Water and Wastewater Protective Coating Systems
Article 03 of 24
Defining Immersion, Headspace, Splash, and Atmospheric Exposure Zones
A single tank, basin, wet well, or treatment structure may contain several different coating environments. Each exposure zone must be identified before the protective system is selected.
One Structure Does Not Mean One Exposure
It is tempting to describe an entire structure as water service, wastewater service, interior service, or exterior service. Those descriptions are too broad for reliable coating-system selection.
The bottom of a basin may remain continuously immersed. The area near the liquid line may alternate between wet and dry. Upper walls may receive splash and condensation. The enclosed headspace may collect hydrogen sulfide and corrosive vapors. Exterior surfaces may be exposed to sunlight, rain, temperature changes, and industrial contamination.
Each zone can impose different requirements for chemical resistance, permeability, flexibility, film thickness, surface preparation, detailing, inspection, and maintenance.
Zone 1: Continuous Immersion
Continuous-immersion surfaces remain submerged during normal operation. Examples include tank floors, lower walls, clearwells, wet wells, channels, basins, clarifiers, process tanks, and submerged pipe.
An immersion coating must maintain adhesion and barrier performance while exposed to the actual liquid, temperature, pressure, chemistry, solids, and biological conditions. A coating described only as moisture resistant or waterproof is not automatically suitable for continuous immersion.
Information Required
- Potable water, raw water, process water, wastewater, sludge, or chemical solution
- Normal and maximum liquid temperature
- Expected pH and chemical composition
- Continuous versus intermittent immersion
- Flow velocity, turbulence, grit, and abrasion
- Cleaning and disinfection chemicals
- Required cure time before filling or returning to service
For potable-water contact, the selected system may also require current NSF/ANSI/CAN 61 certification for the particular use and application conditions.
Zone 2: Wet-and-Dry Cycling
The wet-and-dry zone lies between normal operating levels or in areas periodically drained, filled, flooded, washed, or exposed during process changes.
This zone can be more demanding than steady immersion. Repeated wetting and drying may move moisture and soluble material through a porous substrate. Deposits may concentrate as water evaporates. Temperature changes can create movement and condensation. The surface may also receive ultraviolet light when the structure is open.
The contractor should document normal, minimum, maximum, and upset liquid levels. A single line shown on a drawing may not represent the full operating range.
Zone 3: Splash, Spray, and Washdown
Splash-zone surfaces are not continuously submerged, but they receive frequent contact from turbulence, aeration, leaking equipment, overflow, spray, washdown, rain, or maintenance activities.
The exposure may be intermittent, but it should not be treated as mild. Splash can carry wastewater, treatment chemicals, oils, solids, and microorganisms onto walls, ceilings, equipment supports, walkways, and structural steel.
Common Splash-Zone Locations
- Around aeration basins and mechanical aerators
- Near pumps, valves, weirs, and rotating equipment
- Above channels and open tanks
- At chemical-feed and chemical-storage areas
- Walls and curbs exposed to routine washdown
- Areas subject to overflow or process upset
Zone 4: Wastewater Headspace
The headspace is the area above the liquid inside an enclosed or partially enclosed wastewater structure. It may experience condensation, high humidity, hydrogen sulfide, methane, corrosive vapors, biological activity, and limited air movement.
Under certain conditions, hydrogen sulfide generated in wastewater enters the headspace. Microorganisms on damp concrete surfaces can convert sulfur compounds into sulfuric acid. This process, commonly associated with biogenic sulfuric-acid corrosion, can severely attack concrete and steel.
The upper portion of a structure can therefore deteriorate faster than the continuously immersed area. A visual survey should include the crown, ceiling, access hatch, upper walls, pipe penetrations, supports, and surfaces behind equipment.
Hydrogen sulfide is also a serious worker hazard. Exposure-zone mapping does not replace atmospheric testing, ventilation, permit-required confined-space procedures, respiratory protection, or emergency planning.
Zone 5: Interior Atmospheric Exposure
Interior atmospheric surfaces do not normally contact the process liquid, but they may still experience high humidity, condensation, washdown, chemical vapors, dust, and temperature changes.
Examples include pump rooms, galleries, equipment rooms, pipe tunnels, covered treatment areas, filter buildings, and chemical-handling buildings.
The exposure should be evaluated by location. A dry control room and a humid gallery beside an open wastewater channel should not automatically receive the same coating system merely because both are inside buildings.
