Coal Tar Epoxy for Wastewater Sumps, Trenches and Tank Linings

Walk through an older wastewater plant or an industrial pretreatment system and a lot of the steel and concrete you see under the water line is black. That's usually coal tar epoxy, and much of it has been doing its job for twenty years or more. Coal tar epoxy earned that track record because of its very low water permeability, strong adhesion even on less-than-perfect surfaces, and resistance to the bacteria and dilute chemicals in wastewater.

It's still specified today. Carboline's Bitumastic 300M lists tank linings, piping, trenches, and sumps as core uses. But it isn't the right answer for every wet environment, and knowing where it stops working is as important as knowing where it works.

Where Coal Tar Epoxy Works Well

• Industrial sumps and trenches carrying process water, wash water, and dilute chemical runoff.
• Clarifier and settling tank steel such as rake arms, weirs, baffles, and walkway supports below the water line.
• Lift station and wet well steel in the submerged zone.
• Non-potable water and process tanks running at or below 120°F.
• Exterior of buried pipe and tanks, including those under cathodic protection.
• Containment areas where splash and occasional immersion are expected.

The Hard Limits

The Bitumastic 300M data sheet sets several limits that decide whether it belongs on your job:

• Immersion temperature: 120°F maximum. Hot process water, CIP rinse, or heated tanks need a different lining.
• Potable water: Not allowed. Drinking water service requires an NSF/ANSI 61-certified lining.
• Topcoats: Not recommended. You can't color-code or brighten it with a finish coat.
• Strong chemicals: Coal tar epoxy handles dilute chemicals and wastewater well, but concentrated acids, caustics, and solvents call for novolac epoxy or vinyl ester. See Novolac Epoxy Tank Lining and Vinyl Ester Linings.

The Hydrogen Sulfide Problem

The toughest area in any wastewater system is usually not what's underwater. It's the headspace above the water line in wet wells, manholes, and covered channels. Bacteria convert hydrogen sulfide gas into sulfuric acid on the concrete surface, and that acid eats concrete fast. This is called microbially induced corrosion (MIC).

In severe H₂S headspace exposure, coal tar epoxy is usually not the best choice. Those areas typically get 100% solids epoxy, novolac epoxy, or polyurea systems built for acid resistance. Coal tar epoxy still does well in the submerged zone below, where there is little oxygen and the acid-producing bacteria can't thrive.

“The worst corrosion in a wet well is usually above the water line, not below it. Spec the headspace and the submerged zone separately.”

Concrete Prep for Sumps and Trenches

Most sumps and trenches are concrete, and coating failures on concrete almost always come back to moisture and prep:

• New concrete must cure 28 days at 75°F / 50% RH before coating.
• Clean and abrade per ASTM D4258 and ASTM D4259. Remove laitance, oil, and old failed coatings.
• Fill voids and bugholes. Pinholes in concrete outgas through a fresh coating and leave holidays.
• Thin the first coat of 300M (up to 25 oz/gal Thinner #10) so it penetrates the concrete.
• Build to at least 16 mils DFT overall. In abrasive trench service, go heavier, up to the 35-mil maximum.

Existing sumps that have been in service are usually saturated and contaminated with oils. They need thorough degreasing and time to dry before any coating will bond. Pressure washing alone rarely gets there.

Steel Prep for Immersion

For steel in immersion, the data sheet calls for SSPC-SP10 near-white metal blast with a 2–3 mil profile. Stripe coat all welds, edges, and bolt heads by brush before the full spray coat. Edges are where immersion coatings fail first.

Cure Time Before Returning to Service

Downtime is usually the biggest cost on a sump or tank job, so plan around the cure schedule. For Bitumastic 300M at 16 mils:

• 50°F: 14 days to immersion
• 75°F: 7 days to immersion
• 90°F: 5 days to immersion

For heavy builds of 25–35 mils, Carboline recommends force curing: hold the steel at 150°F for 8 hours and it's ready for immersion. In cold weather, temporary heat and ventilation in the structure can bring a 14-day cure closer to 7.

Before return to service, holiday test per NACE SP0188: wet sponge below 20 mils, high-voltage spark above 20 mils.

Is Coal Tar Epoxy Still the Right Call?

For submerged wastewater steel and concrete at normal temperatures, coal tar epoxy is still a proven, economical system. The questions to ask are:

• Does the spec or owner allow coal tar? Many municipal specs now don't. See Coal Tar Regulations and Alternatives.
• Is it potable water? If yes, coal tar is out.
• Is it hot, over 120°F? If yes, look at novolac.
• Is it H₂S headspace? If yes, use an acid-resistant system there.
• Is it secondary containment for chemicals? Spray polyurea may be the better fit.

For more on painting treatment plants in general, see Wastewater Facility Painting.

Frequently Asked Questions

Is coal tar epoxy good for wastewater?

Yes, for submerged steel and concrete at normal temperatures. Coal tar epoxy has very low water permeability and resists bacteria and dilute chemicals. It is less suited to severe hydrogen sulfide headspace, which usually needs a more acid-resistant system.

What is the maximum temperature for coal tar epoxy immersion?

Carboline lists a maximum immersion temperature of 120°F for Bitumastic 300M.

How long does coal tar epoxy need to cure before immersion?

Bitumastic 300M needs 7 days at 75°F, 14 days at 50°F, or 5 days at 90°F before immersion. Heavy builds can be force cured at 150°F for 8 hours.

Can coal tar epoxy be used in potable water tanks?

No. Coal tar epoxy is not approved for potable water. Drinking water tanks require an NSF/ANSI 61-certified lining.

How should concrete sumps be prepared for coal tar epoxy?

Concrete must be cured 28 days, cleaned and abraded per ASTM D4258 and D4259, and have voids filled. The first coat should be thinned so it penetrates the concrete.