The Pressure Is On: How High-Stress Sewage Tank Environments Promote Concrete Corrosion and Structural Damage

Wastewater treatment plants (WWTPs) are essential for processing sewage, often dealing with environments of extreme pressure and harsh conditions. Despite their robust design, the high-stress environments within these plants can lead to significant issues over time.

The Problem

After only two years of operation, pre-treatment tanks at a wastewater facility exhibited severe issues. The epoxy coatings designed to protect the concrete were failing over large areas, and the concrete itself showed deterioration extending more than 2 cm deep. This early and significant damage prompted a thorough investigation to determine the root causes.

Research Approach

To uncover the underlying issues, the research team conducted an extensive analysis, including:

- Design Documentation Analysis: Reviewed the original design and construction plans to verify the intended materials and specifications.

- Environmental Tests: Evaluated the conditions within the tanks by measuring:

- Temperature distribution on tank walls.

- Composition of the wastewater.

- Levels of CO2, H2S, and NH3 in the wastewater vapors.

- Laboratory Tests: Conducted on samples taken from the tank walls, these tests included:

- Compressive strength and freeze-thaw resistance assessments.

- Analysis of sulfate-ion content and pH distribution.

- Phase composition research and scanning electron microscope (SEM) observations.

- In-Situ Diagnostics: Directly tested the tank structures with pull-off and rebound hammer tests to assess the condition of the coatings and concrete.

Findings

The environmental tests indicated that while typical municipal sewage substances were present, they were not the primary cause of the problem. Instead, the high-pressure environment of the tanks played a crucial role:

- Microbiologically Induced Concrete Corrosion (MICC): The intense conditions led to the proliferation of sulfate-reducing bacteria (SRB) in the wastewater, which produced hydrogen sulfide (H2S). In the tank's headspace, H2S was oxidized to sulfuric acid (H2SO4), leading to concrete corrosion. The high vapor pressure and insufficient resistance of the epoxy coating allowed sulfate ions to penetrate the concrete rapidly, causing it to expand and leading to coating delamination and concrete degradation.

Corrosion Mechanism

The corrosion process involved several interrelated factors:

1. H2S Formation: Produced by SRB in the wastewater's organic-rich biofilms and sediments.

2. H2S Release: H2S entered the gas phase and diffused into the condensate film on the concrete surface.

3. Decrease in Concrete Alkalinity: H2S dissolved into the concrete, reducing its alkalinity.

4. Sulfide Oxidation: The oxidation of H2S to sulfuric acid further reacted with concrete hydration products, leading to deterioration.

Factors Influencing Corrosion

Several factors exacerbated the corrosion under high-pressure conditions:

- Wastewater Conditions: Variations in pH, biological oxygen demand, and dissolved oxygen levels.

- Environmental Conditions: High concentrations of gaseous H2S, flow velocity, turbulence, temperature, and relative humidity.

- Concrete Properties: Concrete’s permeability and composition significantly influenced its susceptibility to damage.

Preventive Measures and Conclusions

The study reveals that while the concrete was well-designed and constructed, the main issue was the coating’s inadequate resistance to sulfuric acid and high vapor permeability. To address these challenges, the following strategies are recommended:

-  Improved Protective Technologies: Developing solutions with better resistance to sulfuric acid and lower vapor permeability.

- Improved Concrete Mixes: Use mixes less prone to sulfate attack.

- Regular Maintenance: Implement routine inspections and maintenance to catch and address issues early.

Conclusion

This case study highlights the impact of high-pressure environments on wastewater treatment plant infrastructure. By understanding the mechanisms of concrete corrosion and coating failures, we can develop better materials and strategies to ensure the durability of these critical structures. Proactive measures and advanced technologies are essential for withstanding the extreme conditions faced by wastewater treatment plants.

For more insight, check out this article on Science Direct.