H2S Corrosion Protection for Sanitary Manholes: An Engineer’s Options Guide

Hydrogen sulfide (H2S) corrosion concentrates in a small number of
sanitary structures, and an engineer can protect those structures at
modest cost by specifying protection at design. This guide covers how
the corrosion works, where it is critical, and the five tools available
to the design engineer, from a $0 hydraulic change to a cast-in HDPE
liner. We wrote it as a reference for engineers in Northwest Montana,
North Idaho, and Southern Alberta.

H2S corrosion is biological: bacteria living on the concrete
above the waterline convert hydrogen sulfide gas into sulfuric acid,
which dissolves the cement paste and thins the wall. It is
often called microbially induced corrosion, or MIC. Most gravity
manholes on a fresh, well-aerated line never see it. Force main
discharge manholes and lift stations often do.

Download the full guide as a PDF for your spec
file.

How H2S corrosion works

The sequence, as summarized in Purdue research (Ding,
2015):

1. Anaerobic bacteria in the wastewater produce dissolved sulfide.
2. Turbulence releases it as H2S gas into the headspace.
3. Sulfur-oxidizing bacteria (Thiobacillus species) colonize
   the moist crown and walls and oxidize the H2S to sulfuric acid.
4. The acid dissolves the cement paste and forms gypsum and ettringite.
   Aggregate loosens, the wall thins, and structural capacity is lost.

Two points drive design decisions.

The bacteria hand off in stages. Fresh concrete sits
near pH 12 to 13. Milder bacteria lower the surface pH first, and T.
thiooxidans takes over only once the surface is very acidic (ConSeal
/ ACI webinar). Interrupting the early stages is far easier than
resisting the late one.

Severity has a threshold. In the Purdue tests,
antimicrobial admixtures and calcium aluminate cement improved
resistance under moderate conditions, with the concrete surface above pH
2.0. Below pH 2.0, no change to the concrete mix prevented degradation,
and lower gas-phase H2S concentration improved concrete resistance (Ding,
2015).

Where H2S protection is
critical

Corrosion concentrates where septic wastewater meets turbulence. Most
gravity manholes on a fresh, well-aerated line need no special
protection. These structures do:

Septic tanks: residential concrete septic
tanks vented through the building’s plumbing stack are typically low
risk, because the house sewer and roof vent give the tank continuous
airflow. Risk rises for pump tanks and dosing chambers, large commercial
or community systems with long retention, and tanks that are sealed or
have no vent path.

Risk indicators to check at design: force main
retention time, wastewater temperature, a measured headspace H2S reading
at an existing comparable structure, a rotten-egg odor history, and
visible crown damage on nearby structures. A one- to two-week data
logger reading at an existing discharge manhole is the most direct way
to size the protection.

The engineer’s five options

An engineer has five ways to protect a concrete sanitary structure.
They work in different ways: reduce the H2S, make the concrete hostile
to the bacteria, or put a barrier between the acid and the concrete.
They stack, and the strongest designs combine at least two.

1. Design it out

H2S that never reaches the headspace cannot corrode anything.

• Bring force main discharges in at or below the water surface instead
  of dropping and splashing.
• Minimize drops, hydraulic jumps, and sharp direction changes at the
  discharge.
• Size force mains and wet wells to limit retention time.
• For severe force mains, consider source chemical control at the pump
  station (magnesium hydroxide, nitrate, iron salts, air or oxygen
  injection). It stops sulfide from forming but carries an ongoing
  operating cost.

Hydraulic changes cost almost nothing at design and cannot be
retrofitted cheaply after construction.

2. Antimicrobial admixtures

An antimicrobial admixture is added to the concrete at
batching and becomes a permanent part of the cement matrix, stopping the
early bacterial stages before the surface turns acidic. Because
it is in the concrete rather than on it, it cannot be punctured,
delaminate, or wear off the way a surface product can. Several
EPA-registered products are on the market, including ConBlock MIC
(ConSeal), Bio-San C500 (Xypex), and ConShield (ConShield Technologies).
Two compare as follows:

Sources: ConBlock
MIC technical data sheet; Bio-San
C500 product data sheet.

