Advances in the management of corrosion in sewers

Corrosion of sewer assets is a worldwide phenomenon, particularly in countries with a warm climate. It is estimated that concrete sewer pipes in many areas of Australia are being corroded at an average rate of 1-3 mm per year or more. Hence, instead of providing service for 50 to 100 years as recorded in water utilities asset registers, sewer pipes are failing after 20 years or less. Internal surveys by several major water utilities in Australia show that the abnormally fast depreciation of assets and the mitigation of corrosion problems alone are costing the Australian water industry hundreds of millions of dollars a year.

Sewer corrosion and odour are closely linked. Hydrogen sulfide (H₂S) is generated under anaerobic conditions in sewers, mainly in long rising mains, and this is the main source of obnoxious odours emanating from sewers. This same H₂S is also the main cause of accelerated corrosion in sewers due to the work of bacteria which convert the H₂S under aerobic conditions, mainly in the headspace of gravity sewers, to sulfuric acid.

Management of corrosion in sewer systems is experiencing significant new challenges. Restricted water use in many areas results in considerably reduced flows. This has resulted in more concentrated sewage and increased hydraulic retention time resulting in higher H₂S concentrations. Also, the expansion of sewer networks in many cities to service fringe developments and new estates are making hydraulic retention times in sewer networks longer and more subject to anaerobic conditions, which is exacerbated by greater reliance on pumping stations to connect these new areas.

In an effort to address these challenges, most of the major water utilities in Australia are jointly funding, along with the Australian Research Council (ARC), a major project - the Sewer Corrosion & Odour Research (SCORe) Project which started in late 2008 and will run for five years with a total budget of around $20 million.

The project comprises four themes, which are:

• Theme 1 - Corrosion processes,
• Theme 2 - Gas phase technologies,
• Theme 3 - Liquid phase control, and
• Theme 4 - Knowledge management.

Nine inter-linked subprojects (SP) have been designed under these themes, each has distinctive foci and is being undertaken by a dedicated research team of people who are located at one or more research centres around Australia.

Research on the SCORe Project covers both corrosion and odour; however, this article focuses on the significant advances in the management of corrosion in sewers being achieved by the project. A full coverage of the outcomes being delivered to the water industry by the SCORe Project is available on the project website www.score.org.au.

Prediction of H₂S generation in sewers

Improved understanding through the SCORe Project of physical, chemical and biological processes occurring within sewers has led to the development of an advanced mathematical model which is capable of predicting both spatial and temporal variations in H₂S concentration as well as other sewer parameters including GHG emissions.

This sulfide generation model, the SeweX Model, has been linked to sewer hydraulic models such as MOUSE to predict the dynamic changes in sulfur compounds within the sewer system as a result of changing sewer characteristics such as diurnal variations.

Bacterial activity and the rate of corrosion

The SCORe Project has used newly developed genomic analytical methods to identify the bacterial communities present during corrosion of concrete sewers below pH 4. These studies have identified a far greater diversity of bacteria, which suggests that Acidithiobacillus thiooxidans (the traditionally thought of culprit) may not be the sole bacterium responsible for corrosion of the sewers at low pH and that other processes may also be involved. This research is continuing and promises to reveal a far greater fundamental understanding of corrosion processes at low pH.

Also, a better understanding of concrete corrosion processes at neutral and high pH is being developed. The role of carbon dioxide in the early corrosion processes has been found to be far less important than previously thought and that H₂S concentration is the far more dominant factor in the early corrosion processes. Temperature and relative humidity also influence the rate of corrosion at neutral and high pH.

Concrete corrosion in sewers is being studied both in the field, with specially prepared coupons in six locations in Sydney, Melbourne and Perth, and under controlled conditions in 36 corrosion cabinets in a laboratory. In addition, historic records of corrosion and environmental conditions are being analysed with the objective to develop a new fundamental process-based corrosion model to be able to accurately predict concrete corrosion rates in sewers under all conditions.

Liquid phase control methods

A recent survey carried out by the SCORe Project identified that there are five chemicals added to sewer flows for the control of gaseous H₂S concentrations in sewers that are now popularly used by the Australian water industry:

• Magnesium hydroxide,
• Sodium hydroxide,
• Nitrate,
• Iron salts, and
• Oxygen.

