Water Journal : Water Journal March 2011
78 MARCH 2011 water technical features Abstract In 2003 Sydney Water commenced the implementation of a long-term corrosion and odour management strategy. This strategy set out the issues, causes, solutions and targets in the trunk sewer system feeding the Malabar treatment plant. A risk analysis had estimated that without the effective implementation of this strategy, $570 million would be needed over the next 25 years for rehabilitation as a result of corrosion. A benefits review of the strategy was undertaken in 2009--10 and found that the key strategic target of reducing levels of hydrogen sulfide (H2S) to medium to low risk was met. This benefit was obtained by using chemical dosing and forced ventilation with odour treatment, combined with the application of source control in a largely industrial catchment. Malabar System The Malabar wastewater system services over 1.5 million people and transports over 400ML per day of wastewater to the Deep Ocean Outfall Wastewater Treatment Plant (WWTP) at Malabar. More than 150 pumping stations are located in the system. Over time the system has grown, with industrial and residential development occurring in the upper reaches of the catchment. One of the consequences of this is increased travel and residence time of the wastewater and the associated increased risk of septicity occurring in the network, leading to odour and corrosion. Background Sydney Water, in collaboration with Sinclair Knight Merz, completed a major strategic review of the Malabar System, including the Southern and Western Suburbs Ocean Outfall Sewer (SWSOOS), the trunk sewers leading into the Malabar WWTP. It began with a Risk Reduction Program (RRP) in 1997 and resulted in the development of an Implementation Strategy (IS) in 2003. The RRP was initially triggered by concerns over the hydraulic capacity, as well as the structural integrity, of the SWSOOS in being able to meet its service requirements. The IS had four components -- odour and corrosion management, flow management, silt management, and inspection and rehabilitation. The objectives of the corrosion and odour component of the strategy were to achieve H2S concentrations of less than 10ppm in the gas phase and 0.5mg/L in the liquid phase, corrosion rates of <1mm per year of concrete and a reduction in odour complaints. These were to be achieved by chemical dosing, removal of H2S in the gas phase by ventilation, and trade waste controls. This paper presents the findings of a review of the odour and corrosion component of the implementation strategy. The review particularly compared the effectiveness of the strategy during the warmer months of the year (November to March), when sulfide formation was at its peak. Odour and Corrosion Strategy To reduce odour complaints and corrosion rates and achieve the strategic targets for this program, the following multi-faceted approach was employed: • Chemical dosing to reduce the amount of hydrogen sulfide (H2S) available for release into the gas phase; • Improved ventilation and odour treatment to reduce H2S levels in the sewer gas phase and reduce odour complaints; • Trade waste controls to reduce high organic discharges into the network. Figure 1 is a map of the entire system, showing the locations of the various units. Chemical Dosing Strategy The chemical dosing strategy was primarily based on using the mechanism of binding dissolved sulfide. Thirteen chemical dosing units were installed in the Malabar System, all except one of them dosing ferrous chloride. Calcium nitrate was dosed at the Sandringham wastewater pumping station (SPS196). L Vorreiter, A Soliman, J Campbell, M Whalan, C Gabus Review of the benefits of implementation over six years MALABAR WASTEWATER SYSTEM ODOUR AND CORROSION MANAGEMENT STRATEGY sewer processes Figure 1: Map of chemical dosing units (CDUs), odour control units (OCUs) and monitoring locations.
Water Journal April 2011