In Altena in Sauerland, space is a scarce commodity. The houses in the valley are lined up close together along the Lenne and the city’s sewage treatment plant is encircled between the river and the railway tracks. Yvonne Schneider from the Ruhrverband shows the banana-shaped property in an aerial photo. “The lack of space was definitely a problem when it became clear a few years ago that the technology and concrete pool had to be renewed. Especially since operations were to be continued in parallel,” says the head of the “Central Technical Departments” division.
A planning team calculated various renovation options according to economic criteria. The task of the location was also up for debate. In the end, a technology that was patented in the Netherlands did best, which requires little space and is already in use in more than 20 systems worldwide, but not so far in Germany. It replaces the common activated sludge process, the biological cleaning stage that clarifies the water with the help of brownish bacterial flakes. Operation in Altena is expected to start in the summer with provisional approval for a two-year test phase. The Federal Environment Ministry is funding the project with 1.4 million euros.
Bacteria are also at the center of the new process, although they are packed into compact, dark brown, and small-like, lentil-sized spheres. These so-called granules are a kind of three-in-one combination product, because each granule contains different bacteria with different abilities. In interaction, they convert faeces and urine into carbon dioxide and biomass, defuse nitrogen compounds and remove phosphates from the water. Since this all happens in one go, fewer concrete pools are needed. The operator can also do without the usual chemicals for phosphorus removal, at least to a large extent. “And we’re hoping for energy savings because less pumping and mixing is required,” adds Schneider. This also helps with the energy transition.
In the new plant, the sewage flows through the bacterial clumps in three seven-meter-high concrete reactors from below and the mixture is repeatedly bubbled up with air. The alternation of oxygen supply and withdrawal ensures that all types of bacteria are activated once in each cycle. And the gas bubbles literally grind the spherical microbial communities smooth, making them more stable. For continuous operation, the reactors work at different times.
“Many sewage treatment plants have to be renovated or expanded because more and more people are moving to the cities”
Mari Winkler of Washington University in Seattle has long been a fan of microbial communities. “In the USA, not only do many sewage treatment plants have to be renovated, they also have to be expanded because more and more people are moving to the cities. But often there is not enough space for this or you have to intervene in intact nature,” she explains. Wastewater treatment with granules can solve such problems and can also be integrated into existing processes. “In the granules we have a lot of microorganisms in a very small space. That alone makes the wastewater treatment more effective,” she says. In addition, the compact balls sink to the bottom faster than the light flakes in activated sludge systems and can be removed more easily once the cleaning work has been completed. The huge round settling tanks of classic sewage treatment plants are simply no longer needed. The researcher recently reported in the science journal that space savings of up to 75 percent are possible Science.
The bacteria for the new process are contained in the wastewater and do not need to be added once the process has started. A little help is needed to ensure that the microbes form balls instead of flakes. “It works through a kind of forced selection,” says Winkler. Compact formations of bacteria, which lie on the ground after just a few minutes, are repeatedly introduced into the cleaning process, and everything that remains floating in the water for a longer period of time is discarded. A particularly nutritious sewage diet also helps, because it primarily promotes the slow-growing, clump-forming types of bacteria. However, it can take a long time until a batch of granules suitable for operation is obtained with such tricks. In Altena, the process is therefore started with so-called inoculation sludge, with stable granules from Dutch plants.
The technology could soon get an additional boost, because by the next decade at the latest, European sewage treatment plants will not only have to remove phosphorus-containing substances from the sewage, but also recycle them according to the Sewage Sludge Ordinance. “The granules can be thickened relatively easily and also release the phosphate again,” says Winkler. Then it could be processed into fertilizers with a comparatively small amount of chemicals. “But that’s still a long way off,” she emphasizes.
“The time was simply ripe for an innovation”
Harald Horn from the Karlsruhe Institute of Technology promoted wastewater treatment with granules many years ago and successfully tested it in a pilot plant in Garching. “The time was simply ripe for an innovation that would have been very good for Germany as a technology location,” he says. After all, the classic activated sludge process is over 100 years old. At the time, the researcher would have wished for more openness in German wastewater management.
“Then a system like the one in Altena would have been possible ten years ago and funding from innovation pots would have been appropriate. Today, the process is established on the market worldwide.” However, it is not suitable for every sewage treatment plant, for example not for waste water that is heavily diluted by seeping groundwater. A well-maintained sewer system is an important requirement, according to Horn.
The construction of the new plant in Altena has now been completed. The necessary technology and the three reactors are in a long, narrow building next to the existing clarifier. The operation should be fully automated and remote controlled. And in the first few years, it is accompanied by a measurement campaign to repeatedly and precisely check the quality of the water and granules. “For us, the whole thing is a kind of pilot project,” says Yvonne Schneider. “We want to see if the technology meets our expectations and, if so, recommend it for other wastewater treatment plants to emulate.”