Why Wastewater Structures Fail Above the Waterline

Wastewater treatment plants are some of the hardest coating environments there are. The reason is not that the water is dirty. It is what the bacteria do with it.

How the damage happens

Sewage carries sulfate. In the low-oxygen, submerged parts of a pipe, wet well or channel, sulfate-reducing bacteria convert it to hydrogen sulfide (H2S), the rotten-egg gas. The gas rises into the headspace above the waterline. There, a second group of bacteria living on the damp concrete consume the hydrogen sulfide and give off sulfuric acid. The pH at the surface can drop to 1 or 2.

This is microbiologically influenced corrosion, or MIC. It does not just wear concrete down. It dissolves it. Structures can lose an inch of wall section in a few years, and the loss is almost always above the waterline, not below it.

What that means for the coating

A wastewater lining has to resist acid where the structure is driest, not only where it is wet. It also has to stop hydrogen sulfide from passing through the film to the concrete behind it. That is why wastewater systems are a different class of product. 100% solids, high-build epoxies are formulated specifically to resist H2S permeation, and they are applied anywhere from 15 to 500 mils depending on the structure and the service.

Surface preparation of the concrete follows SSPC-SP 13/NACE No. 6. Corroded concrete usually needs to be cut back to sound material and resurfaced before any lining goes on.

A useful question during an assessment

When an owner says the old coating "didn't last," ask where it failed. If the answer is "up top," the cause is almost certainly biogenic sulfide corrosion, and the replacement system has to be chosen for it.

What to check in your spec

• Is the lining rated for H2S and sulfuric acid exposure, not just water immersion?

• Does the spec call out the headspace and splash zone as the critical areas?

• Is concrete prep per SSPC-SP 13/NACE No. 6, with resurfacing where section loss exists?

• Is the film thickness matched to the severity of the exposure?

Further reading: Tnemec Technical Bulletin 06-56, Odor Control and Biogenic Sulfide Corrosion. Standards: NACE SP0112, SSPC-SP 13/NACE No. 6.

Talk to a consultant. Atlantic Coating Consultants helps owners, engineers and contractors in PA, NJ, DE, MD, DC, WV and OH with specification writing, on-site condition assessment, training, product recommendations and field support. Call 484-644-5303 or email atlantic@tnemec.com.