But does a fourth treatment stage even make sense from the state government's point of view? After all, the Mainz sewage treatment plant has a particularly large receiving water body in the form of the Rhine. This means that the treated wastewater is further diluted when it returns to the natural cycle. Nevertheless, the opinion of the responsible Ministry of Environment, Energy, Food, and Forestry (MUEEF) is clear in this case.

"... The following reasons, among others, speak in favor of establishing a fourth treatment stage at the Mainz wastewater treatment plant:

• the trace substance load of the treated wastewater from the Mainz wastewater treatment plant. Currently, around 100 tubes of diclofenac, for example, still enter the Rhine with the treated wastewater.
• Together with the other large wastewater treatment plants on the Rhine that have already been retrofitted or are under construction or in the planning stage, the pollution of the Rhine with trace substances can be reduced to a relevant extent.
• Efficiency reasons (cost/benefit) for a large wastewater treatment plant."

Excerpt from a letter from the MUEEF to the municipal utility company.

A fourth treatment stage for sewage treatment plants is not (yet) required by law. However, because subsidies alone will not be sufficient, fees are necessary to pay for the remaining construction costs and the operation of the fourth treatment stage. Is this possible? Yes, says MUEEF after consulting with the Ministry of the Interior and Sport.

"... I can therefore confirm to the business that the reasons will be included in the water law decision, together with the requirements for trace substance elimination."

Dr. Erwin Manz, Head of Water Management, MUEFF

Politically not a must! Technically not mature! Unnecessary if only what is actually allowed to enter the wastewater ends up there! There are many concerns about a fourth treatment stage at this point in time. "And we have, of course, considered these concerns and weighed up the pros and cons," emphasizes Jeanette Wetterling. Ultimately, however, one argument outweighed all doubts: "The will to do something," says the CEO, "for water protection, environmental protection, and thus also the protection of each and every one of us."

This refers to a technical addition to existing sewage treatment plants. With its help, trace substances, i.e., tiny pollutants such as pharmaceutical residues, hormones, microplastics, but also multi-resistant germs, can be removed from wastewater more effectively. Plants that are already equipped with a fourth treatment stage have relied on either activated carbon or ozonation.

Among other things, the TU study found that both processes can be used together in Mainz. This allows their respective advantages to be combined, thereby optimizing the treatment performance.