GREENHOUSE GASES FROM WASTEWATER TREATMENT SYSTEMS
The energy versus nitrous oxide emissions nexus
DW de Haas
Publication Date (Web): 4 May 2018
Understanding of greenhouse gas emissions related to wastewater handling has improved a lot in the last ten years thanks to the efforts of researchers, including significant contributions from Australia. The existing NGER scheme in Australia for reporting nitrous oxide (N2O) emissions from wastewater handling is flawed and needs to be updated. It provides no incentive to design and operate wastewater treatment processes that produce less N2O. It makes provision for one fixed emission factor for wastewater treatment with origins in an estimation method that is more than ten years old.
Compared with the factor for wastewater treatment in the NGER (2016) guideline, the actual N2O emission factor (if measured) might range from a much smaller value (around half or less) to much larger value (around five times greater). Paradoxically, some of the ‘novel’ wastewater treatment processes, which are currently receiving a lot of attention in the industry for reasons of cost, energy and/or effluent nitrogen (N) reduction, have a significantly higher risk of N2O emissions. These include processes with ‘nitritation’ (partial nitrification to deliberately form nitrite), followed by deammonification (anaerobic ammonium oxidation or anammox processes), as well as more conventional activated sludge processes with high ammonium oxidation rates and/or low internal recycle rates (e.g. step-feed systems).
Engineering WWTPs through the use of such processes to promote N removal and/or reduce electrical energy use, may be misguided in terms of life cycle GHG emissions. Without active mitigation measures, it is likely that increased N2O emissions will cancel out (or exceed) the benefits of reduced Scope 2 (indirect) emissions associated with lower use of grid electricity. If renewable energy continues to replace fossil-fuel based sources of electricity production in future, the apparent greenhouse benefit of these processes related to energy savings could be cancelled out if they have an increased N2O emissions profile.
At the very least, the determination of N2O emissions under the NGER scheme, should make provision for one or more alternative methods, such as direct measurement of emissions (where feasible), or a range of emission factors according to types of nitrogen processes used, based on information that is periodically updated from the literature.
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