Water and Wastewater Protective Coating Systems
Article 06 of 24
Hydrogen Sulfide, Biogenic Sulfuric Acid, and Microbiologically Influenced Corrosion
Hydrogen sulfide can be both a deadly atmospheric hazard and the beginning of a biological corrosion process that severely damages concrete, steel, equipment, and wastewater infrastructure.
Safety Comes Before Corrosion Inspection
Hydrogen sulfide, commonly abbreviated H2S, is a colorless, flammable, highly hazardous gas. It may be present in sewers, manholes, wet wells, lift stations, digesters, tanks, vaults, enclosed treatment structures, and other wastewater environments.
Hydrogen sulfide may initially smell like rotten eggs, but the sense of smell can quickly become fatigued. Odor must never be used to determine whether an atmosphere is safe.
The gas can accumulate in low areas and confined spaces. High concentrations can rapidly cause collapse, unconsciousness, respiratory failure, and death. It can also contribute to a flammable atmosphere.
No one should enter a suspected hazardous atmosphere merely to inspect coating damage. Entry requires the employer’s hazard evaluation, atmospheric testing, ventilation, permit-required confined-space procedures when applicable, trained personnel, appropriate respiratory protection, communication, attendants, and a workable rescue plan.
The Corrosion Process Is a Chain of Conditions
Biogenic sulfuric-acid corrosion does not begin with sulfuric acid being poured onto the concrete. It develops through a sequence involving wastewater chemistry, microorganisms, hydrogen sulfide, moisture, oxygen, and the concrete surface.
- Wastewater contains sulfate and biodegradable organic material.
- Under low-oxygen or anaerobic conditions, sulfate-reducing microorganisms can produce dissolved sulfide.
- Hydrogen sulfide can be released from the wastewater into the air above it.
- The gas reaches moist concrete and other surfaces in the headspace.
- Sulfur-oxidizing microorganisms can colonize suitable damp surfaces.
- Biological oxidation produces sulfuric acid at the surface.
- The acid reacts with and progressively destroys susceptible cementitious material.
This chain helps explain why the most severe concrete loss may occur in the headspace above the wastewater rather than below the liquid line.
Where Hydrogen Sulfide Problems Develop
Hydrogen-sulfide generation and release are influenced by wastewater composition and operating conditions. The contractor is not expected to design the treatment process, but should understand why some locations are more aggressive than others.
Conditions that may contribute include:
- Long wastewater detention times
- Low dissolved-oxygen conditions
- Warm wastewater temperatures
- High organic loading
- Sulfate availability
- Stagnant or slow-moving wastewater
- Force-main discharge and pressure release
- Turbulence, pumping, drops, and aeration that release dissolved gas
- Enclosed headspaces with limited ventilation
- Moist surfaces that support biological activity
Common High-Risk Locations
Collection System
- Manholes
- Interceptors
- Force-main discharge structures
- Junction structures
- Siphons and enclosed pipelines
Pumping Facilities
- Wet wells
- Lift stations
- Valve vaults
- Pump rooms with connected process openings
- Covered channels and transitions
Treatment Plant
- Headworks
- Covered primary-treatment structures
- Digesters
- Sludge-handling areas
- Odor-control ductwork and equipment
How Sulfuric Acid Damages Concrete
Portland-cement concrete contains alkaline cement paste that binds the aggregate together. Sulfuric acid attacks susceptible cementitious compounds, producing reaction products and progressive loss of the paste.
As deterioration advances:
- The surface may soften
- Cement paste may be lost
- Fine aggregate may become exposed
- Coarse aggregate may become prominent or loosen
- The surface profile may become extremely irregular
- Section thickness can be reduced
- Reinforcing steel may become exposed
- Structural capacity may eventually be affected
A coating cannot restore lost structural capacity. Severely deteriorated concrete must be evaluated by qualified personnel and repaired to a sound, properly configured substrate before the protective system is installed.
How the Environment Affects Steel and Equipment
Hydrogen-sulfide environments can also damage carbon steel, galvanized components, electrical equipment, instrumentation, fasteners, ladders, supports, ventilation systems, and other metallic components.
Moisture, condensation, acidic deposits, chlorides, process chemicals, and dissimilar metals can combine to create severe localized corrosion. Thin edges, welds, bolts, crevices, horizontal ledges, and areas that retain condensate deserve particular attention.
The contractor should not assume that a lining selected for concrete is suitable for every metal component. Steel preparation, primers, stripe coats, film thickness, flexibility, and transition details may require a separate system.
Recognizing Possible Biogenic Corrosion
Visual symptoms can indicate a problem, but laboratory analysis, pH testing, material evaluation, structural assessment, and process data may be needed to establish the cause and severity.
Possible indicators include:
- Soft, friable, or paste-like concrete surfaces
- Exposed aggregate
- Loss of concrete at the crown or upper walls
- White, yellow, gray, or dark deposits
- Low surface pH
- Corrosion concentrated above the normal liquid level
- Blistered, cracked, undercut, or detached coatings
- Rusting fasteners, ladders, supports, and equipment
- Recurring damage near turbulent discharge points
Do not coat over deteriorated concrete because the surface appears dry or hard in isolated locations. The complete area must be evaluated to determine the depth and limits of unsound material.
MIC Is a Broad Term
Microbiologically influenced corrosion, often abbreviated MIC, describes corrosion or deterioration affected by microorganisms and their activity. The microorganisms do not necessarily consume the substrate directly. They can change the local chemical environment, create corrosive products, form deposits, or establish differential conditions at the surface.
