Water treatment plant coatings: basins and clarifiers

In a water treatment plant, the coating is chosen by zone: headworks and channels exposed to H2S need an acid-resistant lining over repaired concrete; basins and clarifiers in immersion need an immersion-grade epoxy over well-prepared concrete or steel; chlorine contact chambers and potable water structures need products compatible with the disinfectant and, for drinking water, compliant with NSF/ANSI/CAN 61. The work is done while a basin is out of service, with drying and cure planned in.

Municipal and industrial maintenance managers run structures that almost never stop: a basin taken offline often has to return to service within a tight window. This guide helps prepare a tender or quote for basins, channels and clarifiers. It builds on our concrete protection page and our reference page on protective coatings.

Which zones of a water treatment plant are attacked most?

The most attacked zones are those where wastewater releases H2S (headworks, grit chambers, covered channels, turbulent areas), followed by basin waterlines, where wetting and drying alternate, and chlorine contact chambers. Permanently submerged zones often suffer less than the splash band and the air space above the water.

The H2S attack mechanism is covered in our article on wastewater concrete corrosion, and potable water requirements in our guide to potable water tank coatings and NSF 61.

How is basin concrete prepared before coating?

Concrete is prepared down to sound, clean, rough concrete under SSPC-SP 13 / NACE 6, with a surface profile defined by the ICRI 310.2R references. Laitance, degraded concrete and non-adherent old coatings are removed, then repairs are made before the system goes on.

• Removing degraded concrete. By hydro-blasting or abrasive blasting down to sound concrete; hydro-blasting limits dust and micro-cracking.
• Cracks. Active or leaking cracks are treated by crack injection before coating, or they will telegraph through it.
• Repairs and resurfacing. Compatible repair mortars and a bughole filler, to avoid pinholes in the coating.
• Moisture. The concrete of a drained basin stays damp for a long time; moisture is checked, for example under ASTM D4263, and the product must tolerate the measured condition. See our article on concrete moisture before epoxy.

Which coatings resist H2S and chlorine?

Against H2S, choose acid-resistant resins (novolac epoxies, vinyl esters, some immersion-grade polyurethanes or polyureas per the data sheet), applied at high build over repaired concrete. Against chlorine, choose a product whose data sheet confirms compatibility with the disinfectant at its service concentration.

• Acid resistance. Our comparison of vinyl ester vs novolac epoxy helps choose by acidity and temperature.
• Thickness and continuity. In immersion, the smallest pinhole becomes an entry point; continuity is checked by holiday test.
• Microbial corrosion. Deposits and stagnant zones promote bacterial corrosion; regular basin cleaning limits it.

A practical point for tenders: describe each zone separately, with its substrate, its exposure and its condition. A single line item for the whole basin hides very different work between a sound submerged wall and a degraded waterline, and makes quotes hard to compare.

How do you protect clarifier steel and mechanisms?

Clarifier steel is protected as an immersion structure for its submerged parts (rake arms, scrapers, centre column, baffles) and as a severe atmospheric structure for its parts above water (bridge, walkway, drive, launders and weirs). The preparation required is generally near-white blasting for immersion.

• Edges and welds. Arms and trusses have many edges, bolts and weld beads; they receive a brushed stripe coat before each full coat.
• Dissimilar metals. Stainless scrapers or fasteners in contact with carbon steel create a galvanic couple; contacts are isolated or both metals are coated.
• Weirs and launders. The splash band and deposits concentrate corrosion there; these parts are coated in place or removed depending on access.
• Drive mechanism. It is protected during blasting and painting, and moving parts are not painted.

How do you organize the work during a basin outage?

Work is organized around the outage window: draining and cleaning, inspection and repair survey, preparation, repairs, coating, cure, then testing before refilling. Actual duration depends mostly on concrete drying, weather for outdoor basins and the cure time before immersion stated on the data sheet.

• Redundancy. One basin is treated at a time while the others carry the flow; phasing is decided with operations.
• Weather and season. Outdoor basins are best done in mild weather; in the cold season, a heated and dehumidified enclosure is needed. See our article on cold-weather application.
• Confined space and atmosphere. Covered channels and chambers are confined spaces: gas detection, ventilation and entry procedure.
• Contingency for hidden damage. Degraded concrete and thinned steel often turn out larger than the survey showed once the basin is clean. Unit rates for extra repair quantities, agreed in the quote, avoid stopping the work to negotiate.
• Coordination. Our project management service coordinates trades and inspection hold points during the outage.

Plan the return to service as carefully as the start. Operations need a firm date to bring the basin back online, and the coating needs its full cure before immersion. Building the cure time into the outage schedule from the outset avoids the pressure to refill early, which is one of the most common causes of premature lining failure.

What checks come before refilling a basin?

Before refilling, the coating's thickness, continuity and cure are checked and the results are recorded. These checks form the structure's reference file and support the warranty.

• Thickness. On steel to SSPC-PA 2; on concrete, by the method set in the specification.
• Continuity. Holiday testing by the specified method and voltage, on steel and on concrete prepared for it.
• Adhesion. Pull-off testing on witness panels or chosen areas, for example under ASTM D7234 on concrete.
• Cure. Time and temperature respected before immersion; for potable water, flushing and commissioning per the applicable requirements.

These records also serve the next outage. Knowing which zones were repaired, with which products and at what thickness, lets operations plan the next intervention basin by basin instead of starting again from a full survey.

Frequently asked questions

Can a basin be coated without draining it?

For an immersion lining, no: the substrate must be dry, clean and prepared. Some repair products can be applied underwater, but they do not replace a full system.

Does new basin concrete need a coating?

Not always. It does when water, H2S or treatment chemicals attack it, or when watertightness must be improved. New concrete must have cured before coating.

Do potable water and wastewater need different products?

Yes. Potable water requires a product compliant with NSF/ANSI/CAN 61; wastewater mostly requires resistance to H2S and acids.

Do clarifier mechanisms need replacing?

Not necessarily. After blasting and inspection, sufficient remaining steel thickness often allows a new coating system; parts that are too thin are replaced.

How do you get a quote?

A quote starts from the zone survey, the condition of the concrete and steel, and the available outage window. Our protective coatings service covers basins, channels and clarifiers; our crews and qualified subcontractors work from LaSalle, including in Montérégie and the Quebec region. Send photos, drawings, dimensions and the outage window through the request a quote form: pricing on consultation, reply within 24 business hours.