Concrete is the most commonly used material in sewers, especially for large sewer pipes, which can be attributed to its strength, durability and relatively low maintenance costs. However, due to the nature of concrete and the unique sewer atmosphere, corrosion of concrete pipes and associated infrastructure (e.g. manholes and pumping stations) has been widely recognised as a significant challenge for the utility industry.
Concrete corrosion in sewers is caused by a series of abiotic and biotic corrosion processes. Acidification of concrete caused by carbonation and gaseous hydrogen sulfide initiates the first stages of corrosion, before developing into the highly destructive, microbially induced corrosion process.
The chemical reactions of carbon dioxide and gaseous hydrogen sulfide with the concrete lower the surface pH of the concrete to about eight, enabling the growth of sulfide oxidising bacteria.
As the surface pH gradually decreases, the sulfide oxidising bacteria colonise the concrete surface and produce large amounts of biogenic sulfuric acid, which can directly attack the cementitious materials in concrete.
Collectively, gaseous hydrogen sulfide and microbial activities are the major causes of concrete corrosion in sewers.
Professor Zhiguo Yuan, Director of The University of Queensland’s Advanced Water Management Centre, said that concrete sewer corrosion is a long-standing and costly problem for the water industry.
“The corrosion and deterioration of concrete sewers require costly rehabilitation and replacement by utilities. An annual expenditure of billions of dollars was estimated for repairing and replacing corroded sewers worldwide, which is expected to increase as the aging pipes proceed to fail,” Professor Yuan said.
“In the US alone, corrosion is causing sewer asset losses estimated at around $14 billion per year. The concrete corrosion process can result in the rapid loss of concrete mass, which can be up to 10mm per year and reduce 50-100 years of expected service life to less than ten years in some extreme conditions.”
Currently, various technologies have been developed to control or mitigate the concrete corrosion process in sewers. The technologies can be divided into four commonly used strategies:
- Preventing hydrogen sulfide production in the liquid phase, or minimising its transfer to sewer air through the dosing of chemical agents such as magnesium hydroxide, iron salts, hydrogen peroxide, nitrate, caustic and free nitrous acid
- Using ventilation systems to reduce hydrogen sulfide and humidity levels in sewer air, and slow the corrosion process. In some cases, the ventilated sewer air needs to be treated with physical adsorption, chemical scrubbing or biofilters to reduce hydrogen sulfide levels and prevent odour issues
- Treating concrete surfaces exposed to sewer air with antimicrobials like silver-loaded zeolite to inhibit the activities of bacteria and reduce sulfuric acid production, or protective epoxy coatings to prevent the direct contact of sewer gas with the concrete pipes
- Adding alternative binders, such as fly ash and silica fume, to the cement for new sewers, and alternative aggregates like ground limestone, in order to increase its corrosion resistance
Professor Yuan said that all of these strategies can help, but at a cost.
“The first two strategies require continuous or frequent chemical dosing or operational efforts to achieve effective corrosion control, which incurs continual, substantial costs,” Professor Yuan said.
“For the third strategy, the coating and surface treatment are temporary approaches that need regular reapplication, disrupting the operation of the sewer and inducing hefty costs.
“With the fourth strategy, the addition of alternative binders and aggregates doesn’t show significant improvements in corrosion resistance in very acidic conditions.
“As such, there is a need to develop a long-lasting, effective and environmentally-friendly antimicrobial agent to be added into concrete, since the microbial process plays a critical role in the corrosion process.”
Novel approaches to overcoming corrosion challenges
Professor Yuan said that the Advanced Water Management Centre’s Sewer Research Group (SRG) has been working very closely with the Australian water industry for over 15 years on sewer corrosion mitigation.
“We have developed modelling tools and various technologies to support the industry in its constant battle with sewer corrosion. We are currently undertaking two projects.”
Reducing concrete corrosion rate using free nitrous acid
This novel method controls microbial concrete corrosion in sewers by using a brand new, low-cost and environmentally-friendly antimicrobial agent: free nitrous acid (FNA).
Since microbial processes play critical roles in the corrosion process, one potential solution is to suppress the corrosion-inducing microbes by blending antimicrobial agents into cement.
The SRG proposed the use of FNA as an antimicrobial agent based on its previous groundbreaking research, which demonstrated that FNA, at parts per billion (ppb) levels, is a metabolic inhibitor to a broad range of microorganisms, and at parts per million (ppm) level, is a potent biocidal agent, causing cell death/paralysis.
