The problem of the corrosion of sewage systems is especially severe in some areas. As usual, this is a result of a number of physical and chemical factors.
The first factor has a hydrodynamic nature: as a rule, it is water streams that flow at a high speed or fall fr om a high height that sometimes reaches up to 60 meters. It has be optimized by proper configuration of the sewage system and flows.
The second factor is usually a complex biochemical process. When the initial effluent has an acidic reaction and carries sulfur-containing compounds, for example, sulfates, hydrogen sulfide (H2S) is released from water into the gas phase with the appearance of a strong unpleasant smell. The released gas is adsorbed on the concrete walls of the sewage and is gradually oxidized by thiobacteria to sulfuric acid (H2SO4), which begins to corrode the concrete, as evidenced by a drop in its pH. If this process is uncontrolled, then the result is the destruction of the sewage. The speed of this process depends on the concentration of hydrogen sulfide, the grade of concrete and the presence/absence of protective layer.
Schematically, this process can be represented as follows:One of the most effective solutions to this problem is to adjust the pH of waste water from acidic to basic area. As a result, the equilibrium of the reaction below shifts to the left, contributing to the formation of aqueous S2- and not gaseous H2S.
S2- + 2H+ = H+ + HS- = H2S↑
Aqueous phase Gas phase
Below you can find a phase diagram showing the concentration of all substances depending on pH:
As can be seen at the pH of 8.5, the release of gaseous H 2S drops almost to zero. If there is no release of hydrogen sulfide into the gas phase, then there is no biochemical corrosion of the sewage system.
Typically, the reagent is introduced into sewage system wh ere the wastewaters are collected and start its way to municipal wastewater treatment plant. In this case, wastewater does not emit H2S along its entire way and coming with already optimum pH of 8-8.5 for biological treatment.
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