Home page // Technical & Scientific Corner // Biogenic corrosion principles

Biogenic corrosion principles... Or how tiny bacteria can destroy your sewers

Biogenic Corrosion, or Microbiolocaly Induced Corrosion (MIC), occurs when specific strains of bacteria "colonize" the sewer surfaces and produce sulfuric acid that "eats away" the materials like concrete, steel, and cast iron. Although microscopic, such bacteria can cause significant damage over time.

In fact, Biogenic Corrosion is driven by the biological activity of several strains of "acidophilic" bacteria, i.e. bacteria that need an acidic environment to grow. A tiny biofilm of Thiobacillus Thiooxidans - also named Thiobacillus Concretivorus a few decades ago because they "eat away" concrete - can produce enough sulfuric acid to corrode up to 25 mm of concrete within a single year when the worst conditions are present. In order to choose the proper solution to this problem, it is essential to understand the biogenic corrosion mechanism.

Biogenic Corrosion: A four step process involving two range of bacteria

Biogenic Corrosion involves a complex ecosystem feeding on the nutrients found in septic waste water. This ecosystems "productivity" varies depending on the environment conditions (ambient temperature, relative moisture, retention time in the sewer, etc.), the more favorable, the conditions, the more sulfuric acid is produced. This ecosystems life can be summarized with the four step process presented here.

STEP 1: Production of H2S gas

Wastewater itself is rarely corrosive. However, under reducing conditions spurred on by anaerobic bacteria, the decomposing organic material in the wastewater produces hydrogen sulfide gas (H2S). In this first step, the H2S gas remains dissolved in the effluent and concentration builds in the laminar flow of sewage.

H2S: you need food and time

The production of H2S gas by anaerobic bacteria in the sewerage depends on several parameters: the availability of organic matter, enough time to digest it, and a favorable temperature for bacteria growth. Temperature of the sewerage is only one of the factors that contribute to maximizing the efficiency of bacteria to transform the organic matter. Biogenic corrosion can be a problem even for cities in colder climates because the sewer temperature is largely influenced by daily domestic and commercial use (warm water from showers, cooking, dishwashers, air conditioning cooling, etc.). Networks in areas with large seasonal variations of population - sunny leisure spots - can suffer because of much longer retention times during the off-season because the enhanced by the much lower flow of sewage. Flat terrain also makes it more difficult to have the necessary efficient slope and rapid, continuous flow, giving more time for bacteria to produce the detrimental H2S.

Release of H2S gas in the areas above the flow line

STEP 2: Release of H2S gas in the areas above the flow line

Eventually the dissolved H2S will seperate from the collected sewage either because the dissolved concentration in the effluent becomes too high, or because of turbulent flow conditions in pipes, manholes, pump stations or at the treatment plant. H2S gas is heavier than air; it tends to accumulate above the flowline in the sewer. H2S can sometimes have an odor like "rotten eggs", it is responsible for the "sewer smell" often sensed close to sewer vents. H2S is also a deadly gas at level as low as 15 ppm. This explains why the access to the confined space of a sewer requires very specific safety measures.

Feeding acidophilic bacteria

STEP 3: H2S feeding acidophilic bacteria

When the H2S gas comes into contact with the moist sewer surfaces above the flowline, a fraction of the H2S can combine with dissolved oxygen (02) to directly form sulfuric acid (H2SO4). But in general, most of the H2S is decomposed into elemental sulfur S. This is a source of energy for the Thiobacillus bacteria family; they are oxidizing the elemental sulfur and their metabolic waste is H2SO4. This sulfuric acid lowers the biofilm pH and corrodes the concrete. Hence the expression "Microbiologically Induced Corrosion" (MIC).

Thiobacillus Thiooxidans become predominant

STEP 4: Thiobacillus Thiooxidans become predominant

As their name indicates, "acidophilic bacteria" actively develop in acidic conditions. There are many different strains of "acidophilic bacteria" and, as shown in the graph, each strain has its own pH range for optimal metabolic activity. Over time, as the surface pH decreases, one strain will replace the preceding one as it creates a harsh environment that no longer supports the life of that particular strain. Around a pH of 5-6, Thiobacillus Thiooxidans will start to breed and multiply until the pH level reaches around 1, i.e. that it will try to produce enough sulfuric acid to maintain its environment at this optimal point. Neither Portland cement concrete, steel or cast iron can survive very long in such conditions. An expert microbiologist would mention that other acid producing living organisms are found in sewers, such as fungus; however, current understanding is that Thiobacillus Thiooxidans is the most common strain responsible for severe biogenic corrosion.

Each strain has its own pH range for optimal metabolic activity