Green is the color of hope.

If in a few years what is currently hidden in an inconspicuous green container on the site of Wiesbaden's main sewage treatment plant has been replicated on a large scale, then the wastewater from the almost 300,000 residents, commuters and companies could be discharged into the Rhine much cleaner than has been the case up to now allow three cleaning levels.

A good two and a half years after the start of a pilot project for wastewater treatment, Susanne Lackner from the Technical University of Darmstadt draws an encouraging interim balance.

Oliver Bock

Correspondent for the Rhein-Main-Zeitung for the Rheingau-Taunus district and for Wiesbaden.

  • Follow I follow

The two-stage process is promising: the so-called after-runs of the three cleaning stages are mixed with activated carbon in a first step and pumped through a plastic membrane in a second step.

According to previous analyzes, only ten out of 100,000 bacteria survive, and the microplastics still in the water are even filtered out 100 percent.

"We don't want them there anymore"

Scientists are convinced that the fourth (activated carbon) and fifth (membrane) cleaning stages can save lives.

Because they get the dangerous multi-resistant, pathogenic germs and viruses out of the water.

"We don't want them there anymore," says Lackner, even if the legislator does not yet explicitly prescribe the cleaning level.

However, those responsible at the Wiesbaden waste disposal companies assume that sooner or later the European Water Framework Directive will prescribe the "bacterial" treatment of the wastewater after the mechanical and biological ones.

At the disposal companies, the management has already thought about the costs for a large-scale solution.

A rough estimate, non-binding "house number" is 30 to 35 million euros.

In addition, the electricity requirement of the sewage treatment plant could increase by ten percent.

Broken down to the consumer, Lackner expects additional costs of a maximum of ten euros per person and year.

The remotely monitored test facility, through which up to 25 cubic meters of Wiesbaden waste water flow every hour, is to be in operation for another nine months in order to gain further experience and to further optimize the technology, processes and procedures.

Above all, Lackner wants to find out which fragments can still get past the membrane and why.

Because the plastic membrane has pores with a permeability of only 0.04 microns.

However, a bacterium is one to two micrometers in size, so it is actually impossible to get through.

But parts of the bacterium can still be dangerous and pass on their destructive DNA message.

Expect at least five years for expansion

It is still uncertain whether and when the expansion of the sewage treatment plant will take place.

Five to seven years can probably be estimated once the technical path has been clearly clarified.

The main wastewater treatment plant in Wiesbaden is one of the largest of its kind in Germany and can treat up to 7,000 cubic meters of wastewater per hour.

Around 50,000 cubic meters are cleaned every day and fed into the Rhine.

So far, this process has focused on mechanically removing undissolved contaminants and biologically breaking down dissolved substances using microorganisms.

Microplastics and drug residues as well as multi-resistant germs and other trace substances have so far not been taken into account because conventional sewage treatment plants are not designed for them.

However, Lackner is convinced that the membrane-supported powdered activated carbon process can bring the solution.

Because the germs and trace substances attach themselves to the activated carbon and can be easily removed.

Although this produces a small amount of further sewage sludge, it can be incinerated.

The research project is supported by the federal government and funded with 400,000 euros.

In fact, the project should have been completed by now.

But a virus called Corona also led to delays, as did the accident and subsequent blasting of the Salzbach Viaduct, because the green container was in the particularly vulnerable radius.

In the meantime, the test operation is running at full speed.

And Lackner is satisfied and optimistic: "It works."