We use ConBlock MIC as our default antimicrobial because it delivers
the protection these structures need at a lower cost than the
alternatives we have evaluated. When an engineer specifies another
product, we supply it.

Limits: independent testing shows antimicrobials
lose effectiveness once the concrete surface falls below pH 2.0 (Ding,
2015). Admixtures do not address odor or gas exposure, and in grease
interceptors they do not fully address direct fatty-acid attack.

3. Passive venting

A vent stack exhausts H2S and moisture. A dry crown starves the
acid-producing bacteria, and a lower H2S concentration improves concrete
resistance.

• Size: a 4 in. stack moves roughly 15 to 35 cfm
  passively, depending on wind. That is about one air change every 2 to 4
  minutes in a 4 ft diameter, 5 ft deep manhole (63 cubic feet). These are
  our first-order estimates, not tested values.
• Height: terminate about 12 ft above grade. In
  simple dispersion models, ground-level concentration falls roughly with
  the square of release height, so a 12 ft stack gives on the order of 9
  times lower ground-level H2S than a 4 ft vent. This is also a
  first-order estimate.
• Make-up air: the structure needs an inlet path (lid
  pick holes, an inlet pipe, or the upstream line). A single opening with
  no inlet exchanges very little air.
• Limits: the passive draft weakens on calm summer
  days, when sulfide production peaks. Where vent odor could reach homes,
  provide for a carbon canister. A vent reduces corrosion; it does not
  stop it at high sulfide.

4. Embedded plastic liners

A cast-in liner is an HDPE or PP sheet with anchor studs on
the back, placed against the form before the pour so the studs lock it
into the concrete; the seams are extrusion welded after stripping to
form a continuous acid barrier (AGRU
Sure-Grip).

• Material: HDPE, PP, PVDF, or ECTFE, 2 to 12 mm
  thick, with 13 mm V-shaped anchor studs for new precast. A coextruded
  signal layer is available for damage detection.
• Performance: HDPE is essentially inert to sulfuric
  acid at any pH, so the liner covers the severe cases where admixtures
  fall short. It also resists fatty-acid attack in grease
  interceptors.
• QC: every seam is spark (holiday) tested. Integrity
  depends on weld quality and penetration detailing.
• Limits: highest first cost of the concrete-side
  options. The precast plant installs a cast-in liner during production;
  field crews cannot retrofit one.

5. Applied coatings

• Field- or plant-applied coatings (epoxy,
  polyurethane, acid-resistant cementitious): an effective barrier, but
  bond and holiday (pinhole) quality control matter, and coatings
  typically need recoating over the structure’s life.
• Calcium aluminate cement mortars: better acid
  resistance than portland cement under moderate conditions (Ding,
  2015).

A note on all-plastic
structures

HDPE and other thermoplastic manholes avoid acid attack entirely, but
they trade it for structural limits. Thermoplastics creep under
sustained load, so long-term deflection under soil and traffic loads
governs the design. They depend heavily on backfill quality, need
anti-flotation measures in high groundwater, and have lower load
capacity than reinforced concrete at depth or under traffic. For most
buried sanitary structures, protected concrete combines acid resistance
with structural capacity.

Selecting the right
level of protection

Match the protection to the severity. ConSeal frames it as three
severity levels, from low bacterial population and activity (Level 1) to
high population and high activity (Level 3) (ConSeal
/ ACI webinar). The H2S ranges below are our working guidance for
local structures, not a published standard.

For grease interceptors, treat the structure one step more severe
than its H2S reading suggests, because fatty-acid attack adds to the
sulfide attack.

Our mix and our
recommendations

Our standard sanitary mix is self-consolidating concrete (SCC) at a
0.36 water-to-cementitious ratio. That is below the strictest maximum in
ACI 318 Table 19.3.2.1 (0.40, for severe freeze-thaw class F3, chloride
class C2, and one option under very severe sulfate class S3) and below
the 0.45 limit for severe sulfate exposure (S2). Confirm the limits
against the code edition your jurisdiction has adopted. SCC needs no
vibration, so it fills tight corners and the areas around penetrations
without voids or honeycombing, which is where acid attack usually
starts. Mix quality improves resistance at moderate severity, but it
does not stop attack once the surface falls below pH 2.0, so the five
options above remain the primary protection.

