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GREENHOUSE GAS INVENTORIES FROM WWTPs – THE TRADE-OFF WITH NUTRIENT REMOVAL

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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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Document Type: Research Article

Publication date: 2008-01-01

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