Water and Wastewater Protective Coating Systems

Article 02 of 24

Mapping the Water and Wastewater Treatment Process

A contractor cannot properly estimate, plan, coat, or inspect a treatment facility without understanding where the water travels, what happens at each process stage, and how the exposure changes from one structure to the next.

Begin with the Process—not the Paint

Every treatment facility has a process. Water, wastewater, solids, treatment chemicals, and gases move through connected structures and equipment. The coating system must perform within that operating process.

A drawing may identify a structure as a tank, basin, channel, vault, or building, but that name alone does not define the coating exposure. The contractor must determine what the structure contains, whether it is normally wet or dry, what chemicals are present, whether gases accumulate, how it is cleaned, and how the facility will be isolated for the work.

Process mapping converts a large facility into understandable coating work areas. It allows the estimator, project manager, superintendent, applicator, inspector, engineer, and owner to discuss the same locations and exposures using the same language.

A Typical Drinking-Water Process

Not every drinking-water facility uses the same treatment sequence. The source water and required treatment determine the process. A conventional surface-water facility may include the following general stages:

1. Raw-water intake: Water enters from a river, lake, reservoir, or other source.
2. Screening or preliminary treatment: Large debris and material are removed before the water reaches downstream equipment.
3. Coagulation: Chemicals are introduced to destabilize suspended particles.
4. Flocculation: Gentle mixing encourages small particles to combine into larger floc.
5. Sedimentation or clarification: Heavier floc settles and is removed.
6. Filtration: Water passes through granular media, membranes, or another filtration system.
7. Disinfection and chemical adjustment: Treatment controls microorganisms and adjusts water chemistry as required.
8. Finished-water storage: Treated water is held in a clearwell, reservoir, or storage tank.
9. Distribution: Pumps, pipe, valves, and storage facilities deliver water to the system.

The coating exposure can change at every stage. Raw water may contain sediment and biological material. Treatment chemicals may create localized chemical exposure. Clarifiers and basins experience immersion, splash, and wet-and-dry cycling. Finished-water structures require materials appropriate for potable-water contact.

Potable-Water Contact Must Be Identified

Mark every surface that will contact finished drinking water. These surfaces may be subject to project requirements involving NSF/ANSI/CAN 61-certified products, cure times, disinfection procedures, water temperature, minimum tank size, film thickness, and other limitations.

Do not assume that every structure inside a drinking-water facility is a potable-water contact surface. Conversely, do not assume that a surface is exempt because it is located inside a building. Trace the process and obtain written clarification when the service is uncertain.

A Typical Wastewater Process

Wastewater facilities also vary, but many municipal treatment plants follow a general sequence that includes collection, preliminary treatment, primary treatment, biological treatment, clarification, disinfection, discharge, and solids handling.

1. Collection system: Sewers, manholes, interceptors, wet wells, and lift stations transport wastewater to the treatment plant.
2. Preliminary treatment: Screens, grinders, and grit-removal systems remove debris and abrasive solids.
3. Primary treatment: Primary clarifiers remove settleable solids and floating material.
4. Secondary or biological treatment: Aeration basins, activated-sludge systems, trickling filters, or other biological processes reduce organic loading.
5. Secondary clarification: Biological solids are separated from the treated liquid.
6. Advanced treatment: Additional filtration, nutrient removal, membranes, or other processes may be used.
7. Disinfection: Chlorine, ultraviolet light, ozone, or another process reduces pathogens before discharge or reuse.
8. Effluent discharge or reuse: Treated water is discharged or directed to a reuse system.
9. Solids handling: Sludge is thickened, digested, dewatered, stored, treated, or removed.

The Collection System Is Part of the Facility

The treatment process begins before wastewater reaches the plant. Manholes, wet wells, pump stations, force mains, junction structures, and interceptors may experience some of the facility’s most aggressive conditions.

Long detention times, turbulence, septic wastewater, temperature, and limited ventilation can contribute to hydrogen-sulfide generation and accumulation. Where hydrogen sulfide is converted into sulfuric acid on moist surfaces, severe concrete deterioration and corrosion can occur above the normal liquid level.

A coating survey that examines only submerged surfaces may therefore miss the most aggressive exposure. The headspace, crown, access opening, pipe penetrations, bench, invert, and changing liquid line must all be mapped.

Map Exposure Zones Within Each Structure

One structure may contain several coating environments. Divide each structure into exposure zones rather than assigning one general description to the entire area.

