Prof. Dr. Markus Röhricht and his team are researching a new type of solar-powered sewage treatment plant. Flush the toilet and the waste disappears. A complex cleaning process then begins - not visible to most people. The importance of functioning wastewater treatment is shown by disasters such as the floods in the Ahr Valley or the situations in refugee camps around the world, where untreated wastewater spreads epidemics and contaminates drinking water. There is no or very limited opportunity to purify wastewater in a functioning sewage treatment plant.

So far, mobile systems have been the solution in humanitarian aid. Some of these are solar powered, but they are always equipped with a power supply via generators in order to continuously maintain the wastewater treatment process. “The expenditure on fuel to power the generators seems marginal. “The question for the aid organizations is whether they should spend the money on diesel or on soap,” says Christoph Stein, summing up the current situation. He is a graduate of the Technische Hochschule Mittelhessen (THM) and a founding partner and authorized representative of Saygin & Stein engineering GmbH. The company offers solutions to critical and complex water technology problems, including water treatment options for aid organizations.

“In order to save fuel costs, we want to develop a wastewater treatment system that runs solely on solar energy,” explains Stein. What sounds simple at first is highly complex. At this point, the expertise of Prof. Dr. Markus Röhricht was asked at the THM .

To purify wastewater in a wastewater treatment plant, the right microorganisms and oxygen are needed. Oxygen enters the wastewater by swirling it. Electricity is required for this. Normally, this process runs in a sewage treatment plant regardless of the time of day so that the microorganisms are always supplied evenly. “Our task now is to find out under what conditions the microorganisms form a biofilm that remains stable even when no electricity is generated at night due to a lack of sunlight and therefore no oxygen can be introduced into the wastewater,” explains Röhricht.

While there are already enough microorganisms in an existing sewage treatment plant to clean new wastewater, they first have to be introduced into mobile plants. Röhricht and Peter Jungkurth, master's student and research assistant in this project, are also researching how this works best. Reeds show small foam cubes, about one cubic centimeter in size. These are manufactured by the project partner EMW filtertechnik GmbH. “This is the carrier material onto which the microorganisms are vaccinated. We put these cubes in the wastewater and they should be swirled there so that the microorganisms can then develop within a few days,” he explains.

By the end of this year, he and Jungkurth will be investigating how large the bodies should be and what pore size is optimal so that they have the same density as water and can therefore be easily swirled. And they want to find out how the microorganisms react when they are not supplied with oxygen for a certain period of time. “We cannot initially test this under real conditions, but instead use synthetic wastewater for our test series,” explains Röhricht.

A separate system is to be built in the Lollar sewage treatment plant at the beginning of 2025. “We want to see how the microorganisms and the carrier material behave within an entire annual cycle. “You don’t want them to give up in the German summer,” says Stein. In the Saygin & Stein bioreactor, which is separate from normal sewage treatment plant operations, research will then be carried out using activated sludge and real wastewater from the sewage treatment plant under conditions that are as realistic as possible. “This composition comes closest to the highly concentrated wastewater that we find in camps for refugees in war and crisis regions,” explains engineer Stein. For comparison: In rich industrialized countries, each person produces around 120 to 140 liters of wastewater per day, including by flushing toilets, washing clothes, showering or bathing. In humanitarian precarious situations such as in the large camps, only around ten liters of wastewater are collected per person per day. These are often pure human feces.

The entire research project is located at the Centre of Competence for Sustainable Engineering and Environmental Systems (ZEuUS) at THM . The total costs are a good 628,000 euros. The Federal Ministry of Education and Research is funding THM with 242,500 euros. It is scheduled to last two years. In addition to the project partners Saygin & Stein, THM and EMW filtertechnik GmbH, the Lollar-Staufenberg association is an ideal project partner.