The state of sewer vents – and the hidden risks you need to know about

Vent shafts are often ‘hidden’

As we recently reported in our article on the history of sewer vents, vent shafts are often ‘hidden’ in plain sight. And while some see their inconspicuousness as an advantage, there’s another unseen aspect that’s not so favourable. With thousands of sewer vents around the country potentially close to – or well past – their use by date, there are veiled risks you should be aware of.

As Duncan Reynolds, Research and Development Manager for SVSR observes, “New South Wales alone has more than 15,000 sewer vent pipes and chimneys. And they are spread across other cities including Adelaide, Brisbane and Melbourne. They date from the early 1800s and many around today may be 70, 80 or even 100 years old.”

And with age often come problems that need addressing by experts in sewer vent shaft design, fabrication, installation… and maintenance.

The State of Sewer Vents

The state of sewer vents

Vent cowls and shafts play a crucial role in wastewater networks, by providing adequate ventilation, but not everyone realises that, says Duncan. Everybody thinks that vent shafts are a relic
of the past. Everybody wants to get rid of them. Sensible asset
managers understand the reality is that we will always need
them. Ventilation of sewer networks is as much a part of our future as our past.”

Sewer pipelines often produce toxic gases including hydrogen sulphide (more widely known as ‘rotten egg gas’) which sewer vent pipes release. While odour is one aspect, it’s not the only one.

“Odour and corrosion are almost always found together. Under typical sewer conditions, odorous gases chemically react to form acids, which cause corrosion of our valuable and critical pipes,” explains Duncan. “If the problem persists, this leads to leaks and even complete collapse of the sewer system, with catastrophic consequences.”

When sewer stacks corrode, it leaves them structurally unsound and even prone to wild weather. “A lot of vent shafts tend to be made from reinforced concrete, which is concrete and steel. When the concrete corrodes, it exposes the steel core and becomes a very good conductor,” explains Duncan. “We’ve seen vent shafts hit by lightning and even explode. And after electrical storms, it’s not uncommon to see them a tonne pipe strewn across three or four backyards.”

armidale corroded vent photo

Unseen risks

Yarra Valley Water in Victoria has a network of around 1200 sewer vent pipes, typically nine to twelve metres tall, between 70 and over 100 years old, and found in the inner suburbs of Melbourne. They have conducted regular visual inspections and had a rehabilitation plan in place for many years, with their records indicating on average less than one vent fail per year due to structural conditions.

Due to the possibility of damage or personal injury with vent collapse – and feedback from technicians – the company decided to evaluate their asset management … and the results were alarming. They discovered after reanalysing their data that there was in fact an average of five sewer vent failures per year.

The problem was that the damage from corrosion of the vent shaft was unseen. It was generally found to occur below the seal, between the vent tube and base of the vent shaft. This corrosion was found to be the main cause of structural failures – and it was impossible to identify through standard visual checks.

A detailed analysis of 20 vent tubes found that corrosion was often severe – with, in many cases, vents having only 1-2mm remaining wall thickness.

A common issue

A common issue

The results of the Yarra Valley Water investigation concluded that i) indication of structural failure was hidden from inspection, and ii) this meant there was a major inadequacy of existing visual inspection techniques.

Duncan is not surprised by this conclusion.

“I wouldn’t be surprised to find that across many councils and utilities companies. Most of the sewer vents we see are 70 or more years old – and we have found both the concrete and steel reinforcement often have severe corrosion.

The trouble is that the corrosion is internal. The pipes tend to only be about 30cm wide without light, and it is extremely difficult to assess them without taking them apart. Non-destructive testing (NDT) is unsuitable for this type of product. And without knowing what exactly is going on inside the pipe, a patch is likely to be ineffective in the long term.”

Sewer vent solutions

Sewer vent solutions

Another factor for consideration is that sewer vent shaft lifespan can vary enormously, depending on location, climate and the quality of construction.

And those without a cowl may be blocked with leaves, rubbish and even dead animals. “As such, there is no easy way of telling how much life is left in a pipe,” explains Duncan.

Failing or clogged sewer vents may cause further corrosion through the wider network, or pose a risk to people or property via collapse. The options are to pay for a very expensive assessment, which involves dismantling the sewer stack pipe, or to get a replacement, which is what most customers opt for.

“It’s much more cost effective, and you have a shaft that is significantly better quality,” says Duncan. “Today’s versions are fabricated in high-grade stainless steel – which provides better durability and requires minimal maintenance. And we can add rotating educt vent cowls, induct cowls or custom-designed cowls for added efficiency.”

Options for sewer vent shafts and vent cowls

Options for sewer vent shafts and vent cowls

The Yarra Valley Water project identified that sewer vents present a significant risk of structural failure due to hidden corrosion. By identifying the mechanism of vent structural failure and by completing a full asset base survey, it was able to improve reporting methods, maintenance and replacement techniques.

SVSR believes that replacement with a quality vent shaft is the best solution for aging sewer vents, and regular maintenance is recommended to monitor existing sewer networks. We’re Australia’s leading provider of sewer vent shaft design, fabrication and installation… and we provide monitoring services for clients including Sydney Water and regional councils.

If you need sewer vent solutions – including shafts, rotating educt cowls and induct cowls – then we have the expertise to provide a premium solution.


Enquiry for: Sewer Vent Design, Fabrication, Installation

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29 Shepherd St, Liverpool
NSW 2170

02 8798 8788