Water Journal : Water Journal December 2012
refereed paper asset management water DECEMBER 2012 91 • Relative humidity (RH) and that the corrosion of concrete sewers involved a succession of chemical and biological processes as the pH of the concrete surface is progressively lowered. Further, it was understood that Thiobacillus concretivorus (now known as Acidithiobacillus thiooxidans) was responsible for the generation of sulphuric acid when the pH of the concrete surface dropped below pH 4, and that an RH of greater than 87% and a concrete moisture of greater than 4% was needed for the bacteria to be active. The highest rate of activity was understood to be at approximately 30°C. The SCORe Project has used newly developed genomic analytical methods (Cayford, 2012) to identify the microbial communities present during corrosion of concrete sewers below pH 4. These studies have identified a far greater diversity of microbes, which suggest that A. thiooxidans may not be the sole microbe 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 (Joseph, 2012). The role of carbon dioxide in the early corrosion processes has been found to be far less important than previously thought and H2S concentration is the far more dominant factor in the early corrosion processes. Temperature and RH also influence the rate of corrosion at these neutral and high levels of 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 (Figure 3). 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 (Wells et al., 2011). Corrosion and odour control methods The methods for controlling odour and corrosion in sewers has not changed significantly since the 1989 Manual, as shown in Figure 4, which is reproduced from the 1989 Manual. Although the methods have not changed, the popularity of various chemicals used by the water industry in Australia to control odour and corrosion has seen some changes. A recent survey carried out by the SCORe Project (Ganigue et al., 2011) identified that there are five chemicals 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 (Gutierrez et al., 2008; Zhang et al., 2009; Jiang et al., 2009; Gutierrez et al., 2009; Pikaar et al., 2011) 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. Development of the Sewex model for predicting sulfide 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 (Sharma et al., 2008). To further optimise the dosing of chemicals, on-line control strategies have been developed for the five popular chemicals using a level of sophistication of sensors appropriate to the application (Ganigue et al., 2012). Savings in chemical use of up to 50% have been achieved with the use of on-line control. In addition to optimising the use of the popular chemicals for control of odour and corrosion, the SCORe Project has developed two new methods: • Free Nitric Acid (FNA); and • In-sewer electrochemical generation of chemicals for control of sulfide. FNA has a strong biocidal effect on the biofilm in sewer pipes that generate the sulfide (Jiang et al., 2011). This control method is very cost effective, as the FNA can be dosed intermittently as the biocidal effect has been found to reduce sulfide generation by more than 50% for up to 14 days. An exciting new method for control of odours in sewers is by generation of chemicals such as sodium hydroxide and oxygen within the sewer by electrochemical process to control the generation of sulfide (Pikaar et al., 2012). 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 (Gutierrez et al., 2011) can now be used to evaluate prospective odour control additives for sewer. 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. Figure 3. Laboratory Corrosion Chambers (a) and Field Corrosion Coupon Installations (b), (c) and (d).
Water Journal February 2013
Water Journal November 2012-1