We recommend an antimicrobial admixture as the minimum protection for
every force main discharge manhole and lift station; our default is
ConBlock MIC. These options are not included by default. We provide them
when the engineer of record specifies them, and the tiers below are a
starting point for that specification:

Both admixture manufacturers publish specification language. For
ConBlock MIC, ConSeal publishes a CSI
specification (Section 03 05 00) and a standard
specification. For liner projects, contact us early so we can
confirm materials and lead time.

For the structures themselves, see our precast
manhole components guide, the difference between storm
and sanitary sewer manholes, and our precast
lift stations.

Frequently asked questions

What causes H2S
corrosion in concrete manholes?

H2S corrosion is caused by bacteria, not by the gas alone. Anaerobic
bacteria in the wastewater produce dissolved sulfide, turbulence
releases it as hydrogen sulfide gas, and sulfur-oxidizing bacteria on
the moist concrete above the waterline convert that gas into sulfuric
acid. The acid dissolves the cement paste, loosens the aggregate, and
thins the wall until structural capacity is lost.

Which manholes
need H2S corrosion protection?

The highest-risk structures are force main discharge manholes and
lift station wet wells, followed by grease interceptors, drop manholes
and hydraulic jumps where flow is septic, manholes at the end of long
flat or low-flow runs, and junctions downstream of industrial or
food-processing discharges. Most gravity manholes on a fresh,
well-aerated line need no special protection.

Do antimicrobial
concrete admixtures work?

Yes, at low to moderate severity. An antimicrobial admixture such as
ConBlock MIC or Bio-San C500 stops the early bacterial stages before the
concrete surface turns acidic, and it is permanent because it is part of
the concrete. Independent Purdue testing found antimicrobials lose
effectiveness once the surface falls below pH 2.0, so severe structures
need a liner, venting, or source control as well.

When does a manhole need
an HDPE liner?

A cast-in HDPE liner fits severe sulfide conditions, such as
sustained headspace H2S of 100 ppm or more, grease interceptors exposed
to fatty-acid attack, and any project whose specification requires a
liner. HDPE is essentially inert to sulfuric acid at any pH. The liner
must be cast in at the precast plant during production; it cannot be
retrofitted in the field.

Are
residential septic tanks at risk from H2S corrosion?

A residential septic tank vented through the house plumbing stack is
typically low risk, because the house sewer and roof vent keep air
moving through the tank. Risk rises for pump tanks and dosing chambers,
large commercial or community systems with long retention, and tanks
that are sealed or have no vent path.

Protect the structures that
need it

H2S corrosion is concentrated, predictable, and cheap to address at
design. Design the hydraulics so sulfide stays in solution, add an
antimicrobial admixture to every force main discharge manhole and lift
station, vent where odor and exposure matter, and line the structures
that will see severe sulfide. You build it once, so build it right.

Glacier Precast Concrete is an NPCA-certified plant, Montana-owned
and owner-operated, producing in Kalispell since 1989. For a
project-specific question, call (406) 752-7163, send us your drawings
for an estimate, or start with the complete
guide to precast concrete manholes.

References

• ConBlock MIC
  product page, Concrete Sealants, Inc.
• ConBlock
  MIC technical data sheet (version 12 February 2026)
• ConBlock
  MIC CSI specification, 03 05 00 and standard
  specification
• ConBlock
  MIC installation instructions and quality
  assurance plan
• Microbially
  Induced Corrosion of Concrete: Efficacy of Antimicrobial and Colloidal
  Silica Admixtures, S. Lines, ConSeal, ACI webinar
• Assessing
  the performance of antimicrobial concrete admixtures in concrete
  subjected to microbially induced corrosion, L. Ding, M.S. thesis,
  Purdue University, 2015
• Xypex
  Bio-San C500 product data sheet, Xypex Chemical Corporation
• AGRU Sure-Grip
  concrete protective liners, AGRU America
• SST
  Installation Guide, USDA Forest Service (passive vent stack
  background)

Codes and test standards cited: ACI 318 Table 19.3.2.1, ASTM C1904,
ASTM C1898, ASTM D4783, ISO 22196.