ABSTRACT

Rapid deterioration of concrete sewer pipes due to microbial activities has a major impact on environmental health and safety, particularly in countries with warm climates, such as the USA and Mexico. This is a process termed "Microbial Induced Concrete Corrosion" (MICC).

An experimental system was set-up to demonstrate the mechanism of MICC by measuring pH, sulfate formation, and loss of material in an accelerated setting and to later investigate conditions that are similar to those commonly found in sewer collection systems.

In a system with the continuous water circulation and sulfur oxidizing microorganisms (SOM) + Acidiphilium cryptum inoculation, significant pH drop (10 to 3), mass loss of 0.39%, sulfate formation, and calcium release were observed within 100 days. A continuous hydrogen sulfide concentration in the system keeps the MICC process going. It seems the corrosion is initiated by neutrophilic sulfur oxidizing microorganisms (NSOM). Based on these results, the mechanisms for the MICC can be explained by hypothesizing that the growth of NSOM provides favorable conditions for the subsequent acidophilic sulfur oxidizing microorganisms (ASOM) growth in a new concrete sewer pipe.

INTRODUCTION

Microorganisms that produce sulfuric acid catalyze the deterioration of concrete sewer pipes in the process known as the Microbial Induced Concrete Corrosion (MICC). MICC progresses through three distinct stages, which encompass changes in active microorganism populations and concrete properties that can be the basis for modeling MICC.

In the first stage, chemical reactions with carbon dioxide (CO2) and other acidic gasses in the headspace of sewers cause the pH of moisture condensing on concrete pipes to drop from values of about 13, to about 9; this pH drop is the result of abiotic processes and no microorganisms have been associated with this initial stage of MICC.

The second stage of MICC is the colonization of condensate on the pipe crown surfaces by sulfur oxidizing microorganisms (SOM). This population of bacteria utilizes the reduced sulfur compounds, such as hydrogen sulfide (H2S) and other species emitted from sewage, producing elemental sulfur and polythionic acids, which further lower the pH of the concrete surface to below neutral conditions. As the pH of the concrete drops, it has been hypothesized that neutrophilic sulfur oxidizing microorganisms (NSOM), such as Thiobacillus neapolitanus and Thiobacillus thioparus, continue colonization, producing even more acid until the low pH limits their growth (pH below 3). Beyond this, a population of acidophilic sulfur oxidizing microorganisms (ASOM), such as Acidithiobacillus thiooxidans, and acidophilic heterotrophic bacteria such as Acidiphilium cryptum becomes established and continues the oxidation of hydrogen sulfide (H2S) to sulfuric acid. Autotrophic ASOM can sustain this oxidation at pH levels below 3.

The growth of ASOM in a biofilm on the concrete surfaces is accompanied by copious acid production, which is the actual cause of sustained corrosion by the solubilization of minerals in the concrete matrix. The acid produced by bacteria diffuses into fresh concrete, reacts with concrete binders and produces ettringite and gypsum that severely compromise the permeability and strength properties of the pipe.

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