Alternative strategies for the reduction of TDS in the western sewer system, Melbourne, Australia

Abstract

Over the past decade shrinking freshwater supplies have forced the Victorian government to investigate alternatives sources of water for domestic and industrial uses. A significant source of alternative water is treated effluent from wastewater treatment plants. Currently the concentration of total dissolved solids (TDS) has become a major factor limiting the usefulness of effluent for irrigation from wastewater treatment facilities such as the Western Treatment Plant (WTP). This project was designed to develop alternative strategies to the current approaches being employed by Melbourne Water and City West Water to reduce TDS at the WTP. Detailed knowledge of the western sewer system was required in order to develop strategies for reduction of TDS at the WTP. This was undertaken by characterising the influent at WTP and the significant sources, domestic and industrial by applying an intensive sampling regime. The analysis of the data from the sampling investigation revealed key information required to propose TDS reduction strategies. Two strategies were presented in this study. The first strategy became apparent through the data analysis of the inputs into the WTP. It was found that diverting a proportion of the influent at times when TDS concentrations were at the highest to either a sacrificial land site or a separate lagoon system could result in significant reductions in the TDS of the recycled water. The second strategy investigated was an at-source approach. Industry was shown to have the highest contribution of TDS at the WTP. Since a significant amount of TDS comes from the pre-treatment of trade waste before discharge, an approach was investigated to eliminate neutralization prior to discharge in certain circumstances. The concept involved using the buffer capacity of the sewer to neutralise the acidic trade waste rather than using chemicals which contain high amounts of soluble inorganic ions. Two industries were chosen for case studies to investigate the feasibility of this concept. The industries chosen were located in different catchments. The first industry was located in a domestic catchment whereas the second industry was in an industrial catchment. There were two elements to this concept, the first was conducted in the laboratory and the second was conducted in the field. The laboratory component consisted of titrating both the trade waste prior to neutralisation and the receiving sewer with a strong acid and base. This demonstrated the capability of the receiving sewer flow to cope with either strong acid or base additions. The field work consisted of discharging uneutralised trade waste into the receiving sewer. The pH of the trade waste and the receiving sewer were recorded throughout the event to show the impact from the addition of the acidic trade waste. Both strategies investigated in this project were effective in demonstrating the potential of reducing TDS at the WTP