Evaluation of the spatial and temporal variability of nitrous oxide (N2O) emissions at two different full-scale aerobic treatment systems used in the post-treatment of UASB effluents in Brazil

Abstract

Monitoring of nitrous oxide (N2O) emissions from wastewater treatment plants (WWTP) correlated to different operational conditions plays an important role in proposing mitigation strategies. However, few full-scale studies are available so far correlating N2O emission factors (EF) from wastewater treatment systems to different structural, operational and environmental configurations. In this context, the aim of this study was to evaluate the variability of N2O emissions from two aerobic treatment systems, activated sludge (AS) and low-rate trickling filter (TF), operated as post-treatments in upflow anaerobic sludge blanket reactors (UASB), under different operational conditions: (1) employed aeration system, (2) biomass growth and (3) environmental conditions, with a main focus on the generation of N2O EFs for these types of systems. Both studied systems displayed high temporal variability in N2O emissions associated with different operational and environmental conditions, such as: (1) aeration intermittency and high amplitudes of influent N loads for the AS system, (2) recirculation of treated wastewater and different wastewater temperatures for the TF system. In addition, the temporal variability of methane (CH4) emissions was also significant, with greater amplitudes for the AS system, due to the greater air-stripping effect intensified by the combination of intermittency and excess aeration. Finally, regardless of the employed operational condition, the average EFs (N2O and CH4) determined for the WWTP-1 aeration tank (AT) (0.04 % and 0.054 %) and WWTP-2 TF (0.005 % and 0.015 %) were substantially lower than the N2O and CH4 EFs (1.6 % and 0.75 %) suggested by the IPCC, for centralized WWTPs with aerobic systems.