Controlled treatment with nitrate in sewers to prevent concrete corrosion

It has generally been accepted that concrete corrosion is caused by bacterial oxidation of hydrogen sulphide in sewage systems. Costs related to sewer replacement and remediation are quite high, but there is limited knowledge and documentation on the relationship between hydrogen sulphide levels and corrosion rates. This is necessary information in order to select the appropriate means of hydrogen sulphide control and to conduct cost-benefit evaluations. This pilot scale study shows that the concrete corrosion rate can be modeled by a Monod type function with K_s=2 ppm H₂S in gas and a maximum concrete corrosion rate at 25°C of 16 mm/year. Complete hydrogen sulphide control with 0 ppm H₂S in water and gas is required to prevent concrete corrosion. This can be achieved by controlled treatment with nitrate using the Nutriox^® Concept where the nitrate dose is based on flow, temperature, sewer design, and sewage concentration. The local conditions will be important for the cost-benefit evaluations, but in general terms, one can say that the longer the hydraulic retention time is, the more cost effective will a controlled treatment with nitrate be for corrosion control.