How Does Hydrogen Sulphide Damage Storage Tanks
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How Does Hydrogen Sulphide Damage Storage Tanks?
Hydrogen sulphide is the reason sewage tanks fail from the top down. In the liquid, sulphate-reducing bacteria generate sulphide. In the gas space, a different set of bacteria converts it to sulphuric acid on the damp wall. The acid then eats the crown and the upper shell - the structural elements nobody sees and nobody inspects - while the immersed surface below looks perfectly sound.
Understanding the mechanism changes the specification. If the damage happens above the waterline and is biological in origin, then the answer is not a thicker wall; it is a surface that the acid cannot attack, plus a cover that keeps the gas out of the atmosphere. Center Enamel specifies glass-fused-to-steel for exactly this reason on sewage and sludge duty.
1. Where Does the Hydrogen Sulphide Come From?
From anaerobic bacterial activity. Sulphate-reducing bacteria convert sulphate and organic sulphur into hydrogen sulphide wherever oxygen is absent - in sewage, sludge, leachate and digesters.
· Sulphate-Reducing Bacteria: In anaerobic conditions these bacteria use sulphate as an electron acceptor and release sulphide as a by-product.
· Conditions That Increase Production: Warm temperature, high strength organic load, long retention and stagnant zones all raise sulphide generation.
· Where It Concentrates: Inlet works, equalisation tanks, sludge holding tanks, digesters, leachate storage and long sewage rising mains.
· It Is Also a Safety Hazard: Hydrogen sulphide is toxic at low concentrations and rapidly dangerous at higher ones, which is why containment and ventilation are regulated.
2. How Does It Turn Into Acid?
In the headspace. Hydrogen sulphide gas dissolves in the moisture film on the wall above the liquid, and sulphur-oxidising bacteria convert it to sulphuric acid.
· The Biological Step: Sulphur-oxidising bacteria, principally Thiobacillus species, metabolise the sulphide into sulphate, producing acid.
· Why Above the Waterline: The reaction needs both gas and oxygen, which is only available above the liquid line - hence the classic crown corrosion pattern.
· The Effect on Concrete: The acid dissolves the cement paste, lowering surface pH and exposing reinforcement, which then corrodes and expands, spalling the concrete further.
· The Effect on Steel: Acid and sulphide accelerate general corrosion and pitting, and sulphide can promote cracking in high-strength steels.
· Observed Rates: In severe, warm, high-strength sewage, concrete crown loss of several millimetres per year is well documented - enough to breach a roof within a decade.
3. How Do You Stop It?
Three routes, best applied together: keep the gas out of the atmosphere, remove the sulphide, and make the surface inert.
· Gas-Tight Covers and Treatment: Sealed aluminium dome roofs with gas collection to a biofilter, scrubber or the site's gas system remove both the odour and the exposure.
· Ventilation Where Covers Are Not Used: Dilution and controlled ventilation keep gas concentrations below the level at which corrosion and hazard become serious.
· Inert Surface in the Splash and Headspace Zone: Glass-fused-to-steel is unaffected by the acid, so the crown and upper courses are protected for the life of the tank.
· Source Control: Dosing iron salts to precipitate sulphide, nitrate dosing to suppress sulphate-reducing bacteria, and reducing retention in stagnant zones.
· Routine Inspection: Inspect the underside of the roof and the upper shell on a defined cycle; crown corrosion is invisible from the ground and catastrophic once advanced.
Tank Zone | Dominant Mechanism | Protection Measure |
Below the liquid line | Sulphide in solution, organic acids, chlorides | Inert lining; GFS resists all three |
Splash zone at the waterline | Alternating wet and dry, oxygen available | Hard, non-porous surface; no coating disbondment |
Headspace above the liquid | Biogenic oxidation to sulphuric acid | Gas-tight cover plus GFS on the crown |
Roof underside | Acid condensation, worst corrosion rate | Fused glass or sealed membrane; inspect routinely |
External atmosphere | Odour release, safety exposure | Gas collection and treatment |
Engineering Assurance and Project Support
Every tank delivered by Shijiazhuang Zhengzhong Technology Co., Ltd. (Center Enamel) is engineered against AWWA D103-09 and EN 1090 with finite element verification of shell, roof and nozzle loads, fused at 820-930°C under ISO 9001 and ISO 45001 control, holiday tested at 1500 V across one hundred percent of the surface, and assembled with Grade 8.8 bolts and manufacturer-certified sealant. Sewage and sludge tanks are supplied with gas-tight aluminium dome roofs, gas collection nozzles, sealed penetrations and a fused glass lining that protects the crown and upper courses where biogenic acid forms.
Frequently Asked Questions (FAQ)
Where does hydrogen sulphide come from in tanks?
From sulphate-reducing bacteria acting on sulphate and organic sulphur under anaerobic conditions. Concentrations rise with temperature, organic strength, retention time and stagnant zones, so inlet works, equalisation, sludge holding and digesters are the worst affected.
Why is corrosion worst above the waterline?
Because converting hydrogen sulphide to sulphuric acid requires oxygen. That only happens in the gas space, on the moisture film coating the wall and the roof underside - which is why crowns fail before floors.
How fast does biogenic corrosion progress?
In warm, high-strength sewage, concrete crown losses of several millimetres per year are well documented. That is enough to expose reinforcement and compromise a roof within ten years if nothing is done.
How do you stop sulphide corrosion?
Combine measures: a gas-tight cover with gas collection and treatment, controlled ventilation where covers are impractical, an inert surface such as glass-fused-to-steel in the splash and headspace zones, source control by iron salt or nitrate dosing, and routine inspection of the roof underside.
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