Map the Substrates

The exposure map must be combined with a substrate map. The same process area may contain several materials requiring different preparation, repair, primers, and coating details.

• Cast-in-place concrete
• Precast concrete
• Concrete masonry
• Carbon steel
• Galvanized steel
• Stainless steel
• Ductile iron
• Fiberglass-reinforced plastic
• Existing coatings and linings
• Sealants, elastomers, repair mortars, and transition materials

Record substrate changes, embedded metals, repairs, joints, penetrations, pipe supports, ladders, fasteners, equipment bases, and dissimilar-material transitions. These details frequently determine where failures begin.

Map the Operational Conditions

The contractor needs more than a process description. The work plan must reflect how the facility actually operates.

• Normal operating level: Where is the usual liquid line?
• Maximum and minimum levels: Which areas cycle between wet and dry?
• Flow and turbulence: Where will the coating experience abrasion, impact, or high velocity?
• Process temperature: What temperatures occur during normal service, cleaning, and upset conditions?
• Chemical exposure: What treatment, cleaning, or process chemicals may contact the surface?
• Cleaning methods: Will surfaces be pressure washed, chemically cleaned, scrubbed, or mechanically cleaned?
• Shutdown duration: How much time is available for cleaning, preparation, application, cure, inspection, and repairs?
• Redundancy: Can the structure be removed from service, or must temporary bypass and isolation systems be provided?

Map the Hazards and Access Conditions

Process mapping is also safety planning. Tanks, wet wells, manholes, vaults, pipelines, digesters, and enclosed basins may be permit-required confined spaces. Wastewater areas may contain hydrogen sulfide, methane, oxygen-deficient atmospheres, biological hazards, engulfment hazards, or active process equipment.

The map should identify:

• Entry and exit points
• Permit-required confined spaces
• Atmospheric-monitoring locations
• Ventilation supply and exhaust locations
• Isolation, blanking, blocking, and lockout points
• Fall hazards and retrieval requirements
• Rescue access and emergency communication
• Material, abrasive, hose, and waste-removal routes
• Areas that must remain operational during the project

A facility drawing does not replace the employer’s hazard evaluation, confined-space program, entry permit, atmospheric testing, rescue plan, or required training.

Convert the Map into Work Packages

Once the process, exposure zones, substrates, operating conditions, access, and hazards are identified, divide the project into manageable work packages.

A useful work package should identify:

• Structure and exact limits of work
• Measured surface area
• Substrate and existing condition
• Exposure zone and service conditions
• Cleaning and decontamination requirements
• Repairs and detail treatments
• Specified surface preparation
• Complete coating or lining system
• Access, containment, ventilation, and environmental control
• Required inspection and testing
• Shutdown, cure, disinfection, and return-to-service schedule

A Practical Facility-Mapping Worksheet

For each structure, record the following information during the preconstruction review:

1. Structure name and facility identification number
2. Purpose in the treatment process
3. Contents and expected chemical exposure
4. Normal and maximum operating levels
5. Immersion, wet-and-dry, splash, headspace, atmospheric, and exterior zones
6. Substrate type and condition
7. Existing coating type and condition, when known
8. Cracks, joints, penetrations, edges, welds, and repairs
9. Access, scaffold, containment, and ventilation requirements
10. Isolation and shutdown requirements
11. Known atmospheric and safety hazards
12. Specified coating system and required thickness
13. Inspection and testing requirements
14. Unresolved questions requiring written clarification

Article 02 Knowledge Check

1. Why should the contractor understand the treatment process before selecting or applying a coating?
2. Name the major stages of a conventional drinking-water treatment process.
3. Name the major stages of a typical municipal wastewater treatment process.
4. Why can the headspace be more corrosive than the immersed portion of a wastewater structure?
5. What six general exposure zones should be considered when mapping a structure?
6. How does a facility map help the estimator convert a large project into defined work packages?

Technical References and Further Study

• U.S. Environmental Protection Agency: Primer for Municipal Wastewater Treatment Systems.
• U.S. Environmental Protection Agency: Drinking-water treatment and public-water-system educational resources.
• AMPP: Water and wastewater corrosion-control, coating-selection, surface-preparation, application, quality-assurance, and inspection resources.
• OSHA: 29 CFR 1910.146, Permit-Required Confined Spaces.
• OSHA: Hydrogen-sulfide workplace guidance, exposure evaluation, atmospheric monitoring, ventilation, and confined-space precautions.

Facility processes and hazards vary. Always use current drawings, process descriptions, owner information, project specifications, safety procedures, and field verification when planning the work.

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