In wastewater concrete, the term is often associated with microbially influenced concrete corrosion or biogenic sulfuric-acid corrosion. For metals, MIC may involve different organisms, deposits, electrochemical mechanisms, and localized attack.
Do not use MIC as a substitute for investigation. Identify the substrate, exposure, process conditions, location, deposits, microorganisms when relevant, corrosion form, and contributing factors.
The Protective System Must Separate the Exposure from the Substrate
A protective lining acts as a barrier between the concrete or steel and the corrosive wastewater environment. For severe service, that barrier must remain bonded, continuous, chemically resistant, and sufficiently thick to tolerate the surface profile and operating exposure.
Possible systems may include:
- High-build epoxy linings
- Polyurethane systems
- Polyurea and hybrid lining systems
- Vinyl-ester systems
- Reinforced lining systems
- Cementitious or polymer-modified resurfacers beneath a protective lining
- Other specialized systems supported by the manufacturer for the defined exposure
The chemistry name alone does not establish suitability. Selection must consider chemical resistance, permeability, film thickness, flexibility, adhesion, substrate moisture, crack behavior, temperature, application conditions, detailing, cure, inspection, repair, and documented service history.
Surface Preparation Begins with Removing Unsound Material
A high-performance lining cannot compensate for weak, acid-damaged, contaminated, or improperly repaired concrete.
The preparation plan should address:
- Cleaning and decontamination before evaluation
- Removal of all unsound and chemically deteriorated concrete
- Evaluation of remaining concrete and reinforcing steel
- Structural repairs where required
- Compatible repair and resurfacing materials
- Required surface profile, cleanliness, and soundness
- Treatment of cracks, joints, penetrations, and transitions
- Concrete moisture and outgassing
- Written acceptance of the prepared substrate before lining application
Process Control and Coatings Work Together
A coating can protect the substrate, but it does not eliminate hydrogen-sulfide generation from the wastewater process. Owners may use operational, chemical, biological, ventilation, or collection-system measures to reduce sulfide generation, gas release, odor, or corrosion.
The coating contractor should understand the intended control strategy because it can affect:
- Expected exposure severity
- Ventilation and isolation during construction
- Chemical contamination on existing surfaces
- Shutdown and bypass planning
- Future inspection and maintenance access
- Whether an operating change could increase exposure after the coating is installed
The most durable protection often combines appropriate facility operation, corrosion control, surface rehabilitation, a correctly selected lining, controlled installation, inspection, and planned maintenance.
Pre-Work Investigation Checklist
- Identify the process function of the structure.
- Review wastewater characteristics and available hydrogen-sulfide data.
- Identify immersion, wet-and-dry, splash, and headspace zones.
- Review ventilation and odor-control systems.
- Establish safe access and atmospheric-monitoring requirements.
- Clean the surface sufficiently to expose its actual condition.
- Map concrete deterioration, exposed aggregate, cracks, joints, and reinforcement.
- Map corrosion of steel, equipment, fasteners, and supports.
- Determine whether structural evaluation is required.
- Define removal, repair, resurfacing, preparation, and lining requirements.
- Confirm the coating manufacturer’s written suitability for the exposure.
- Establish inspection, testing, repair, documentation, and maintenance requirements.
Never Attempt an Unplanned Rescue
Hydrogen-sulfide incidents can injure or kill the first worker and then kill coworkers who enter without protection to attempt a rescue.
Rescue capability must be planned before entry. Workers should follow the employer’s confined-space program, entry permit, atmospheric-testing requirements, ventilation plan, communication procedures, respiratory-protection program, and rescue arrangements. This article is educational and is not a substitute for required training or a site-specific safety program.
Article 06 Knowledge Check
- Why must odor never be used to determine whether hydrogen sulfide is present at a safe concentration?
- Describe the chain of events that can produce biogenic sulfuric-acid corrosion on concrete.
- Why may the wastewater headspace deteriorate faster than the continuously immersed area?
- What visual conditions may indicate acid-related concrete deterioration?
- Why should MIC be treated as a starting point for investigation rather than a complete diagnosis?
- Why must unsound concrete be removed before a protective lining is installed?
- Why should process control, protective coatings, inspection, and maintenance be treated as connected parts of corrosion management?
Technical References and Further Study
- U.S. Environmental Protection Agency: Hydrogen Sulfide Corrosion in Wastewater Collection and Treatment Systems and related detection, control, and rehabilitation resources.
- OSHA: Hydrogen Sulfide Safety and Health Topics, workplace hazards, exposure evaluation, controls, and applicable standards.
- NIOSH: Pocket Guide to Chemical Hazards - Hydrogen Sulfide.
- OSHA: 29 CFR 1910.146, Permit-Required Confined Spaces, and other standards applicable to the work.
- AMPP: Technical resources addressing biogenic sulfuric-acid corrosion, concrete deterioration, protective coatings, and wastewater infrastructure.
- AMPP: SSPC Guide 27, Recommended Performance Properties for Liquid-Applied Organic Polymeric Coatings and Linings for Concrete Structures in Municipal Wastewater Facilities.
- Coating manufacturer: Written chemical-resistance, substrate-preparation, application, cure, testing, and service-suitability requirements for the proposed system.
Exposure limits, regulations, standards, and technical guidance change. Employers must evaluate the actual workplace and follow current requirements governing hydrogen sulfide, confined spaces, respiratory protection, ventilation, atmospheric monitoring, and emergency response.
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