Is hydrogen sulfide dangerous in water

If the sewers smell like rotten eggs, then hydrogen sulfide is involved. As long as you can smell the poisonous gas, there is still no danger. If the proportion of hydrogen sulfide in the air you breathe exceeds 200 millionths, i.e. 200 ppm, it numbs the sense of smell. "At 500 ppm the colleague falls unconscious," said Frank Lessig, head of the OEWA branch, drastically. Then there is a mortal danger. That is why an employee never steps into a sewer without a rescue harness and a colleague to secure it. The hydrogen sulfide content is measured beforehand. "The measuring devices are checked every two months," said Lessig.

There are problems with the toxic gas in the waste water network of the AZV Döbeln-Jahnatal. And it's not just dangerous for employees. It also attacks the concrete of the pipes. So much so that the OEWA has to think about technical changes and concrete renovation. The water service provider commissioned a study for this.

In several places high levels of hydrogen sulfide have been measured in the sewer system. For example at the WelWel sports and leisure center, where a lot of warm water is discharged. “The bacteria like that,” said Lessig. Up to 400 ppm hydrogen sulfide were found there. Much less is enough to cause damage to the concrete. In connection with water, an acid is formed that dissolves the lime. Even at a relatively low concentration of 20 ppm, up to 15 millimeters of concrete are dissolved per year. When three to four centimeters have been removed, the steel reinforcement of the shafts is exposed. Values ​​of up to 570 ppm were found during the measurements in various shafts.

Hydrogen sulfide is mainly produced where wastewater has to travel for hours in the absence of air. And these are the pressure pipes in the sewage system. Bacteria release the oxygen from the sulfur sulfate it contains. The sulfur combines with hydrogen, explained Lessig. When the wastewater relaxes in the so-called spring shafts at the end of the pressure pipes, the hydrogen sulfide produced escapes. The more wild the wastewater is whirled around, the more so. That is why it is helpful if the drainage conditions are improved, says Lessig.

In the sewer system at the WelWel sports and leisure center and in the Roßweiner Strasse pump shaft, the damage is so great that the concrete has to be renovated. In the Mockritz industrial park, where the problem was first noticed, the OEWA successfully tested the chemical treatment of the wastewater. The hydrogen sulfide can be neutralized by adding ferric chloride through an automatic dosing system. The test in Mockritz achieved very good values, says Lessig.

The OEWA can hardly change the basic problem, the long pressure lines. The wastewater would not travel that long in thinner pipes. But in order to be able to clean the pressure lines, they must have a width of at least 80 millimeters, said Lessig. When the plants were built ten to 15 years ago, there was hardly any experience with hydrogen sulfide. “Today we are further,” says Lessig. In general, plastic pipes are much more resistant to the influence of acids. But concrete pipes are cheaper.

The OEWA operates all wastewater systems for the AZV Döbeln-Jahnatal. The 287 kilometers of sewer pipes end in six sewage treatment plants. 30 pumping stations create the wastewater for cleaning. Last year the OEWA flushed 28 kilometers of sewer and inspected 8.7 kilometers with a camera. With a cleaning capacity of 1.8 million cubic meters of wastewater per year, the Döbeln-Masten sewage treatment plant is the AZV's largest.