced emissions of climate-damaging nitrous oxide. Mohr and his team now want to implement their project results on a large scale and are working with a larger sewage treatment plant in the region.
Source: Fraunhofer Institute for Interfacial and Biochemical Engineering IGB