Australian sewers are being corroded partly because of an additive used in the drinking water treatment process. In some cases the lifetime of concrete pipes is being reduced by up to 90%.
But much of that corrosion could be reduced by a simple change in the chemicals used to treat the water, we report in a paper published today in Science.
Our urban-based societies are completely dependent on extensive infrastructure support to function effectively.
Wastewater collection in sewer networks is arguably one of the most critical elements in today’s cities. It helps to protect public health and enable the productive economic activities that are crucial for our high living standards.
The corrosive element
But the effective operation of these critical sewer networks is under constant threat due to a simple but powerful molecule: H2S or hydrogen sulfide.
Sulfide is generated in the sewage from sulfate and organic waste and can be stripped as hydrogen sulfide gas into the sewer atmosphere.
On the upper walls of the sewers, microbes take up the H2S and oxidise it with air to form sulfuric acid, H2SO4. This is extremely powerful in corroding concrete, which is the most common material used in large sewer pipes.
This corrosion process converts solid concrete into a soft, crumbling powder at a rate of up to 10mm/year or more in extreme cases. This can reduce the useful lifetime of sewer pipes from the expected 50-100 years to as little as 10 years!
Given the huge costs of building and maintaining extensive infrastructure networks such as wastewater collection systems (typically 70% of total infrastructure value is in the pipes), it is crucial to manage this corrosion process effectively to ensure a long service life of sewer pipes.
Corrosion a costly matter
Sewer maintenance and repair or replacement is already costing Australian water utilities hundreds of millions dollars per year. A similar amount is spent on various mitigation efforts such as chemical dosing and ventilation, to minimise the corrosion process. So any deterioration of our ageing sewer infrastructure is a major concern.
Why can we not reduce the hydrogen sulfide generation in the first place? Well, maybe we can if we manage the whole urban water system in an integrated way, and particularly look at the water treatment processes far upstream!
Our study was done over several years and is the first to reveal this surprising connection between water treatment and wastewater management.
To reduce the sulfide formation we would need to reduce either the sulfate or the organics in the sewage. The latter is not possible due to the continuing waste discharges from households and industries, which is indeed a major function of the sewer system.
Treatment part of the problem
But when we carefully identified the sources of the sulfate in the wastewater we made the startling discovery that 50% or more could be added in the purification process at the drinking water treatment plant.
Chemical coagulants are added in most water treatment processes to remove turbidity from the source water.
While the aluminium binds to the particles in the water and is removed in the process, the sulfate is soluble and remains in the treated drinking water.
This is of no concern for human health but can have major impacts in the sewer systems downstream due to the sulfide-induced corrosion.
In our study, we have clearly identified that in systems with low sulfate levels in the raw water (as is typically the case with dam-based water supplies), the sulfate added in the drinking water treatment can cause significant additional sulfide formation in sewers.
Interestingly, the production of desalinated or recycled water typically eliminates sulfate from the final product water. That creates a potentially very valuable benefit for downstream sewer protection.
Why didn’t we know before?
The main reason this surprising connection has not been discovered earlier is likely due to our institutional separation of the urban water system into water and wastewater sections. These are often run by different organisations.
Therefore a more fully integrated urban water management approach is necessary to identify such interactions and determine the most optimal long-term solution for the overall system, rather than primarily minimising costs locally.
In our current situation, a change to non-sulfate coagulants could be quite easily done. This would dramatically reduce concrete corrosion – by 35% after just 10 hours and 60% over longer durations.
It would likely incur only marginal additional treatment costs at the drinking water plant but could generate significant overall savings across the whole water system by reducing corrosion.
Some cities, such as Sydney, are already using non-sulfate coagulants in their water treatment, although usually due to local availability or cost benefits rather than the whole-system impacts demonstrated in our study.
Nevertheless, this demonstrates that these alternative coagulants are just as effective and efficient in water treatment operations.