Wastewater concrete corrosion: how H2S attacks concrete, and how to repair and line wet wells, pits and channels

Wastewater concrete corrosion is mostly an acid attack that happens above the liquid. Hydrogen sulfide (H2S) released from the effluent dissolves in the moisture film on walls and crowns, where bacteria oxidize it to sulfuric acid. That acid dissolves the cement paste, so the concrete softens, loses section and eventually exposes its reinforcement.

The mechanism is best known from municipal sewers, but it is just as active inside a plant: effluent pits, process drain channels, treatment basins, in-plant wet wells and covered tanks receiving organic, sulfate-bearing effluent. This guide is written for those industrial structures.

How H2S concrete corrosion works

Biogenic corrosion is a chain of steps. Breaking any link reduces the attack; a lining breaks the last one.

1. Sulfide forms in the liquid. In sludge, sediments and the slime layer on submerged surfaces, sulfate-reducing bacteria convert sulfate into dissolved sulfide. Long retention, warm effluent, high organic load and settled solids favour it.
2. H2S escapes to the air space. Dissolved sulfide is released as gas wherever the liquid is agitated: drops, cascades, pump discharges, the outlet of a force main. A lower effluent pH also shifts sulfide towards the gaseous form.
3. The gas reaches a damp surface. Walls and crowns above the waterline stay wet with condensation, and H2S dissolves in that film.
4. Bacteria make sulfuric acid. New concrete is too alkaline for most microorganisms. As carbonation and H2S lower the surface pH, successive populations of sulfur-oxidizing bacteria colonize it and oxidize the sulfide to sulfuric acid.
5. The acid attacks the cement paste. Sulfuric acid dissolves the calcium hydroxide and calcium silicate hydrates that bind the aggregate. The main product is gypsum, a soft layer with no strength; gypsum can react with the cement's aluminate phases to form expansive ettringite, which cracks the concrete behind the attack front.

The result is the signature of microbiologically influenced corrosion on concrete: a soft white or grey layer that scrapes off, aggregate standing proud, then exposed reinforcement. The bacteria involved on steel are described in our article on microbial corrosion (MIC); on concrete, the damage comes from the acid they produce.

Where concrete corrodes in wastewater structures

The attack follows the gas and the condensation, not the liquid. Concrete that stays permanently submerged is usually in far better condition than the wall just above it.

In a plant, the most exposed structures are often the least visited: a covered pit, a channel under grating, an automatic wet well. Damage is found when a cover gives way or a pump support loosens.

Diagnosing biogenic corrosion of concrete

Assessment is done after isolation, ventilation and cleaning, under a confined space entry procedure. It answers three questions: how deep the damage goes, whether the reinforcement is affected, and what substrate will remain once the damaged layer is removed.

• Visual inspection. Soft paste layer, exposed aggregate, loss of the original surface, exposed reinforcement, cracking, damaged inserts. Photograph each zone with a scale.
• Scraping and sounding. A scraper or hammer separates the soft layer from sound concrete. Hollow sound indicates delamination, often over corroding reinforcement.
• Surface pH. Measured on the moistened surface with indicator paper or a surface electrode, it shows whether acid attack is active. A phenolphthalein indicator sprayed on a fresh fracture or core shows the depth to which the concrete has lost its alkalinity.
• Section loss and cover. Measured against drawings or a reference point; a cover meter locates the reinforcement.
• Cores. Where capacity is in question, cores give depth of attack and strength. Structural conclusions belong to the engineer responsible for the structure.

The diagnosis also looks at the cause: where H2S is released, how the structure is ventilated, what the effluent contains.

Repairing H2S-damaged concrete before lining

A coating cannot be applied over the soft layer. Gypsum has no strength, and acid-contaminated concrete under it keeps a low surface pH that compromises adhesion.

Remove the damaged concrete. High-pressure water jetting, hydrodemolition or chipping removes soft and contaminated concrete down to sound, alkaline substrate. ASTM D4259 covers abrading concrete before coating, and SSPC-SP 13 / NACE No. 6 covers concrete preparation for protective coatings. Washing then removes sulfate residues; surface pH is checked against the data sheet.

Treat the reinforcement. Exposed bars are cleaned around their full circumference, assessed for section loss by the responsible engineer, then protected per the repair system. ICRI 310.1R is the reference guide for preparation when concrete is repaired because of reinforcement corrosion.

Rebuild the section. Lost thickness is restored with repair mortars compatible with the lining that will follow: polymer-modified cementitious mortars, calcium aluminate mortars, often chosen in sewer service for their resistance to biogenic acid, or epoxy mortars for thinner rebuilds. Each mortar has its own cure and moisture requirements before it can be coated.

