Impacts of water demand management on sewer resilience

Abstract

Sewers, which dispose our wastes and serve to protect our health and environment, are an integral component of our transition to a more water sustainable society. Sewers have the capacity to generate large quantities of hydrogen sulfide gas which is toxic, odorous, and highly destructive to concrete sewer infrastructure. Its generation, transport and loss in sewers is highly dependent on hydraulics and wastewater quality. There is the risk that improper implementation of water sustainable practices and infrastructure could have consequences for sewer resilience by increasing hydrogen sulfide concentrations, potentially costing tens of billions of dollars in remedial costs annually in Australia. For sustainable water practices to be successful, the impacts to sewers must be understood and accounted for.This thesis set out to identify the sections of sewer networks most vulnerable to upstream changes in water use based on changes to hydrogen sulfide generation, oxidation and mass transfer in pressure mains and gravity sewers. Fully quantifying the overall impacts to sewer sulfide dynamics has remained a challenge since the transition towards water sustainability in municipalities remains a long term process that is still ongoing. As a result, field studies are practically difficult at the timescales required for the study period but adequately scaling the interactions between the physical, chemical and biological sewer processes for laboratory based bench top experiments is equally challenging. This study addressed the limitations faced by field and laboratory sewer studies by deploying a large scale sewer pilot system with over a kilometre of sewers that operated at field scale flows. In conjunction with analysis support from a well proven sewer transformation model, the effect of changes in hydraulics, wastewater quality and the integration decentralized treatment systems on sewer hydrogen sulfide were investigated and expended to determine potential impacts on network wide resilience. Sulfide generation in pressure mains is highly dependent on the hydraulic conditions and the operational behaviour of the pumping system employed. The experimental investigation determined that reducing pumping frequency in pressure mains greatly decreased sulfide generation. Reductions from once every 30 minutes to once every 60 minutes decreased the average sulfide generation by 25±9%. The results provide a framework minimizing sulfide generation by optimizing pump timing, as well as, determining the consequences of reduced influent flows on effluent sulfide concentrations. Reductions in water consumption are also expected to result in more concentrated wastewater. An experimental and computational model-based assessment quantified the impact on sulfide generation and oxidation from water conservation based changes in sewage quality. The model predicted that hydrogen sulfide generation only exceeded 0.1 mg S L-1 km-1 in pressure systems with low initial organic carbon concentrations. At concentration above 150-200 mg CODs L-1, further increases in organic carbon did not influence generation. In gravity mains, the largest detrimental impacts were limited to sewer sections with low slopes, low flows and high pre-existing concentrations of influent sulfide. In such cases, downstream dissolved sulfide concentrations increased by up to 0.35 mg S L-1.A systematic sewer design analysis was completed to quantify the combined effects of changes to wastewater hydraulics and wastewater quality on downstream hydrogen sulfide concentrations. In pressure mains, conservation based reductions in flows decreased the number of pumping events per day, and the daily mass of sulfide discharged, however effluent dissolved sulfide concentrations increased. The impact was greatest in smaller capacity stations. At 5 L sec-1 stations the sulfide discharge concentration increased by nearly 50% after a 40% reduction in daily flow. Gravity pipes receiving lower flows experienced more rapid sulfide depletion in both the liquid and gas phase. The beneficial impact was due to lower in-pipe wastewater volumes improving re-aeration. High slope, low capacity pipes experienced the greatest beneficial impacts from reduced flows. By employing multivariable regression, the entire modelled dataset was used to derive sets of simplified empirical equations that provide the capacity to rapidly estimate changes in liquid phase and gas phase effluent sulfide concentrations in gravity and pressure mains. As part of the treatment process, decentralized treatment systems create wastes and sludge that can be disposed of through existing sewer infrastructure. The disposal of decentralized wastes in sewers is a concern if its contribution to the overall oxygen demand is sufficient to increase the prevalence of hydrogen sulfide in downstream infrastructure. Results confirmed that for sludge with an oxygen uptake rate of less than 3 mg DO g VSS-1min-1, the decrease to dissolved oxygen concentrations in sewage were insufficient to change sulfide dynamics in most gravity sewers. A downstream increase in dissolved sulfide of > 1 mg S L-1 was only predicted for gravity sewers with a high existing sulfide build-up of > 10 mg S L-1 and slopes of 0.001 m m-1 or less.