Water Journal : Water Journal March 2011
technical features 70 MARCH 2011 water sewer processes the more rapid depletion of the oxygen in the wastewater and stimulates the generation of H2S by the sulfate- reducing bacteria (SRB). 4. Trade waste regulations have reduced the amount of heavy metals (and other pollutants) that is discharged to sewers, thereby reducing the precipitation of metal sulfides in sewers. Precipitation of the heavy metal sulfide would reduce the H2S concentration in the sewage. In addition, it is believed that the heavy metals have an inhibitory effect on the microbes responsible for H2S production. There do not appear to be any off- setting system changes to reduce the odour and corrosion problems, which from inferred evidence are becoming more severe. In fact, there is a suggestion that odour complaints are increasing due to a lowered tolerance to odours by the general public, which exacerbates the problem further. Knowledge Gaps The fundamental processes of odour and corrosion that are covered by the Hydrogen Sulfide Control Manual, 1989, are shown in Figure 1. Over the last few decades development of odour and corrosion knowledge has been left mainly to the practitioners, who have applied this knowledge to their specific sewer systems where odours and/or corrosion problems occurred (WERF, 2007). It is seldom feasible to completely prevent the bacterial biofilm activity that leads to problems in sewers. Control strategies usually focus on H2S as it is the major primary odorous product of the biofilm, although other odorous compounds such as organic sulfides (mercaptans, dimethyl sulfide, etc) and volatile organic compounds (VOCs -- aldehydes, ketones, hydrocarbons, etc) are also a concern in sewers. H2S is a highly odourous (and poisonous) gas that can be readily detected and exists in aqueous equilibrium as dissolved sulfide ions. The common control strategies for H2S involve dosing chemicals that either oxidise it to less problematic forms (for example, sulfate), or "lock" it into forms that are not volatile (HS-/ S2- ions that dominate the equilibrium at alkaline pH, or metal precipitates such as iron sulfide). H2S(aq) has very low solubility in water and will volatise into the headspace above the wastewater, particularly under turbulent conditions. H2S in the headspace may be oxidised to sulfuric acid by bacteria which grows on surfaces under moist, aerobic conditions. This sulfuric acid can cause corrosion of concrete, mortar or metal sewer infrastructure. Steel and alloys of copper are readily corroded to flakes of metal sulfide. Basic knowledge gaps still exist in our fundamental understanding of odours and corrosion, including the following: • Corrosion Processes & Control: The estimation of the corrosion rate and the life expectancy of pipes are very difficult to predict and almost entirely based on empirical data about the past performance of pipes under similar conditions. • Gas Phase Technologies: It is difficult to quantify and characterise odours from sewers without relying purely on costly, problematic and time-consuming human olfactometry 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. • Liquid Phase Technologies: 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, Fe2Cl3, 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. New Approach to Research Optimal corrosion and odour management has clearly been hindered by limited understanding of several key in- sewer processes contributing to the problems, and the lack of tools and reliable technologies to support strategic decisions and cost-effective sewer operations. By filling in the above gaps, the SCORe Project aims to provide knowledge and technology support to the Australian water industry for cost-effective and efficient corrosion and odour management in sewers. The size of this problem is reflected in the broad support being provided to the Project by water utilities throughout Australia. There are 11 Industry Partners and five Research Partners. The Project started in late 2008 and will run for five years with a total budget of around $20 million, including the highest ever Linkage Project Grant from ARC to the water industry of $4.7 million. A new approach is being taken by this project. Field experience is being used to identify problems, which are then mimicked in lab systems to reveal fundamental scientific understanding of odour and corrosion processes under controlled conditions. This new knowledge is being integrated with mathematical models using lab results for calibration. The mathematical models are then being validated with field studies and used to optimise odour and corrosion control systems. This new industry linkage methodology is shown in Figure 2. Figure 2: Research methodology for the SCORe Project.
Water Journal April 2011