Evaluation of Fugitive Greenhouse Gas Emissions from Decentralized Wastewater Treatment Plants

Fugitive emissions of methane (CH4) and nitrous oxide (N2O) are known to be produced and released by wastewater treatment processes. CH4 and N2O emissions from large-scale, high-volume centralized wastewater treatment plants (WWTPs) have been well documented and quantified by online gas analysis instrumentation and grab sampling techniques. However, there is minimal information available on the extent of the fugitive emissions released at small-scale decentralized WWTPs treating smaller daily volumes of sewage, particularly those running a treatment regime with a membrane bioreactor (MBR) system. To ascertain the amount of fugitive emissions produced by these decentralised WWTPs, a series of CH4 and N2O gas flux measurements were performed at four different Australian small-scale WWTPs over the months of autumn and winter in 2012. Three of these WWTPs were using an activated sludge treatment scheme, and one employed an MBR system. It was found that under certain biochemical conditions, decentralized activated sludge WWTPs can emit greatly variable, but still significantly high, levels of CH4 and N2O (17 g CH4 m-2 d-1 and 12 g N2O m-2 d-1 at their peak), most particularly from aeration tanks. In addition, CH4 emissions as high as 2.5 g CH4 m-2 d-1 were measured from the anaerobic tank of the MBR system, which were not predicted by previous modelling studies.