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Greenhouse gas emissions are likely to rise faster than growth in population and more than double for water supply and wastewater services over the next 50 years in South East Queensland (SEQ), Australia. New sources of water supply such as rainwater tanks, recycled water, and desalination currently have greater energy intensity than traditional sources. In addition, direct greenhouse gas emissions from reservoirs and wastewater treatment and handling have potentially the same magnitude as emissions from the use of energy. Centralized and decentralized water supply and wastewater systems are considered for a scenario based upon a government water supply strategy for the next 50 years. Many sources of data have large uncertainties which are estimated following the IPCC Good Practice Guidelines. Important sources of emissions with large uncertainties such as rainwater tanks and direct emissions were identified for further research and potential mitigation of greenhouse gas emissions.

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Operating data was collected from a number of wastewater treatment plants (WWTPs) in South-East Queensland and used to calculate greenhouse gas emissions from first principles using an inventory-type approach. Emission factors were based as far as possible on relevant data sourced from either the literature or databases used in Life Cycle Assessment (LCA) models. The results were compared with those from a desktop simulation approach for a range of WWTP configurations that formed the basis for an LCA study of the trade-offs between nutrient removal and total environmental burden. The results from the actual operating plants compared well in general terms with those from the desktop study, although some differences in points of detail were highlighted. In either case, significant uncertainties in GHG emission estimates were apparent in respect of so-called "fugitive" emissions of nitrous oxide and methane from WWTP operations since both of these gases are major greenhouse contributors. Additional research will be required in this area in order to improve the accuracy of emissions reporting from the wastewater sector. Notwithstanding these uncertainties, the data suggested that imported electrical power and biosolids treatment/ disposal operations are the largest potential sources of greenhouse gas. Opportunities for significant reductions (approx. 20 to 40%) in total GHG emissions exist with the inclusion primary sedimentation, anaerobic digestion and power generation from biogas in the WWTP flow sheet. Addition of chemicals (with embodied GHG emissions) to supplement nutrient removal does not cancel out this reduction. However, from a global perspective, the LCA study has shown that GHG emissions represent only a minor (indicatively <1%) of the normalised total environmental impacts from WWTP operations. Eutrophication and potential human health or ecotoxicity impacts associated with the disposal of biosolids (particularly due to the metals content) are the dominant impacts. This raises the question of how policy directives and environmental regulations from government can best serve the complex (and potentially competing aims) of minimising local environmental impacts whilst also improving sustainability on the widest possible front.

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