FNA can be applied in different ways:
- For new pipes, the SRG proposes that nitrite is incorporated into cement as an admixture. It is hypothesised that when corrosion is initiated and an acidic concrete surface starts to form, nitrite released from the wet concrete surface is acidified to form FNA. The FNA generated in-situ would provide ongoing inhibition to the growth of corrosion-inducing microorganisms throughout the service life of the sewer pipe
- For existing sewers, the SRG proposes spraying nitrite on the concrete surface to instantly kill the corrosion-causing microorganism, via the strong biocidal effects of FNA. The recovery of the microbes will take years, thus intermittent spraying with an interval of several years would be effective to slow down the corrosion process
Recently, the SRG received an Australian Research Council (ARC) Linkage Project grant, enabling it to proceed with its work on reducing sewer corrosion through model-supported ventilation control, in partnership with DC Water (US), Melbourne Water, Urban Utilities and Water Corporation.
It is universally recognised that sewer ventilation influences humidity and hydrogen sulfide levels in sewer air, thus influencing sewer corrosion rate. However, the quantitative dependency of corrosion on sewer humidity has not been well established.
In this project, the SRG is aiming to investigate the impact of sewer wall moisture on the viability and activity of the corrosion-inducing biofilms.
The project will also develop dynamic models to predict sewer humidity, temperature and corrosion rates, and provide ventilation strategies to reduce sewer corrosion, with further outcomes to be revealed within the next three to five years.
Testing out the technology in real-world sewer systems
Professor Yuan said that the SRG has been investigating the biocidal effects of FNA on sewer biofilms to reduce the generation of hydrogen sulfide for over ten years.
“This has led to the development of the Cloevis technology. With this technology, FNA is periodically added to sewage at pumping stations (e.g. 8-24 hours every 2-3 weeks).
“The shock dose of FNA leads to inactivation of sulfide-producing sewer biofilms in the water phase, thus suppressing sulfide formation in the following two to three weeks when another dose is due.
“Since 2013, we have extended the technology to control biofilms on concrete surfaces exposed to air, thus reducing corrosion even in the presence of hydrogen sulfide in the air phase.”
Professor Yuan said that South East Water, City of Gold Coast, Urban Utilities and US-based DC Water all collaborated on the FNA project and the technology has been successfully tested in the City of Gold Coast’s sewer manholes.
“Concrete coupons with calcium nitrite as an admixture were exposed in a sewer manhole, together with control coupons that had no nitrite admixture.
The corrosion process was monitored by measuring the surface pH, corrosion product composition, concrete corrosion loss and the microbial community on the corrosion layer,” Professor Yuan said.
“During the exposure, the corrosion loss of the admixed concrete coupons was 30 per cent lower than that of the control coupons.
“The sulfide uptake rate of the admixed concrete was also 30 per cent lower, leading to a higher surface pH in comparison to that of the control coupons.
“The results obtained demonstrated that this novel use of calcium nitrite as an admixture in concrete is a promising strategy to mitigate microbially-induced corrosion in sewers.
“Work has also been completed at the Luggage Point Wastewater Treatment Plant in collaboration with Urban Utilities.
“It was demonstrated that the calcium nitrite admixture level in the concrete correlates positively with the corrosion mitigation effect, with 17 per cent and 47 per cent reduction in the corrosion rate for one per cent and four per cent calcium nitrite, respectively.”
Professor Yuan said that in addition to the completed field work, there are long-term corrosion experiments happening in the laboratory corrosion chambers.
“Our world-class corrosion chambers are designed to simulate real sewer conditions at crown and tidal regions with controlled levels of hydrogen sulfide, humidity and temperature, which are favourable to investigate the fundamental corrosion mechanism and microbial activities,” Professor Yuan said.
“A three-year laboratory experiment, including more than 220 concrete coupons at different FNA levels (zero per cent, one per cent, two per cent, three per cent and four per cent), was started in 2019 in order to identify the long-term effects of FNA, the underpinning mechanism, the microbial activities and their evolution with time.
“These fundamental studies are extremely beneficial to obtain a thorough understanding of this promising technology, and serve as guidance for further study and practical applications.
“In addition to closely working with our industry partners and disseminating knowledge to these partners directly, we will continue to publish our results at industry conferences and in peer-reviewed research journals to broaden the influence of this technology.
“We are keen to assist industry with constructing new admixed concrete pipes and installing them into real sewer systems.”