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Water and Wastewater Protective Coating Systems | Article 09 of 24
Cleaning, Decontamination, and Preparing Facilities for Coating Work
Coating preparation begins by making the structure safe, accessible, and free from the process residues that can interfere with inspection, surface preparation, and coating adhesion.
Cleaning Is Not the Same as Surface Preparation
Cleaning and decontamination remove materials deposited by the facility’s operations. Surface preparation then creates the cleanliness, profile, and soundness required by the protective coating or lining system.
Abrasive blasting, grinding, scarifying, or waterjetting a surface that is still contaminated can drive oil, grease, biological material, chemicals, or dissolved salts farther into the substrate. It can also contaminate abrasive, equipment, adjacent surfaces, and newly prepared areas.
What May Be Present?
Water and wastewater structures can contain contaminants that are not immediately visible. The contractor should identify the process history and probable contaminants before selecting cleaning chemicals, equipment, personal protective equipment, or disposal procedures.
- Sludge, sediment, scale, mineral deposits, and accumulated solids
- Biofilm, algae, bacteria, fungi, and other biological growth
- Oil, grease, fuel, lubricants, and maintenance residues
- Treatment chemicals, acids, alkalis, disinfectants, and coagulants
- Soluble chlorides, sulfates, nitrates, and other salts
- Sulfide-bearing deposits and corrosion products
- Loose, deteriorated, or chemically attacked concrete
- Rust, mill scale, deteriorated coating, and underfilm contamination
- Unknown residues introduced through industrial discharge or process upset
Safety Comes Before Cleaning
Tanks, wet wells, digesters, clarifiers, vaults, pipelines, and similar structures may qualify as permit-required confined spaces. Cleaning work must not begin until the owner and contractor have identified the applicable entry requirements, hazards, isolation procedures, communication methods, rescue provisions, and responsible personnel.
Wastewater environments may contain hydrogen sulfide, methane, oxygen-deficient atmospheres, biological hazards, and chemical residues. Hydrogen sulfide can rapidly overcome workers, and odor must never be treated as a dependable warning method.
Atmospheric testing, continuous monitoring when required, effective ventilation, trained attendants, appropriate respiratory protection, and a workable rescue plan are parts of the safety system. Ventilation does not replace atmospheric testing or the other requirements of an applicable confined-space program.
Isolate the Work Area
A drained vessel is not necessarily isolated. Pumps can start, valves can leak, gates can move, and connected piping can introduce liquids, gases, chemicals, or mechanical energy.
The facility and contractor should establish a written isolation plan addressing:
- Electrical, mechanical, hydraulic, pneumatic, chemical, and thermal energy
- Lockout and tagout of pumps, mixers, conveyors, valves, gates, and related equipment
- Blanking, blinding, disconnecting, or double-block-and-bleed arrangements where required
- Bypass pumping and temporary process control
- Verification that the isolation is effective
- Responsibility for authorizing shutdown and restoring the facility to operation
Develop the Cleaning Plan
The cleaning plan should be based on the contaminants, substrate, existing coating, configuration, access limitations, environmental restrictions, and requirements of the specified coating system.
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Review the process history. Identify the materials stored, treated, transferred, or discharged in the work area.
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Determine the operating condition. Confirm shutdown, draining, bypass, isolation, access, ventilation, and available cleaning water.
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Characterize the residue. Do not assume that unknown deposits are harmless or compatible with the proposed cleaner.
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Select compatible cleaning methods. Protect the substrate, adjacent equipment, joints, seals, instruments, and areas that will remain in service.
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Plan containment and disposal. Establish how wash water, sludge, removed coating, abrasive, and chemical residue will be collected, characterized, and disposed of.
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Define the acceptance criteria. The parties should agree on how satisfactory cleaning will be inspected and documented.
Common Cleaning Methods
Physical Removal
Shoveling, scraping, vacuum removal, pumping, and other physical methods may be required to remove bulk solids before washing begins. Removing heavy deposits first reduces the amount of contaminated wash water and improves access to the actual substrate.
Water Cleaning
Low-pressure washing, pressure washing, hot-water washing, and waterjet cleaning can remove dirt, salts, deposits, loose coating, and corrosion products. Pressure alone does not reliably remove every oil, grease, or chemical contaminant. The selected pressure, temperature, nozzle, distance, and technique must be suitable for the substrate and cleaning objective.
Detergent and Chemical Cleaning
Compatible detergents, emulsifying cleaners, and other approved cleaning agents may be needed for oils, grease, biological films, and process residues. Cleaners must be used at the manufacturer’s recommended concentration, temperature, dwell time, and agitation level.
The surface must be thoroughly rinsed so that the cleaning agent and loosened contamination are removed. A cleaner left on the substrate becomes another contaminant.
Never Mix Cleaning Chemicals Without Authorization
Water and wastewater facilities may contain acids, alkalis, chlorine compounds, sulfides, ammonia, disinfectants, and industrial contaminants. Combining incompatible residues or cleaning chemicals can generate heat, toxic gases, pressure, fire, or violent reactions.
