Water Journal : Water Journal December 2012
asset management refereed paper technical features 90 DECEMBER 2012 water Identification of Knowledge Gaps The fundamental processes of odour and corrosion occurring in sewers as presented in the 1989 H2S Manual are basically unchanged, as shown in Figure 2, which has been reproduced from the 1989 H2S Manual. However, there are many practical questions where there is inadequate fundamental knowledge to provide robust solutions to water practioners, including the following: • Corrosion Processes & Control: The estimation of the corrosion rate and the life expectancy of pipes are very difficult to predict and are almost entirely based on empirical data about the past performance of pipes under similar conditions. • Liquid Phase Technologies: There is a lack of understanding of the chemical and biochemical transformations that occur in wastewater and the impact of variables such as flow velocity, sediments, and changes in wastewater composition. This makes it difficult to predict the impact of chemicals commonly used to control H2S in sewers such as O2 , NO3 - , Mg(OH)2 , FeCl3, etc. Without closing these knowledge gaps it is not possible to optimise dosing systems for the control of odours in the liquid phase or to reliably predict the impact of dosing systems on the receiving wastewater treatment plants. • Gas Phase Technologies: It is difficult to quantify and characterise odours from sewers without relying purely on costly, problematic and time-consuming human olfactory systems. In addition, applications of odour abatement systems rely on empirical data, with little fundamental understanding of the processes occurring for the removal of the odour. Areas where the SCORe Project is achieving significant advances in knowledge leading to improved control of odour and corrosion in sewers are covered in the following sections. New Developments Prediction of sulfide generation in sewers Prediction of sulfide generated in sewers in the 1989 H2S Manual relied on empirical equations such as: • Pomeroy’s Equation (Pomeroy, 1959); • Boon and Lister’s Equation (Boon and Lister, 1975); and • Thistlethwaite’s Equation (Thistlethwaite, 1972). These equations, such as Pomeroy’s Equation: G = 0.0013 x BOD5 x 1.07(T-20) continue to be used by water practitioners today (Dack, 2011; Shammay, 2010). This equation uses just two parameters, BOD5 and temperature for the prediction of the sulfide generation rate, G, while Boon and Lister’s equation uses COD rather than BOD5, and Thistlethwaite adds a third parameter, sulfate concentration. There has been considerable research into the parameters affecting the sulfide generation rate (Hvitved-Jacobsen et al., 2002; Freudenthal et al., 2005) and these provide significant improvements over previous equations as more biological, chemical and physical processes have been included. However, the kinetic expression for sulfide production still used an empirical approach limiting these models to steady-state conditions. 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 that is capable of predicting both spacial and temporal variations in sulfide concentration as well as other sewer parameters including GHG emissions (Sharma et al., 2011). This sulfide generation model, the Sewex Model, has been linked to sewer hydraulic models such as MOUSE to provide a dynamic model of sewer networks to predict the dynamic changes in sulphur compounds within the sewer system as a result of changing sewer characteristics such as diurnal variations (Wang et al., 2010). Bacterial activity and the rate of corrosion It has been known for some time (Parker and Beer, 1965; MDWB, 1989) that the rate of corrosion depended on: • Sulfide concentration (H2S); • Temperature; and Figure 2. The Sulfide Corrosion and Odour Problem (from 1989 H2S Manual).
Water Journal February 2013
Water Journal November 2012-1