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Concrete Corrosion due to Hydrogen Sulfide
H2S can corrode concrete

  • POST a QUESTION or COMMENT about how hydrogen sulfide contributes to corrosion of concrete tanks, pipes, and other sewer components

Concrete components such as concrete sewer piping may be corroded by hydrogen sulfide and under some conditions may cause costly damage.

This article describes the possible corrosive effects of hydrogen sulfide on concrete tanks and concrete septic system components. It includes research results and possible variables that would affect the level of corrosion due to hydrogen sulfide.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Effects of Hydrogen Sulfide on the Corrosion of Concrete

Collapsing building © Daniel FriedmanResearch indicates that hydrogen sulfide can contribute to the corrosion of concrete, possibly including concrete septic tanks and other components such as concrete piping.

This corrosion may shorten the expected life span of the concrete and, in extreme conditions, might cause enough deterioration in a concrete septic tank cover or lid as to make the system unsafe.

[Click to enlarge any image]

Really? Probably not. Of millions of septic tanks installed around the world, reports of hydrogen sulfide damage are rare.

Biogenic sulphuric acid corrosion is a phenomenon which occurs mainly in sewer pipes.

The process consists of four stages:

  1. the reduction of sulphate to sulphide;
  2. the transition of sulphide to hydrogen sulphide gas in the sewer atmosphere;
  3. the re-oxidation of the sulphide gas to sulphuric acid in an oxidizing environment of the sewer pipe and finally
  4. concrete attack by sulphuric acid.
The most influencing parameters for the model were the temperature, the BOD-content and the pH-value of the waste water, the depth of flow and the detention time. - (Beeldens 2001)

Furthermore, where concrete damage such as loss of material or softening or spalling has been reported at a septic tank or distribution box, we need more information before asserting that the damage is due to hydrogen sulfide.

That's because more common mistakes such as the simple improper mix of concrete when constructing a septic tank, cover, or lid can produce similar spalling or material loss damage.

We will continue to review research to help assess risk of hydrogen sulfide to septic systems; meanwhile there is no cause for panic among concrete septic tank owners.

Types of Concrete Septic Tank Damage & Common Causes

The most common types of concrete septic tank / D-box damage that we have found or that has been reported by our readers are given in the table below, sorted with the most frequent complaint first.

Concrete Septic Tank Damage

Damage Description Usual Causes
Leaks

Cracks as described below
Improper / damaged inlet or outlet piping
Settlement of piping at connections
Settlement of the septic tank

Cracked septic tank walls or bottom

Improper installation on soft fill
Improper installation, out of level
Mechanical damage during installation
Improper construction: insufficient reinforcement, improper mix
Vehicle damage - driving over (uncommon)

Cracked septic tank cover, top, access port cover

Improper handling during installation or removal
Improper construction: insufficient reinforcement, improper mix
Vehicle damage - driving over (uncommon)

Loss of concrete material - spalling

Improper construction: improper mix

Loss of concrete material - microbial corrosion Hydrogen sulfide / sulfuric acid damage?
(Reported in concrete sewer piping but not reported for septic tanks) 3 4

Notes to the table above

  1. This table is an OPINION of the website editor, Daniel Friedman, based on reader Q&A from 2005 to present and from field investigations and septic system inspections at approximately 4500 homes in the Northeastern U.S. between 1986 and 2004, supplemented by a literature review of sources listed at the end of this article.
  2. Watch out: a damaged concrete septic tank cover may be unsafe if its condition means that the cover could fall in or that a person walking on the cover could fall into the septic tank - usually fatal
  3. Since 2005, only one InspectAPedia reader has reported that they suspected that damage to their concrete septic tank has been due to H2S. We note that absence of evidence is not evidence of absence of a condition. H2S damage could occur without being recognized as the cause.
  4. Microbial corrosion in concrete refers to concrete corrosion caused by microbial metabolism, which can lead to surface damage, surface loosening, mortar shedding, aggregate exposure, cracking and steel corrosion in serious cases as well as shorten the service life of concrete structures. - (Xi 2019)
  5. A principal component of concrete septic tank or D-box durability is the quality of the concrete mix itself and the aggregate used (e.g. slag vs stone).

    A low-water concrete mix tends to be stronger and more resistant to biogenic sulfuric acid attack in concrete sewer systems and therefore likely in concrete septic tanks as well.

    Both chemical and microbiological tests showed that the aggregate type had the largest effect on degradation. (Belie 2004)
  6. The rate of material loss or damage compared with the typical thickness of a concrete septic tank is a significant consideration:

    In worst case situations, the degradation is in the order of several millimeters per year. Biogenic sulfuric acid corrosion has been studied since 1945 when Parker [1] discovered that bacteria were involved in the corrosion process.
    ...
    The bacterial and chemical activity in the sewers create a sulfur cycle, which can lead to the bacterial formation of sulfuric acid.


    When anaerobic conditions occur due to long retention time or slow flow of the sewage, sulfate reducing bacteria, e.g., Desulfovibrio, reduce sulfur-compounds to H2S.

    Due to turbulence and pH decrease, H2S escapes into the sewer atmosphere. The transformation of H2S into sulfuric acid occurs under aerobic conditions, after the sorption of H2S from the sewer atmosphere into the concrete or into the biofilm on the surface of the pipelines above the water line.

    The H2S may react with oxygen to elemental sulfur, which is deposited on the sewer wall. Sulfur is a substrate for many thiobacilli such as Thiobacillus thiooxidans, Thiobacillus neapolitanus, and Thiobacillus intermedius [8,9]. Those bacteria metabolise the sulfur into sulfuric acid, which causes concrete deterioration.
    - (Belie 2004)
  7. Diagnostic tip: IF there were H2S damage to a septic tank, it should be visible entirely above the level of the floating scum layer in the septic tank where the presence of oxygen would be critical on the process. (our reading of Belie 2004)

Mechanism of Concrete Sewer Damage from H2S

Biogenic sulfuric acid corrosion has been studied since 1945 when Parker [1] discovered that bacteria were involved in the corrosion process.
...
The bacterial and chemical activity in the sewers create a sulfur cycle, which can lead to the bacterial formation of sulfuric acid.


