Tank Linings to Anaerobic Digesters, Lincolnshire

THE PROBLEM

The two new pre cast concrete anaerobic digesters required tank linings in order to achieve gas tightness and protect the concrete from hydrogen sulphide and biogenic sulphuric acid that would be present when the anaerobic digesters are operational.

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How does Hydrogen Sulphide attack concrete?

Hydrogen sulfide (H2S) is a corrosive and toxic gas that can react with concrete under certain conditions, leading to the degradation of the concrete structure. The attack on concrete by hydrogen sulfide primarily occurs through chemical reactions. Here are the key mechanisms by which hydrogen sulfide can damage concrete:

  1. Sulfuric Acid Formation:
    Hydrogen sulfide reacts with oxygen and water to form sulfuric acid:  H2​S+3/2O2​+2H2​O→2H2​SO4​
    Sulfuric acid is highly corrosive and can attack the cement paste in concrete. It reacts with the calcium hydroxide in the cement, forming calcium sulfate and water:  2H2​SO4​+2Ca(OH)2​→2CaSO4​+4H2​O
  2. Thiobacillus Bacteria Activity:
    Thiobacillus bacteria, which thrive in anaerobic conditions and are often present in sewage or wastewater environments, can further accelerate the degradation process. These bacteria oxidize hydrogen sulfide to produce sulfuric acid, contributing to the acid attack on concrete.
  3. Sulfate Attack:
    The reaction between hydrogen sulfide and concrete may lead to the formation of sulfides, which can react with calcium aluminate phases in the cement, forming compounds such as thaumasite: Ca(OH)2​+Ca3​(Al(OH)6​)2​+2H2​S→Ca3​Al2​(SO4​)(OH)6​↓+8H2​O
    Thaumasite formation can result in the expansion and cracking of concrete, further compromising its structural integrity.
  4. Corrosion of Reinforcement:
    The acidic environment created by the reaction of hydrogen sulfide with concrete can accelerate the corrosion of steel reinforcement within the concrete. Corroded reinforcement can lead to cracking, spalling, and a decrease in the load-bearing capacity of the structure.
  5. Hydrogen Embrittlement:
    Hydrogen sulfide can diffuse into the concrete and react with the cementitious phases, causing hydrogen embrittlement of the concrete matrix. This can lead to a loss of structural strength and integrity.

The severity of hydrogen sulphide attack on concrete depends on several factors, including the concentration of H2S, exposure duration, the quality of concrete, and environmental conditions. Structures located in environments with high concentrations of hydrogen sulfide, such as anaerobic digesters, sewage treatment plants, wastewater treatment facilities, or industrial settings, are particularly susceptible to this form of corrosion.

THE SOLUTION

We proposed the application of our proven anaerobic digester tank lining system that we have installed in over 50 anaerobic digestion industry tanks.

The first stage in the applying tank linings to these type of pre cast concrete anaerobic digester tanks is to make the structure water tight by sealing the gaps/joints between the panels in the roof. All areas to be sealed are high pressure jetted to removed surface laitance and contaminants.

A jointing primer is then applied to the panel edges, backer rod installed and a flexible polyurethane mastic/jointing compound installed. With the works taking place in the winter months the joints had to first be dried using roofing torches and all the application works completed immediately to ensure adhesion.

Internally the first stage of the tank lining process is the preparation of all the precast concrete areas to be coated. This is undertaken by method of high pressure water jetting to remove surface laitance, mould release and contaminants to ensure excellent proper adhesion of the new tank lining.

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All standing water is removed using pumps and wet lift vacuums prior to introduction of heating and dehumidification to dry out the tank internals and ensure the correct climatic conditions for tank lining application.

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Broadly for most tank lining applications the surface to which the tank lining is be applied must be 3 degrees above the dew point and the relative humidity below 85%, but as with all protective coating application you must check the individual tank lining materials technical data sheet for any specific climatic requirements.

As part our standard tank lining quality assurance the climatic conditions were tested and recorded along with the residual moisture content of the concrete.

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As the first stage of achieving gas tightness in the anaerobic digester which is was one of the key requirements for this tank lining application, all panel edges were primed using Remmers MT100 damp tolerant epoxy primer. This excellent epoxy primer can tolerate a residual moisture content of up to 6%, giving great flexibility of application.

Once the epoxy primer has cured all roof to roof joints are bridge using a bond breaker tape. The roof to wall intersection is sealed using a polyurethane jointing compound, which is also continued down the walls in the ‘gas space’.

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To achieve the gas seal SPI Corrolastic UB polyurea is then spray applied to a thickness of 2mm over all previously primed and sealed areas. This pure polyurea is ideal for this gas sealing operation as it can elongate nearly 400% before breaking and being spray applied is seamless. Because this material contains Specialty Products Incorporated ultra bond molecule it can adhere tenaciously to aged polymers that have exceed their overcoat /recoat window without requiring repreparation or re priming. This is of significant benefit in this tank lining application as there is naturally a gap between primer application and spraying of the polyurea due to the taping and mastic application prior to spraying of the polyurea.

Following the application of the polyurea tank lining material Remmers Epoxy Universal was then applied by method of airless spray, in contrasting colours as per best tank lining practice. The first coat is back rolled after spraying to force the material into any pores in the concrete.

As per our standard quality assurance and tank lining best practice the cured coating is then tested for pin holing using a DC Holiday spark tester. Any defects identified were marked prior to making good using the same material applied by brush. This is essential as a single pin hole in an aggressive hydrogen sulphide rich environment such as an anaerobic digester is a point at which degradation of the concrete can commence.

How does a holiday / spark tester work?

A holiday tester, also known as a spark tester or a pinhole tester, is a device used to detect and locate defects or discontinuities in the protective coatings of various materials, such as pipelines, tanks, and concrete structures. These defects, often referred to as “holidays,” can compromise the integrity of the coating and lead to corrosion or other issues. The holiday tester works by applying a high voltage to the coated surface and detecting electrical discontinuities that may indicate breaches in the coating.

Here’s a basic explanation of how a holiday tester works:

  1. High Voltage Source:
    The holiday tester is equipped with a high voltage source, typically provided by a battery-operated circuit within the device.
  2. Voltage Adjustment:
    The user can adjust the voltage level based on the specific requirements of the coating being tested. Different coatings may have different dielectric breakdown strengths.
  3. Probe or Brush:
    The device is equipped with a probe or brush that is brought into contact with the coated surface. This probe is connected to the high voltage source.
  4. Voltage Application:
    When the probe is in contact with the coated surface, the holiday tester applies a high voltage to the coating. This voltage is higher than the dielectric strength of the coating material but not high enough to damage it under normal circumstances.
  5. Detection of Holidays:
    If there is a holiday or breach in the coating, the high voltage can penetrate the defect and create a spark or a current flow between the probe and the underlying substrate.
  6. Audible or Visual Alarm:
    The holiday tester is often equipped with an audible or visual alarm system that signals the presence of a holiday. This alert indicates that the coating has a breach at the location where the tester is applied.
  7. Identification and Repair:
    The user can identify the location of the holiday based on the audible or visual signals. Once identified, the damaged area can be marked for repair or further inspection.

It’s important to note that holiday testing is commonly used for non-destructive testing of protective coatings, such as paints and epoxies, to ensure their effectiveness in preventing corrosion. The method is widely employed in industries like pipeline construction, tank lining, and concrete structure protection.

The voltage level applied by the holiday tester is carefully controlled to avoid causing damage to the coating. The effectiveness of the holiday testing process depends on the skill and experience of the operator, as well as the proper calibration and functioning of the testing equipment.