Chlorine in drinking water: measurements show very low concentration at Offenbach measuring points

To ensure that all Offenbach residents continue to have sufficient drinking water in the future, the city councillors decided in 2023 to join the Hessisches Ried water association. Hessenwasser has also been supplying drinking water to Offenbach since August 13, 2025. Small amounts of chlorine are added to this water for disinfection. The limit value specified in the Drinking Water Ordinance is 0.3 milligrams of chlorine per 1 liter of water. The chlorine concentration in the water supplied is 0.1 milligrams per liter, which is well below the legal limit or the minimum value. Hessenwasser's water is supplied via a transfer point at Kaiserlei and from there into the pipes in Offenbach's municipal area. It mixes with the non-chlorinated water from the ZWO and the mixing and reaction with organic substances in the pipe system continuously reduces the concentration of chlorine. After this water was added, the port area reported that a chlorine smell and change in taste were noticed. This led to concerns and worries in the affected districts and beyond as to whether the drinking water posed a health risk.

Mayor and Head of Health Sabine Groß understands the concerns of the residents: "Water is our most important foodstuff - so I can well understand that citizens are unsettled when the composition changes and smells of chlorine. I also understand the displeasure that the ZWO did not provide information in advance about the supply of chlorinated water and, as part of this, also directly about the amount of chlorine in drinking water, the applicable limits and the effect of chlorine in drinking water. The current measurements confirm that the chlorine concentration is kept to a minimum, that the limit values have not been exceeded at any measuring point and that there is no health risk."

The municipal supply network is subject to monitoring by the municipal health department, which in turn is monitored by the Hessian State Office for Health and Care. The data collected must also be reported to the federal government and the EU, which also ensure compliance with the guidelines. The well and spring areas that are not located in the city of Offenbach are monitored by the regional health authorities.

"I am in close contact with the ZWO Board. The board agreed on a whole package of measures in a closed meeting immediately after it became known," Groß continues. For example, additional measuring points were set up at various points in the northern part of the city where the water quality is regularly determined. These measurements have shown that the chlorine concentration in Nordend and Westend is below the detection limit. In the port, 0.1 milligrams per liter were measured and at one measuring point initially 0.11 milligrams per liter. A further measurement measured 0.08 milligrams. The ZWO is working on a concept to optimize the mixing of chlorinated Hesse water with unchlorinated water from the ZWO.

The Senior Citizens' Council raised the question of a possible health risk from disinfection by-products in drinking water, so-called trihalomethanes (THM).

"The ZWO also tested the water for the substances named in the study. The sampling was carried out in the area where chlorine was still detectable in the water, i.e. in the port area and at the transfer point at Kaiserlei. The results are available to us and at none of the four measuring points was the legally permissible quantity of disinfection by-products exceeded, nor was the much lower quantity from the study referred to by the Seniors' Council," says Groß.

The formation of these disinfection by-products depends on many factors, including the ingredients of the drinking water and the disinfection processes used," says Dr. Faust. The Drinking Water Ordinance, which is based on an EU regulation, takes these products into account and has even issued stricter limit values for them than the EU.

"The samples for measuring these disinfection by-products - trihalomethanes, chlorite and chlorate - were taken on Monday. According to the results, the sum of the trihalomethane concentrations is 5.4 µg/l, which is below the limit value and also below the value of the threshold of 15 µg/l mentioned in the Swedish study as hazardous to health. This is the study referred to by the Seniors' Council," explains Dr. Faust. Dr. Christiane Faust, Head of the Health Department, assures: "The Drinking Water Ordinance sets very strict, toxicologically based limits for chlorine and disinfection by-products in water, and even these are far lower in Offenbach. The Drinking Water Ordinance also stipulates the frequency of testing. Vulnerable groups such as pregnant women, children and senior citizens can use the water".