Preventing Corrosion in Sanitary Collection Systems

Odor and corrosion problems in concrete sanitary sewer systems is a wide spread problem. The corrosion is the result of a two step biological process. Sulfate reducing bacteria present in the collection system convert sulfates into hydrogen sulfide gas. A series of oxidizing bacteria that reside on the sewer crown convert hydrogen sulfide gas into sulfuric acid which will eventually result in a substantial lowering of pH, frequently to values below 2.0, see Figure 1. Low pH conditions and corrosion problems are also similarly experienced in maintenance holes. This acid attacks the concrete, reducing it to a soft putty like gypsum. According to a 1992 EPA report, maintaining a surface pH of 4 or higher is sufficient to prevent an unacceptable corrosion rate.

An effective method to combat the corrosion problem is to spray the corroded sewer crown with a modified technical grade magnesium hydroxide suspension, that is formulated to resist surcharging. Field studies conducted in the City of Los Angeles demonstrated that a single application provided enough alkalinity to protect concrete surfaces for over a year.

The treatment process involves spray painting the technical grade magnesium hydroxide suspension onto the concrete surface requiring protection, to achieve a coating thickness of 100-125 mils. The magnesium hydroxide neutralizes any sulfuric acid present on the surface and raises the pH up to about 10.0. This high pH has the added benefit of deactivating the bacteria responsible for the acid generation, since they cannot tolerate high pH conditions. The magnesium hydroxide coating will also react with hydrogen sulfide gas thus helping to reduce potential odor problems. The coating is intended to be sacrificial, slowly being consumed by hydrogen sulfide, and will need to be replenished on a regular maintenance schedule. It has been estimated that annual treatment of sewers using magnesium hydroxide may extend sewer life by 20 years. Typical spray treatment will cost approximately 200 times less than rehabilitation.

Representative field results from spray treatment of concrete surfaces using THIOGUARD 1 are presented in
Figure 1.

 

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