Lead author: Ollie Whalley, Tonkin & Taylor Limited
Co authors: Warren Biggs, Nelson City Council; Clint Rissmann, Land and
Greenhouse gas (GHG) emissions from wastewater treatment plants (WWTPs) are a growing concern for local authorities in Aotearoa/New Zealand. Significant quantities of climate warming gas are emitted from WWTPs, and these emissions make up a substantial proportion of each operating authority’s carbon footprint. Authorities are therefore grappling with how to better quantify emissions of nitrous oxide (N2O) and methane (CH4) from WWTPs.
While Water NZ has provided helpful guidelines to inform desktop estimates of GHG emissions, there remains a degree of uncertainty with these approaches. This uncertainty led Nelson City Council (NCC) to develop an innovative measurement approach to estimate the GHG emissions for their Nelson WWTP as part of their wastewater emissions reduction programme.
Supported by Three Waters Stimulus funding, NCC, Tonkin & Taylor (T+T) and Land and Water Science (LWS) embarked on a campaign of physical measurements across the WWTP. Flux measurements and gas samples are being taken over a year in each of the spring, summer, autumn and winter seasons. Key plant processes including the biofilter, facultative and maturation ponds and wetlands are being sampled. Measurements are being carried out using an accumulation chamber, including from a boat for the ponds and wetland. Gas samples at each flux measurement are analysed for CO2, CH4 and N2O by gas chromatography undertaken at Lincoln University.
From this data, a refined estimate of the GHG emissions was calculated. Initial results from spring, summer and winter show measured emissions are significantly higher compared with desktop estimated methods, with CH4 from the facultative pond the largest source of emissions for this WWTP.
Based on these findings NCC will install methane sensors to reduce temporal uncertainty and further refine emissions estimates. Ultimately this work aims to improve GHG estimates allowing for climate informed operational and investment and decisions to be made.