Chemically Resistant Sewer Pipelines
In a sewer the concrete is not consumed where the sewage flows: it is consumed above, in the crown, where the liquid never reaches. What eats it is a gas — hydrogen sulphide — which certain bacteria turn into sulphuric acid on exactly the damp surface that no flow ever washes. Designing a durable sewer therefore means dealing with the hydraulics first and with the material only afterwards.
Overview
TAV. 00The process covers the construction of sewers and industrial drains destined for a chemically aggressive environment, from reading the network through to the monitoring plan. The sequence follows the real causal chain: first identify where the sewage turns septic and releases hydrogen sulphide, then correct through network geometry the conditions that generate it, then choose a binder the acid attacks more slowly, and finally protect the crown where exposure remains severe. Commissioning is not limited to the tightness test: it includes a baseline video survey that becomes the reference for every later inspection. The recurring defects — corroded crown, sediment, open joints, ovalised pipes — arise at different stages but all show up in the same way, that is once the pipeline is buried and any intervention costs an excavation.
The site score
TAV. HThe detail of the craftThe gas begins upstream of where it corrodes
Read the network, not the single stretch
Hydrogen sulphide is not generated where it later corrodes: it is generated upstream, where the sewage stands still long enough to turn septic. The places are always the same — stretches with too flat a gradient, rising mains downstream of a pumping station, siphons, chambers with long residence times — and they must be identified on the complete network, not on the stretch under contract. Where a network is already in service the gas is measured in the field at the critical hours, typically in summer and at minimum flow, because those are the conditions in which the problem shows itself.
Analysis of the upstream network identifying the points where hydrogen sulphide is generated, a field measurement campaign under critical flow and temperature conditions, and preparation of the exposure map by homogeneous stretch.site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
PLATE 01
Corrosion is not where the sewage flows
The cycle closes above the wetted surface: the gas rises, condenses on the crown, bacteria oxidise it to acid and the acid consumes the cement matrix. The invert, washed continuously, stays intact.
The diagram illustrates the mechanism. Exposure classes, durability requirements and acceptance criteria are to be taken from the design and the applicable standards.
PLATE 02
Three defences, in this order
Velocity stops the gas from forming, the binder slows the acid that forms anyway, the lining protects where exposure stays severe. Reversing the order means paying for the third because the first was never done.
The three defences are illustrated in their logic. Gradients, exposure classes and lining performance are to be taken from the design and the applicable standards.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QWhy does the invert survive and the crown does not?
Because the acid does not arrive with the sewage, it forms on the wall. The invert is wetted and washed continuously: the bacterial colony cannot establish itself and whatever forms is carried away. The crown is covered by a condensate film that is never renewed, and that is where the bacteria work undisturbed.
Is geopolymer concrete enough on its own?
It slows the attack, it does not eliminate it. The advantage is real and comes from the absence of free lime, which in Portland binder is the first thing the acid dissolves. But if the hydraulic conditions keep producing hydrogen sulphide in quantity, sooner or later the front arrives: the choice of binder is the second defence, not the first.
How often should a sewer be video-surveyed?
There is no single interval: it depends on the exposure class of the stretch. Severe stretches identified in the map need frequent revisiting, those in normal conditions far less often. The monitoring plan exists precisely for this, and it only works if there is a baseline video survey to compare later ones against.
Can a sewer be rehabilitated without reopening the trench?
Yes, with in-situ lining systems: cured-in-place liners, pipes pulled through, sprayed mortars in man-entry sections. These are mature solutions, but they return a reduced effective section and do not correct the gradient. If the original problem was hydraulic, rehabilitation buys time without solving it.