Zone 6: Exterior Atmospheric Exposure
Exterior water and wastewater structures may be exposed to sunlight, rain, condensation, freezing, temperature cycling, windborne contamination, industrial atmosphere, bird waste, landscaping chemicals, and mechanical damage.
The complete system must protect the substrate and retain the required appearance. An immersion-grade lining may not provide adequate ultraviolet resistance or color retention when used as an exposed exterior finish coat.
Steel tank exteriors also require careful treatment of welds, edges, roof details, ladders, platforms, stiffeners, bolts, and areas that retain moisture. Concrete exteriors require evaluation of cracks, joints, moisture entry, previous repairs, and existing coatings.
Transient and Upset Exposures
Normal operation does not describe every condition the coating may encounter. The project team should also consider startup, shutdown, overflow, loss of ventilation, chemical spill, process upset, flooding, high-flow events, emergency cleaning, and extended empty periods.
An upset condition does not automatically determine the coating system, but it should be disclosed to the system manufacturer and addressed by the specification. Written clarification is especially important when the expected exposure falls outside published product limitations.
Zone Boundaries Require Special Attention
Failures frequently begin where one exposure zone, substrate, or coating system meets another. The transition may involve different film thicknesses, chemistries, primers, textures, flexibility, or application methods.
Examples include:
- Immersion lining terminating above the maximum liquid level
- Concrete lining meeting coated steel pipe
- Interior lining meeting an exterior coating at an access opening
- Rigid lining meeting a flexible joint sealant
- High-build lining meeting an uncoated equipment surface
- New coating tying into an existing system
The drawings and specification should define termination locations, overlaps, tie-ins, joint treatments, stripe coats, reinforcement, sealants, and other transition details. The contractor should not invent critical termination details in the field without written approval.
Create an Exposure-Zone Schedule
A practical exposure-zone schedule connects the facility map to the coating specification. Each row should identify one defined work area.
| Structure or Area |
Exposure Zone |
Substrate |
Service Conditions |
Specified System |
| Wet well lower walls |
Immersion |
Concrete |
Wastewater, solids, abrasion |
Per project specification |
| Wet well upper walls |
Headspace |
Concrete |
Condensation and hydrogen sulfide |
Per project specification |
| Exterior access hatch |
Exterior atmospheric |
Steel |
Weathering, condensation, and handling |
Per project specification |
The schedule should be reviewed against drawings, specifications, field conditions, manufacturer recommendations, and owner information before materials are ordered.
How Exposure Zones Affect Estimating
Exposure mapping improves the estimate because it separates areas that require different materials, thicknesses, repairs, preparation, access, containment, ventilation, testing, and cure times.
Do not calculate one total square-foot area and apply one production rate to the entire structure. Separate:
- Floors, walls, ceilings, roofs, and irregular surfaces
- Immersion, splash, headspace, atmospheric, and exterior areas
- Concrete, steel, repairs, joints, and transitions
- Open areas and restricted-access areas
- Spray-accessible areas and brush-or-roller detail work
- Areas requiring separate mobilization, shutdown, or cure periods
Article 03 Knowledge Check
- Why can one structure require more than one protective coating system?
- What conditions make the wet-and-dry zone different from continuous immersion?
- Why can a wastewater headspace be especially aggressive to concrete?
- Why should exterior ultraviolet exposure be considered when selecting a finish coat?
- Why do coating failures frequently begin at zone boundaries and system transitions?
- What information should be included in an exposure-zone schedule?
Technical References and Further Study
- AMPP: SSPC Guide 27, Recommended Performance Properties for Liquid-Applied Organic Polymeric Coatings and Linings for Concrete Structures in Municipal Wastewater Facilities.
- AMPP: Technical resources addressing protective coatings used in wastewater facilities and exposure-dependent coating selection.
- American Water Works Association: AWWA D102, Coating Steel Water-Storage Tanks.
- NSF International: NSF/ANSI/CAN 61 and current certified-product listings for drinking-water contact materials.
- OSHA: Hydrogen-sulfide workplace guidance and 29 CFR 1910.146, Permit-Required Confined Spaces.
- Project documents: Current drawings, process descriptions, operating data, coating schedules, manufacturer data, and owner requirements.
Always verify the required standard edition and obtain current documents from the issuing organization.
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