Modeling the effects of sewer mining on odour and corrosion in sewer systems

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Increasing demand has led to shortage of potable water in many countries. The use of alternative water sources is one of the solutions undertaken to overcome this issue. Alternative water sources can be derived from either collected rainwater, treated greywater or treated wastewater. Sewer Mining is known as an efficient technology which can reduce the cost of wastewater infrastructure required to transport wastewater because the Sewer Mining facility is usually installed close to the site that is using the treated wastewater. The treated water from Sewer Mining has been used as one of the sources of alternative water, particularly to supply water for public open spaces, garden irrigation and toilet flushing. It is conducted by extracting the sewage from sewer pipes and most of the times, disposing the sludge back to sewer pipes. The sewage extraction and sludge disposal back to sewer network is suspected to trigger sewer problems such as blockages. Several applications of Sewer Mining facility have proved to contribute to the increase of blockages in sewer pipes. Furthermore, the changes in the sewage composition after the sewage extraction and sludge disposal location lead to alteration of the sewage biochemical transformation processes, which finally were suspected to change the state of hydrogen sulphide build up. Increase of hydrogen sulphide in sewage and in the sewer atmosphere could also contribute to the problem of odour and corrosion in sewers. This study attempts to model the impact of Sewer Mining on odour and corrosion in sewer systems. A residential area in northern Melbourne was used as a study area. In this study, four Sewer Mining scenarios were considered, consisting of Base Case (BC), Sewer Mining 1 (SM1), Sewer Mining 2 (SM2) and Sewer Mining 3 (SM3). The scenarios were configured based on the volume of sewage extraction. Base Case is the scenario representing the existing condition of water and wastewater use in the study area and there was no Sewer Mining facility in the area. Sewer Mining 1, 2 and 3 represent the scenarios when the Sewer Mining is undertaken and the sewage extraction was adjusted to supply 25%, 50% and 70% of the households in the study area. The location of sewage extraction and sludge disposal was fixed at the middle of main sewer pipe network and was not changed for each scenario. The treated water from the Sewer Mining facility was supplied as water for toilet flushing. Two modeling tools were used for this analysis, the first was an urban wastewater generation model and the second was a sewage transformation (and generation of hydrogen sulphide) model. The output of the first model, namely wastewater flow discharge and contaminant concentration were fed to the second modeling tool. Wastewater generation from the households was simulated in the first modeling tool while Sewer Mining practice was modeled in the second modeling tool. Its impact was analyzed in the sewer pipes downstream of the Sewer Mining facility. The analysis results were obtained by comparing the result from Sewer Mining scenarios 1, 2 and 3 with the Base Case. The difference in total sewer flow, hydrogen sulphide gas production, corrosion rate and the pipe lifetime were the analysis parameters that are discussed in this paper. The results showed that Sewer Mining led to reduction in hydrogen sulphide concentration immediately after the sewage extraction point, but further downstream, the hydrogen sulphide concentration was extremely high. The hydrogen sulphide concentration at the outlet of sewer pipes network for SM1, SM2 and SM3 scenarios had increased up to three, five and eight times respectively, when compared to the hydrogen sulphide concentration in the Base Case. Increase of hydrogen sulphide concentration consequently lead to increase in odour occurrence and corrosion rate which eventually reduce the sewer pipe lifetime. The pipe lifetime at the outlet of sewer network reduced by 204 years, 262 years and 293 years after implementing the scenarios of SM1, SM2 and SM3, respectively. The distance where the extremely high hydrogen sulphide generation occurred was very much determined by the volume of extracted sewage and the location of extracted sewage and sludge disposal.