Research provides solutions for proactive sewer corrosion and odour management

The complexity of monitoring in-sewer processes and the industry’s limited understanding of optimal corrosion and odour management techniques have historically made it difficult to identify an optimal solution for managing sewer corrosion and odour complaints. Recent changes, including the removal of heavy metals from sewage due to tighter controls of trade wastes, the implementation of water-saving measures and devices which are increasing the concentration of pollutants, and rapid population growth and urban sprawl have further exacerbated this challenge, increasing the length and retention time in sewers.

To address this gap and improve industry knowledge of in-sewer processes, the Australian Research Council (ARC) Linkage Program in collaboration with many Australian water utilities funded the Sewer Corrosion & Odour Research Project (SCORe) completed in 2013 - a $21 million, five-year project to uncover beneficial tools and innovative technologies to help water utilities manage odour and corrosion issues within sewer systems. The project, which was completed in conjunction with an ongoing Water Environment Research Foundation (WERF) ventilation research program, leveraged the input of 15 water industry partners (www.score.org.au) and five university partners to develop new or refined techniques for:

• sulfide speciation using ionic chromatography, which provides valuable information on all the sulfur species allowing for accurate mass balance of these compounds;
• determining the importance of relative humidity (RH) for the concrete corrosion process and established a preliminary relationship for sewer corrosions in relation to H₂S concentrations, temperature and RH;
• predicting both natural and forced ventilation air movements in sewers based on the conservation of momentum model which has proven to be much more reliable than previous models;
• understanding of the physical, chemical and biological processes involved with the five most commonly used chemicals used for sulfide control, developing their application guidelines and online control algorithms;
• understanding the most common odorous compounds in sewers, establishing their relative abundance, reproducible methods of analysis and performance of abatement systems for treating these compounds;
• understanding the performance and causes of delamination of epoxy resins and the effective life expectancy of various sacrificial coatings such as calcium aluminate cement (CAC), gunite and calcareous aggregate concrete;
• developing new sulfide control approaches.

The SCORe team developed a powerful modelling tool, which describes all key in-sewer chemical, biological and physical processes. It comprises components that predict:

• biological processes in the liquid phase including the reactions catalysed by the sewer biofilms, leading to the production of hydrogen sulfide, methane and other compounds;
• chemical processes in the liquid phase leading to the oxidation, precipitation of hydrogen sulfide as well as the variation of sewage pH;
• gas-liquid mass transfer of volatile compounds including hydrogen sulfide, methane and oxygen, among other volatile organic and inorganic compounds;
• the oxidation of hydrogen sulfide on concrete sewer walls exposed to sewer air, leading to its corrosion; and
• the movement of  sewer air under forced and natural ventilation conditions.

All these components have been effectively integrated in one modelling platform called SeweX.

The integrated model is able to predict the spatial and temporal variations in the concentrations of hydrogen sulfide and numerous other carbonaceous, nitrogenous and sulfurous compounds, based on which corrosion and odour ‘hotspots’ can be identified. The model also predicts the effects of chemical addition (eg, iron salts, magnesium hydroxide, nitrate and oxygen) and ventilation on sulfide control, which supports the optimal design and operation of these strategies.

To collect inputs for the model, existing data is analysed, including physical sewer characteristics and flow data; data on wastewater depth and flow for hydraulic simulations; sewer airflow and air pressure for airflow simulations; wastewater characteristics such as sulfide speciation for in-sewer biological simulations; gas phase hydrogen sulfide (H₂S) concentrations to fine-tune system-wide H₂S model predications, and in-sewer RH data for corrosion rate assessment of concrete assets. Some examples of the modelling outputs illustrating sulfide and corrosion rates in the gravity sewers for identifying hotspots are presented in Figures 1 and 2.

Figure 1: Dissolved sulfide levels determined by SeweX.

Figure 2: Corrosion rates in gravity sewers determined by SeweX using measured RH values.

Once validated, the model is used to identify hotspots or locations in the sewer network where odour release could result in odour complaints or where high H₂S concentrations could lead to an unacceptable corrosion risk. The team developed and evaluated several potential control solutions using an integrated approach that considered the biological in-sewer processes, ventilation and corrosion risk. The tools and associated guidance documents that were developed as a result of the research findings are helping utilities gather accurate field data and enabling improved designs of sewer system gas phase and treatment systems for greater optimisation and cost savings. The tools allow utilities to identify a preferred management strategy for handling corrosion and odour, after taking into account the costs, benefits and drawbacks, as well as the ability to achieve the utility’s strategic objectives such as: ensuring design asset life is achieved (ie, ‘acceptable’ corrosion rates), lifecycle costs of corrosion and odour management are minimised and no adverse odour impacts are experienced from any network asset.

All of the major industry partners have used the tools developed as part of SCORe, and utilities around the world are already benefiting from the research completed. Notable studies have been undertaken by Sydney Water, which demonstrate how a proactive approach to sewer corrosion and odour management helped the utility achieve a considerable cost saving compared to the conventional approach. In addition to cost savings, this approach produced better outcomes in terms of sewer asset management and reduced odour impact. Other studies by South Australian Water and Gold Coast Water have also resulted in considerable savings from optimised chemical control strategies and better overall outcomes. The savings achieved to date by the partners through the use of the model already amount to several hundred million dollars.

The SCORe project has made a significant contribution to the industry’s understanding of in-sewer processes and enables utilities to support strategic decisions about maintaining sewer systems. The project, recognised for its innovation, received industry recognition from the International Water Association, earning the prestigious 2014 Global Award in Applied Research. By developing a greater fundamental understanding of the in-sewer processes involved in various aspects of managing odour and corrosion, the water industry can take the research from SCORe and progress from a reactive approach to controlling odour and corrosion to a more proactive approach, which will be critical to managing the maintenance and repair costs of ageing infrastructure, as well as minimising odour complaints from communities.

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 operationally by the following key members of the Australian water industry:

Sydney Water Corporation, NSW
Water Corporation, Western Australia
City of the Gold Coast, Queensland
South East Water Limited, 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 teams at:

The University of Queensland
The University of Newcastle
The University of New South Wales
The University of Sydney
Curtin University of Technology

For further information, visit: www.score.org.au.