Create the surface profile. The concrete surface profile is specified under ICRI 310.2R, which defines profiles CSP 1 to CSP 10 and provides replica chips for site comparison. The required CSP comes from the lining's data sheet, not from a general rule. The same preparation logic is detailed in our guide to secondary containment coatings on concrete.

Choosing a wet well coating or concrete lining

Selection starts from three constraints: how aggressive the exposure is, how wet the substrate will be at application, and how long the structure can be taken out of service.

Two rules. Line the full exposed height, from below the low water level to the crown and the underside of covers: a lining that stops at mid-wall moves the attack above it. And specify on the documented sulfuric acid resistance of the actual system, not on the resin family's reputation, as in our guide to industrial tank lining selection.

Concrete moisture and outgassing in wastewater structures

Wastewater concrete is rarely dry: it has been wet for years, groundwater may stand behind the wall, and a dewatered wet well stays humid.

• Moisture testing. ASTM D4263 (plastic sheet) is a quick screening test; ASTM F2170 (in situ relative humidity probes) gives a measurement inside the concrete. Acceptance limits are those in the data sheet of the selected system, for the method it names.
• Damp-tolerant primers. Where the concrete cannot be dried within the shutdown, choose a primer formulated for damp concrete.
• Negative-side water. Water pushing from behind a below-grade wall blisters barrier coatings. Identified at diagnosis, it may call for a system designed for negative-side pressure or a cementitious lining.
• Outgassing. Concrete releases air from its pores when its temperature rises, leaving pinholes in the film. Applying while the concrete temperature is stable or falling, typically later in the day, with a pore filler or scratch coat on open repairs, limits the problem.
• Dew point. Surface temperature is kept above the dew point by the margin in the data sheet; dehumidification and heating are often needed in a closed structure.

Quality control of a concrete lining

The hold points are the same as for any immersion lining, adapted to concrete.

• Before application: CSP compared with ICRI chips, surface pH, moisture test to the specified method, cleanliness, surface temperature and dew point recorded.
• Thickness: controlled with wet-film gauges and consumption per area during application, then after cure with ultrasonic gauges (ASTM D6132) or destructive checks at agreed locations.
• Continuity: a holiday test under ASTM D4787, the practice for continuity verification of linings applied to concrete. Because concrete is a variable conductor, the method and voltage are agreed before application; every holiday is repaired and retested.
• Adhesion: pull-off adhesion testing to ASTM D7234 measures bond to the concrete. The failure mode is recorded with the value; acceptance criteria come from the specification.
• Cure: return to service only once the data sheet cure at the actual temperature is reached.

Confined space and H2S safety

A wet well, pit or covered channel is usually a confined space, and H2S is a toxic, flammable gas that is heavier than air and accumulates in low points. Its odour is not a reliable warning, because high concentrations deaden the sense of smell. Entry requires a written confined space procedure: isolation and lock-out of inflows and pumps, ventilation, continuous atmospheric monitoring, attendant and rescue plan. Exposure limits and entry rules are those of the applicable regulation: in Quebec the Regulation respecting occupational health and safety, in Ontario O. Reg. 632/05 (confined spaces), with CSA Z1006 as the management standard. Lining materials add their own vapour and ignition risks.

FAQ

Why is the concrete above the water more damaged than below it? Because the acid is produced on damp surfaces exposed to H2S in the air space. Submerged concrete is not colonized by the acid-producing bacteria and is attacked far less.

Can a wet well coating be applied without full removal of the corroded concrete? No. The soft, acid-contaminated layer has no strength and keeps a low pH. It is removed to sound concrete, then the section is rebuilt before lining.

Is novolac epoxy always the best choice against H2S? It is among the most resistant to biogenic sulfuric acid, but needs a dry or primed substrate. A damp substrate or a short shutdown can favour another system.

How is a lining on concrete checked for pinholes? By continuity testing to ASTM D4787, with method and voltage agreed beforehand, since concrete conducts variably. Outgassing pinholes are prevented by applying while the concrete temperature is falling.

How often should a lined wet well be inspected? At a frequency set by the owner from exposure and history. Inspections look for blistering, disbondment at terminations, softening and mechanical damage, and life-cycle choices are covered in our analysis of the total cost of ownership of a coating.

In practice

Induscoat removes damaged concrete, rebuilds the section and applies protective linings in plant effluent pits, channels, treatment basins and wet wells through its protective coatings and maintenance and rehabilitation services in Quebec and Ontario, with surface pH, profile, moisture, thickness, continuity and adhesion recorded at each hold point. See also our concrete protection application. Send photos, the structure dimensions, the effluent type and the available shutdown window, and request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.