Effect of parameters of biogenic hydrogen sulfide emission on degradation of concrete wells in sanitary sewage systems

Anaerobic conditions often develop in sewage systems with long wastewater Hydraulic Retention Time (HRT). This applies especially to pressure systems of sanitary sewage systems, where sewage flows through the entire cross-section of the pipe. Such conditions favor the formation of biogenic hydrogen sulfide (H2S).The resulting biogenic hydrogen sulfide may cause odor emissions into the environment, which may cause some nuisance to residents whose properties are located near sewage manholes. However, above all, hydrogen sulfide can cause the degradation of concrete elements of the sewage system, which is caused by sulfate corrosion of concrete.Research was carried out in the municipal sewage system, that discharges sewage from six villages. Significant degradation of concrete sewage manholes was observed after only five years of operation of the system. The degree of destruction varied greatly: from just a slight coating on the concrete surface to layers of corroded concrete several centimeters thick. After another six years, further significant destruction occurred, to the point of threatening the collapse of the well structure.To investigate the reasons for this state of affairs, measurements of hydrogen sulfide concentration were carried out in four selected expansion sewage wells. The measurement system consisted of two sensors: one placed just above the manhole base, the other located about 0.5 m under the manhole cover. The measurement was performed continuously for a period of at least 10 days for each well. The measured hydrogen sulfide concentration values ranged from 0 to values exceeding the sensor range (500 ppm).The operation register of the pumping stations was analyzed. On this basis, information about the expected sewage outflow from the pressure pipeline was obtained.The hydrogen sulfide concentration depended on the location of the well in the sewage system, which determined the residence time of sewage in the pressure pipeline preceding the well and generally influenced the residence time of sewage in the entire sewage system preceding the measurement site.However, the condition of the well was determined not by the maximum concentration of hydrogen sulfide, but by the total exposure of the concrete to hydrogen sulfide.