Because of government mandated industrial pretreatment, longer detention times for waste water due to construction of regional treatment plants, recent air quality regulations, and other factors, concrete structures in waste water treatment plants are exposed to more severe exposure conditions now than in the past. Protective coating systems which had performed successfully for many years no longer provide adequate protection. The result is frequent coating failure and rapid concrete degradation. This paper discusses the more aggressive service conditions today and the changes which have promoted them. The paper will give examples and then present suggested material selection criteria to be used in the future by engineers and operators to ensure successful long term corrosion protection of concrete structures in municipal Waste Water Treatment Systems.
Hydrogen sulfide <H2S) generation in municipal waste water treatment plants has always been present. It has caused corrosion in the form of sulfuric acidic attack of portland cement concrete in sewer collection/treatment systems for over 100 years. Gaseous H2S condenses on aerated, wet concrete surfaces and is metabolized by sulfur oxidizing bacteria or SOB and is oxidized to form dilute sulfuric acid (H2S04). Through the late 1970''s, it was commonplace to periodically encounter H2S concentrations within sewer pipe crowns and other headspaces in conveyance treatment structures as high as 10 to 15 ppm. In such cases, acidic attack of concrete caused a gradual loss of hydrated cement paste. Occasionally, during periods of unusually low sewer flows and elevated ambient temperatures, such as during periods of low rain fall in hot months, even higher H2S concentrations (30 to 50 ppm) were encountered for short periods of time. When frequent, these higher H2S levels accelerated the corrosion of the concrete where up to 1/2''” of concrete was lost in a year.
In general, however, the H2S levels encountered in headspaces were generally below 10 ppm in most municipal sewer system locations prior to the 1980''s and 1990''s. There were obviously exceptions to this in some larger regional systems. Average gaseous H2S concentrations for most systems in the 1970''s were approximately 2 to 7 ppm in pipe crowns and pump station headspaces unless unusual circumstances occurred. This average H2S concentration resulted in concrete corrosion but, it was very gradual. One of the major reasons for these relatively low H2S concentrations was the presence of heavy metals which reduced aqueous sulfide levels in the waste water.
Following the passage of the Clean Water Act in 1980, industrial pretreatment was mandated to reduce or eliminate the presence of heavy metals from waste water discharges. As such, all industrial plants sewering metals such as zinc, lead, iron, mercury, chromium, and copper were required to pre-treat their effluent (for removal of heavy metals) before sending it on to municipal waste water treatment agencies. These heavy metals had been toxins for the anaerobic bacteria found within the slime layers on concrete pipe and tank surfaces.