Detailed laboratory and field testing has been conducted with these five chemicals to gain a better understanding of the physical, chemical and biological processes involved with each chemical to enable:

• Optimal dosing rates,
• Appropriate dosing locations, and
• Mathematical modelling of the processes.

The development of the SeweX model for predicting H₂S generation in sewers can now be used to do desktop evaluation of the performance of various chemicals with various dosing locations to optimise the control method selected.To further optimise the dosing of chemicals, online control strategies have been developed for the five popular chemicals using a level of sophistication of sensors appropriate to the application. Savings in chemical use of up to 50% have been achieved with the use of online control.

In addition to optimising the use of the popular chemicals for control of gaseous H₂S, the SCORe Project has developed two new methods:

• Free Nitric Acid (FNA), and
• In-sewer electrochemical generation of chemicals for control of H₂S.

FNA has a strong biocidal effect on the biofilm in sewer pipes that generate the H₂S. This control method is very cost effective as the FNA can be dosed intermittently as the biocidal effect has been found to reduce H₂S generation by more than 50% for up to 14 days.

An exciting new method for control of H₂S in sewers is by generation of chemicals such as sodium hydroxide and oxygen within the sewer by electrochemical process to control the generation of H₂S. This method has enormous potential as the overall cost is much less than traditional chemical dosing methods and avoids the transport and storage of large amounts of hazardous chemicals.

A laboratory method used in the SCORe Project, called the SCORe-CT method, can now be used to evaluate prospective odour control additives for sewers. This allows new odour control additives (chemicals or biological agents) to be evaluated under controlled laboratory conditions where one wastewater line has the additive dosed and the other wastewater line is used as a control. This removes the natural variations that occur in the field that make evaluation of additives open to interpretation.

Ventilation of sewers

The objectives of ventilation systems in sewers are to:

• Maintain zero relative velocity between wastewater and ventilating air to minimise the rate of H₂S emission and evaporation from the wastewater surface, and/or
• Change the air sufficiently to maintain dry sewer structures at all times, (ie, never allow the air dew point to exceed wall temperature); and
• Minimise the build up of H₂S in the sewer air.

Achieving these objectives with natural ventilated systems has always proved very difficult to predict. In association with Water Environment Research Foundation (WERF) in the USA, the SCORe Project has developed a new ventilation model which has proved in field testing in Adelaide and Perth to be much more reliable than previous models. A spreadsheet-based tool has been developed which can be used to predict both natural and forced ventilation air movements in sewers.This SCORe Ventilation Tool is now being used by many of the water utilities involved in the project.

The new ventilation algorithm is also being used in a supplementary component of the SeweX Model to provide a virtual dynamic prediction of air movements and gas phase H₂S concentrations within a sewer network.

Conclusion

Approximately four years into its five-year lifetime, the SCORe Project has achieved many milestones and has been able to provide to industry partners many valuable deliverables, some of which have already caused major changes to industry practices and decision making.

By developing a greater fundamental understanding of the processes involved in various aspects of corrosion, the water industry will be able to move from a reactive approach of prematurely rehabilitating or replacing valuable assets to being able to control rates of corrosion to allow sewers to achieve their full designed service life.

Acknowledgements: The authors acknowledge the Sewer Corrosion and Odour Research (SCORe) Project LP0882016 funded by an Australian Research Council Industry Linkage Project Grant and supported financially and in-kind by the following key members of the Australian water industry: Sydney Water Corporation, NSW; Water Corporation, Western Australia; Gold Coast City Council, Queensland; South East Water, Victoria; Melbourne Water Corporation, Victoria; Hunter Water Corporation, NSW; South Australia Water Corporation; Barwon Regional Water Corporation, Vic; CH2M Hill Australia; Water Quality Research Australia; Veolia Water, Australia; ACTEW Water, ACT; Queensland Urban Utilities, Queensland; Yarra Valley Water, Victoria; District of Columbia Water, USA; and acknowledge the work done by the Research Partners led by the following Chief Investigators and their teams: Prof Zhiguo Yuan, University of Qld; Prof Jurg Keller, University of Qld; Prof Rob Melchers, University of Newcastle; Prof Richard Stuetz, University of NSW; Dr Phil Bond, University of Qld; Dr Marjorie Valix, University of Sydney; A/Prof Jeffrey Charrois, Curtin University (for more details see: www.score.org.au).

An overview of the project design is shown in Figure 1 - An overview of the SCORe Project.