The aim of the research was to determine changes in the elemental composition of concrete under the influence of exposure to hydrogen sulphide in the existing sewage system. The system operator pointed to significant problems with odours in this system. The research included a fragment of the pressure sewage system. The concentration of gases: hydrogen sulphide, ammonia and methane was measured in selected wells. High concentrations of hydrogen sulphide (over 200 ppm) were recorded in the studies. Concrete samples were taken from the internal walls of the well for testing. The samples were used for a microscopic analysis of their composition, using an electron microscope with an EDS attachment. Also, concrete samples from a new sewage well were tested to compare their elemental composition. Gas measurements confirmed the problem of odors, while the analysis of the elemental composition showed a significant proportion of sulphur (from 7.53% to 42.9%) on the surface of the well compared to the reference sample (0%).

Extraneous water that inflow to the sewage system is basically divided into two streams—accidental water (mainly rainwater) and infiltration water. The aim of the research was to assess the amount of extraneous water inflow to the considered system. Five different quantitative approaches were applied. Three well-known methods were used: the triangle method, the minimum night flow method, and the moving minimum method. The annual balance of water consumption and sewage supply to the wastewater treatment plant were calculated. Also, some analysis of sewage discharge during wet and dry weather was carried out. The study covered data from 6 years from 2014 to 2019. It was established that the main source of extraneous water was infiltration, because three methods which concern both streams (triangle method, minimum night flow, variability in wet and dry weather) confirm the conclusion. Merely the moving minimum method results differ from the others. In this investigation, accidental water (basically rainwater inflow) poses a significantly less share in the total volume of sewage compared to infiltration water. The total amount of extraneous water was estimated as in the range from 38 to 53% of annual sewage supply to wastewater treatment plant, depending on the year. Share of infiltration and accidental water is changing in different methods. Share of infiltration was in a range between 18 and 68%, depending on the year and the method used. Share of accidental water was in a range between 7 and 22%.

The field test of the sewer system in a rural area was made. The results from two different pressure-gravity systems localized in two settlements were compared. The investigated sewer system operates in serial connection; the sewage from one settlement is pumped to the expansion well in the next one. In both systems, the high concentration of hydrogen sulfide, exceeding 200 ppm, and visible concrete wells corrosion effects were recorded. The samples of corroded concrete from the internal walls of the chosen wells were collected. The results of an electron microscope with the EDS attachment (Energy dispersive X-ray Spectroscopy) analysis showed a significant sulfur content in a concrete surface layer from 11.2% to 64.2% for the first system and from 7.53% to 42.9% for the second one. The exposure of concrete to high concentrations of hydrogen sulfide and relatively high content of sulfur on the surface of concrete might be a reason for hydrogen sulfide corrosion of concrete in the investigated systems.

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