Review safety data sheets, obtain the facility’s process information, and secure written authorization for the cleaning procedure. When the residue is unknown, stop and have it evaluated before work proceeds.
Protect the Facility and the Environment
Cleaning can mobilize contaminants that were previously contained on the structure. Wash water should not be allowed to enter operating treatment processes, storm drains, waterways, soil, or public sewers without authorization.
- Establish berms, dams, covers, temporary sumps, or other containment measures.
- Separate contaminated liquids and solids when required.
- Protect instruments, electrical equipment, mechanical seals, bearings, and operating components.
- Prevent overspray and runoff from contaminating adjacent prepared surfaces.
- Characterize waste when required by the owner, disposal facility, or applicable regulation.
- Maintain disposal records, manifests, photographs, and daily quantities when required.
Confirm That Cleaning Is Complete
A surface can look clean while still containing contamination capable of causing blistering, loss of adhesion, osmotic activity, staining, or premature lining failure. Visual examination is necessary, but it may not be sufficient.
Depending on the project, acceptance may include:
- Visual confirmation that sludge, scale, biofilm, oil, grease, deposits, and loose material have been removed
- Inspection under adequate lighting after the surface has been rinsed
- Checking for water beading or other evidence of residual oil and grease
- Soluble-salt testing when required by the specification or coating manufacturer
- pH testing of concrete or rinse water when required
- Additional cleaning of cracks, joints, pits, welds, penetrations, and inaccessible areas
- Written acceptance before mechanical surface preparation begins
Establish a Hold Point
A formal hold point after cleaning protects the contractor, inspector, owner, and coating manufacturer. At this stage, the exposed substrate and the actual extent of deterioration can be evaluated before repair and surface-preparation quantities are finalized.
Work should not advance simply because equipment and personnel are waiting. If contamination remains, the correct action is to repeat or revise the cleaning procedure and document the result.
Cleaning Documentation
The contractor’s daily records should identify:
- The structure and specific areas cleaned
- Observed contaminants and process residues
- Cleaning agents, dilution ratios, temperatures, and dwell times
- Equipment, water pressure, nozzles, and cleaning methods used
- Atmospheric readings and safety controls when applicable
- Waste collection and disposal procedures
- Inspection results, tests, photographs, deficiencies, and corrective work
- Names of the parties who accepted the cleaned surface
Contractor’s Field Checklist
- Has the process history been reviewed?
- Are all contaminants and unknown residues identified?
- Has the space been drained, isolated, locked out, and verified?
- Are confined-space and atmospheric hazards controlled?
- Are the cleaning chemicals compatible with the residues and substrate?
- Are wash water and removed materials contained?
- Have cleaners and loosened contamination been completely rinsed away?
- Has the cleaned surface been inspected and tested as required?
- Has the cleaning hold point been accepted and documented?
Knowledge Check
1. Why should oil, grease, biofilm, and chemical residue be removed before abrasive blasting or grinding?
Because mechanical preparation can spread the contamination, drive it into the substrate, contaminate equipment and abrasive, and prevent proper coating adhesion.
2. Does draining a tank prove that it has been safely isolated?
No. Pumps, valves, gates, connected piping, mixers, and other energy sources must be isolated, locked or tagged as applicable, and verified according to the facility’s procedures.
3. Why is a formal cleaning hold point valuable?
It confirms that contamination has been removed, exposes the true condition of the substrate, and establishes documented acceptance before repair or surface preparation begins.
Technical References and Further Study
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OSHA, 29 CFR 1910.146, Permit-Required Confined Spaces. This standard establishes practices and procedures for protecting general-industry employees entering permit-required confined spaces.
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OSHA, 29 CFR 1910.147, The Control of Hazardous Energy—Lockout/Tagout. This standard addresses control of electrical, mechanical, hydraulic, pneumatic, chemical, thermal, and other hazardous energy.
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OSHA, Hydrogen Sulfide Safety and Health Topics. Consult the current OSHA information addressing hydrogen-sulfide hazards, workplace exposure evaluation, and control measures.
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AMPP SSPC-SP 1, Solvent Cleaning. This standard defines requirements for removing visible oil, grease, and other visible contaminants from metal surfaces before subsequent coating preparation.
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AMPP SSPC-SP 13/NACE No. 6, Surface Preparation of Concrete. Consult the current edition for requirements concerning concrete surface preparation before application of protective coating and lining systems.
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The project specification and the coating manufacturer’s current technical data sheets, safety data sheets, surface-preparation requirements, and written application instructions.
Standards, regulations, and manufacturer instructions may be revised. Confirm the current edition and the requirements applicable to the project location, facility, contract, and type of work before beginning operations.
Professional responsibility:
This article provides foundational education and does not replace a site-specific hazard assessment, written confined-space program, energy-control procedure, environmental plan, project specification, manufacturer instruction, or applicable regulatory requirement. When requirements conflict or site conditions are uncertain, stop work and obtain written clarification from the responsible authority.
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