In a water treatment plant, the air above an open chlorine contact basin is a corrosion chamber. Chlorine gas, water vapor, and concrete surfaces create an atmosphere that accelerates the corrosion of any metal in the headspace. Steel walkways spanning these basins — providing operator access to weir gates and sampling points — are among the highest-maintenance structures in the entire facility.
FRP walkways have been specified into water and wastewater treatment plants for decades now, and the performance record is straightforward: they don't corrode in chlorine-laden humidity. That alone accounts for most of the specification decisions, but there's more to the picture.
The chemical environment inside a treatment plant
Water and wastewater treatment is fundamentally a chemical processing industry. The chemicals are different from a petrochemical plant, but the corrosion mechanisms are equally aggressive:
- Chlorine gas and sodium hypochlorite: Used for disinfection. Chlorine gas in the presence of moisture forms hydrochloric acid. Sodium hypochlorite (liquid bleach) is alkaline and strongly oxidizing. Both attack steel, and the attack is concentrated in the headspace above open channels and basins where the concentration is highest.
- Ferric chloride and aluminum sulfate (alum): Coagulants used in primary treatment. Both are acidic in solution — ferric chloride solutions typically have a pH below 2. Spills and drips around chemical feed areas create localized acid attack on steel grating and supports.
- Hydrogen sulfide: Present in wastewater headworks, primary clarifiers, and sludge handling areas. H₂S combines with moisture to form sulfuric acid on exposed surfaces. This is the same mechanism that corrodes concrete in sewer systems — and it attacks steel grating with equal enthusiasm.
- Sodium hydroxide and lime: Used for pH adjustment. Caustic solutions attack aluminum and can cause stress corrosion cracking in stainless steel under certain conditions. FRP with vinyl ester resin handles both acidic and alkaline exposure without degradation.
Where FRP walkways appear in treatment plants
The walkway locations in a treatment plant are often directly over the process — operators need to access equipment above open tanks and channels. This means the walkway structure is continuously exposed to the headspace atmosphere of the process below:
- Chlorine contact basin covers and walkways: Grating spans across open disinfection channels, exposed to chlorinated water vapor. Steel grating in this application often shows visible rust within 2 years. FRP grating, with vinyl ester resin for maximum oxidizer resistance, is the standard replacement material.
- Chemical feed building platforms: Access platforms around bulk chemical storage tanks and day tanks, where drips and spills are part of normal operation. The combination of multiple incompatible chemicals in one area makes a universal corrosion-resistant material necessary.
- Headworks and screening areas: Raw wastewater entry points where H₂S concentrations are highest and the atmosphere is continuously humid. FRP grating and structural supports resist the combined biological and chemical attack.
- Digester and sludge handling areas: Access walkways around anaerobic digesters and sludge thickeners, exposed to methane, H₂S, and high humidity. Non-sparking property of FRP is an additional safety benefit in areas where flammable digester gas may be present.
Typical specification in water/wastewater service
| Parameter | Typical Value | Application Note |
|---|---|---|
| Grating type | Molded FRP, 38 mm, open mesh, vinyl ester resin | Vinyl ester preferred for combined acid/alkali/oxidizer resistance |
| Cover options | Solid FRP cover plates or checkered plate in chemical feed areas | Containment of drips and spills beneath walkway |
| Support structure | FRP pultruded beams or FRP-encased concrete | No exposed steel anywhere in the walkway assembly |
| Fasteners | FRP bolts with FRP nuts (full non-metallic) | 316 SS acceptable in less aggressive areas; full FRP in headspace over basins |
| Handrail | FRP pultruded system, 1,100 mm height, with toe board | OSHA-compliant for access over open tanks |
| Slip resistance | Grit-top, concave profile available | Wet conditions are continuous; grit-top maintains friction coefficient above 0.6 wet |
| UV resistance | UV-stabilized for outdoor sections | Many treatment plants are partially or fully outdoors |
| Anti-microbial | Available as gel coat additive for critical areas | Optional; not standard but specified in some food-grade or pharmaceutical water systems |
"The steel grating over our chlorine contact basin required replacement every 4 years. The FRP replacement, installed in 2016, has been in service for 9 years with no corrosion — the only maintenance has been hosing off silt deposits during annual basin cleaning."
This page describes where FRP walkways are used in water and wastewater treatment. For the full industrial walkway system overview, see FRP Walkway Systems — Industrial Applications. For corrosion prevention strategies, see Corrosion Prevention Solutions.