Sulfate reducing bacteria

Corrosion by Sulfate-Reducing Bacteria

Corrosion by sulfate-reducing bacteria was first discovered in the Netherlands in 1934. Sulfate-reducing bacteria, abbreviated as SRB, live in aquatic environments, deep underground and in the seas, where dissolved oxygen is minimal. In order to obtain energy and continue life under their hard shell, these organisms secrete special enzymes to break down sulfur compounds such as sulfates and use their oxygen.

What is sulfate-reducing bacteria?

Sulfate-reducing bacteria are a group of obligate anaerobic bacteria that do not require oxygen to live and obtain the energy necessary for their growth from the oxidation of organic matter along with the reduction of sulfate. Instead of oxygen, they consume substances such as sulfate, sulfur and other oxidized sulfur compounds as the final electron acceptor and reduce them to sulfide.

What is the structure of sulfate-reducing bacteria?

Sulfate-reducing bacteria are curved, elliptical, spherical, and filamentous in shape, 1 to 5 and 0.5 to 2 microns long and 0.5 to 2 microns wide, and often have a crown and are motile. Oxygen is harmful to bacteria and although it does not kill them, it causes them to become inactive. The most important sulfate-reducing bacteria are the disulfovibrio bacteria of the Clastidium family.

Sulfate reduction occurs according to the following reaction:

S2-+4H2O→ SO42-+8Hads

If steel is present at the site of the above reaction, the sulfide ion (S2-) formed reacts with iron and is the corrosion product FeS.

FeS+2H+ → Fe2++H2S

Characteristics of corrosion by sulfate-reducing bacteria

Iron sulfide (FeS) is black in color, which can be detected by adding a few drops of hydrochloric acid to it. If the smell of rotten eggs emanates from it, it is a characteristic of SRB bacteria corrosion.

FeCl2+H2S → FeS+2HCl

Their process of action is the reduction of inorganic sulfate and its conversion to sulfide. These bacteria prefer a neutral pH for growth, but their most suitable activity has been reported at pH 5 to 5.9.

What are the living conditions of sulfate-reducing bacteria?

Generally, these bacteria have been reported to live at temperatures of -5 to 104 degrees Celsius. The suitable temperature for their growth is (20-40) degrees Celsius. Freshwater, saltwater, and seawater sediments and mud, soil, sewage, groundwater, animal manure, sulfur springs, oil and gas wells, human and animal digestive tracts, around hot springs, and the depths of the sea are the places where these bacteria grow.

Anaerobic microorganisms

These types of organisms that do not use oxygen in the air are called anaerobic microorganisms. Of course, it has been proven that these types of anaerobic bacteria are not capable of causing disease, but in water extraction operations from deep underground, these bacteria penetrate transmission networks and cooling and heating systems, adhere to the surface of internal walls, and in the fight for survival, they are able to cause severe corrosion of iron metal in water systems. So that they have the ability to accelerate the reduction of sulfate compounds and the release of sulfur, and then the conversion of sulfur into the acidic substance hydrogen sulfide. Sulfate-reducing bacteria (SRB) act as catalysts in the reduction reaction.

Sulfate Reduction Reaction

In order for the reduction reaction to occur, the following four factors must be present simultaneously:

1. Sulfate-reducing bacteria (SRB)
2. Sulfate compounds
3. External energy source in the form of free electrons
4. Water temperature below 65°C

Methods for detecting SRB

Three methods are used to detect SRB in an aquatic system: sediment inspection, pH determination, and biological analysis. All three methods are explained below:

Sediment inspection:

Since hydrogen sulfide is a byproduct of the metabolic processes of microorganisms, if the sediments on the internal surfaces are closely inspected and a foul odor similar to rotten eggs is inhaled, it is almost certain that SRB is present in the environment.

pH determination:

As explained, hydrogen sulfide is a byproduct of the metabolism of microorganisms. In an aqueous system, hydrogen sulfide decomposition produces H+ ions and the environment becomes acidic. This action can reduce the pH of the environment to about 5. The decrease in pH in aqueous environments can be one of the reasons for the presence of sulfate-reducing bacteria in the environment. In order to maintain the pH at about 7, pH-increasing substances such as soda or a pH-adjusting solution should be added to the environment.

Biological analysis:

To determine the presence of sulfate-reducing bacteria (SRB) in the environment using the sampling method, the results are analyzed in two ways:

1. Transferring the sample to the laboratory
2. Adding the sample to a special test kit on site

The procedure for both methods requires adding a water sample or sediment to a device that contains nutrients for bacterial growth and its temperature is kept constant. If bacteria are present in the sample, the culture medium is ready for bacterial proliferation and continuation of its vital activities.

The laboratory method is expensive and requires the sample to be collected in a sterilized bottle and transported to the laboratory within 24 hours. The field method is generally cheaper and does not require extensive training or expertise. The field laboratory kit consists of a syringe and a vial (a small glass bottle, such as a medicine bottle) containing bacterial culture fluid, so that the water or sediment sample is injected into the vial and, if quantitative analysis is desired, the temperature is between 32-37 degrees.

The temperature is controlled at 100°C and if qualitative analysis is desired, the sample is stored at room temperature. If SRB is present in the sample, the concentration of sulfate ions decreases and is converted to sulfide ions, and a black layer of iron sulfide is produced on the sides of the vial within 28 days of the test period.

Reduction of SRB Sulfate Reducing Bacteria

In order to control and reduce the amount of SRB Sulfate Reducing Bacteria in a water system, the following measures are recommended:

1. Maintain water temperature above 65°C.
2. Remove dissolved air from heating and cooling systems to reduce ferrous metal corrosion and inject oxygen corrosion inhibitors
3. Drain system cooling tanks and coils at the beginning of the cold season
4. Circulate the contents of stagnant water throughout the hot and cold water system periodically, especially when the system is out of service
5. Chemical flushing of water system lines if they contain significant amounts of corrosion deposits.