When anaerobic conditions occur due to long retention time or slow flow of the sewage, sulfatereducing bacteria, e.g., Desulfovibrio, reduce sulfur-compounds to H2S.

Due to turbulence and pH decrease, H2S escapes into the sewer atmosphere.

The transformation of H2S into sulfuric acid occurs under aerobic conditions, after the sorption of H2S from the sewer atmosphere into the concrete or into the biofilm on the surface of the pipelines above the water line.

The H2S may react with oxygen to elemental sulfur, which is deposited on the sewer wall.

Sulfur is a substrate for many thiobacilli such as Thiobacillus thiooxidans, Thiobacillus neapolitanus, and Thiobacillus intermedius [8,9].

Those bacteria metabolise the sulfur into sulfuric acid, which causes concrete deterioration.- (Belie 2004)

Diagnostic Questions for Possible Hydrogen Sulfide Damage to Septic Tanks

Photograph of contraditory diagonal "cracks" in a poured concrete foundation wall (C) InspectAPedia & C.C.

Photo above: a cold pour joint in concrete: this is not damage and should not be mistaken for hydrogen sulfide or sulfuric acid attack on concrete.

How Long will Concrete Septic Components like Tanks and Piping Last?

A concrete septic tank can last 40 years to nearly indefinitely, though poor quality concrete or acidic ground water may result in deteriorated baffles or tank components.

The septic tank is only one part of an on-site wastewater system. It is designed to remove solids prior to the effluent entering the soil absorption field, provide for the filtration, digestion of a portion of those solids, and storage of the remaining solids.

Taking care of the septic tank, principally by pumping the tank on a regular schedule, will, however, extend the life of the costly second half of the onsite wastewater treatment system - the absorption system, leach field, or drainfield.

The septic drain field itself has a varying life as a function of the soil percolation rate, drainfield size, and usage level.

We [DF] have seen a septic drainfield, a large one in good soil with a well maintained septic tank, last for more than 50 years.

We have also seen a conventional septic drainfield fail within 24 hours of first use on a new system when piping was poorly installed.

See details of the factors in septic system and component life

at SEPTIC LIFE EXPECTANCY

and

at SEPTIC LIFE MAXIMIZING STEPS

What to Do if you have just moved into a home with a septic system

If you've just moved into a home with a septic tank you may not know the size of the septic tank, its maintenance history, or even where the septic tank is. In this case, you should have the tank pumped and inspected. The company pumping the tank will tell you its size, age, and condition.

Watch out: beware of pumping the septic tank before a septic system inspection or test, perhaps as part of a sale of a home. The purpose of this tank pump-out is to hopefully prevent the inspector from finding evidence of a failed drainfield. An empty septic tank prevents the test-inspector from pushing any effluent into the drainfield.

So perform any septic system loading and dye testing before emptying the septic tank.

But to thoroughly inspect the condition of a septic tank it will need to be pumped empty and its interior washed down with enough fresh water to permit inspection of the tank's sides and bottom for cracks or spalling or other damage.

The septic cover and its safety, access ports, and septic tank baffles should be part of this inspection.

Biogenic Sulfuric Acid Corrosion of Concrete: Research

 




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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2023-02-27 by Inspectapedia Com Moderator

@John M,

We've read, as perhaps you have as well, of H2S corrosion and damage to public sewer systems as well as its ability to corrode and damage electrical components and other metals (explained at CHINESE DRYWALL OUTGASSING HAZARDS)

But H2S Hydrogen Sulfide corrosion of residential septic systems is unusual in my opinion and experience [DF] - as the volume of sewage and sewer gases are so much less than in municipal systems.

What steps have you taken to prove that the problem you're seeing is due to hydrogen sulfide gas? What inspections and tests have been done, and by whom? Any written reports, documentation, photos? If so, what's different about your septic system than others in your area (where are you: country, city) and what do you know about the chemistry of your water supply?

Has anyone inspected for concrete spalling or similiar damage that could be from a poor original mix when that specific concrete septic tank was cast?

Where is the "eating away" of your concrete septic tank visible - or actually - where is it present? To answer, you'll need the tank inspected after it's pumped.

Those diagnostic questions might help us determine what's going on and then what action is needed.

Meanwhile, as we warned before, keep people away from the tank - roping off or barring the area if necessary - lest someone fall into the tank.

Our research suggests that it's quite unlikely that hydrogen sulfide has caused noticeable damage to your concrete septic tank - as you will read in the article above on this page.

So a closer look at your septic tank damage: what is its nature, where is the damage seen, and whether or not there are collapse or other safety issues should be your top questions for an immediate and more complete inspection.

Above on this page we describe inspection points for your septic tank.

Watch Out: there is a very high risk of fatal falls into septic tanks due to open covers, tanks or tank covers in poor condition, and from high levels of methane gas CH4 or hydrogen sulfide H2S.

Watch out: you or someone else could die working around or falling into a septic tank or cesspool.

See more at

SEPTIC TANK SAFETY

On 2023-02-27 by John M - Hydrogen sulfide gas is eating away my concrete septic tank

Hydrogen sulfide gas is eating away my concrete septic tank.

We don't have a garbage disposal and are very careful about what goes down the drain. We pump our 1000 gallon tank every 2 years.

I'm afraid the tank will collapse before there's any problem with the drain field. Is there any way of reducing the level of H2S gas in the tank?


...

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