Full-scale emission results (N2O and CH4)

Abstract

Copyright © 2022 The Authors and Editors. This chapter reviews the studies from N2 O and CH4 monitoring campaigns in full-scale wastewater treatment plants (WWTPs) and sewer networks. The focus is on greenhouse gas (GHG) emissions from WWTPs as more literature is available. The analysis classifies quantified N2 O and CH4 emission factors (EFs), triggering operational conditions and formation pathways for different configurations. Control strategies to minimize N2 O emissions are proposed for different process groups. The main reasons for EF discrepancies are discussed. Overall, N2 O emission factors for processes treating lowstrength wastewater streams range between 0.003 and 5.6% of the N-load (average equal to 0.9% of the N-load). Emissions higher than mainstream process average emissions have been reported in sequencing batch reactors (average equal to 3.6% of the influent N-load) and step-fed plug flow reactors. In full-scale sidestream processes, less than 15 monitoring campaigns have reported EFs (average equal to 2.5% of the N-load). Differences in the EFs among the process groups are partially attributed to disparities in the control strategies (i.e. aeration control), configuration, and operational and environmental conditions that favour the preferred enzymatic pathways. Overall, triggering operational conditions for elevated N2 O emissions in full-scale wastewater treatment processes include (i) increased NH4+ concentrations leading to a high ammonia oxidation rate (AOR) and increased production of intermediates (e.g. NH2 OH, NO−, etc.), (ii) improper aeration control (i.e. inadequate aeration and non-aeration duration, over-aeration, under-aeration), (iii) NO2− accumulation triggering the nitrifier denitrification pathway, and (iv) sudden shifts in incomplete heterotrophic denitrification (i.e. due to excess dissolved oxygen (DO), chemical oxygen demand (COD) limitation etc.). The N2 O monitoring strategies can also influence the reliability of the quantified EFs. Due to temporal variation of N2 O emissions, short-term studies are not sufficient to quantify annual EFs. The analysis showed that the average EF for processes treating low-strength streams monitored for less than a week is 0.66% of the influent N-load. On the other hand, processes monitored over 6 months have an average EF equal to 1.74%. Compared with N2 O, CH4 quantification from full-scale WWTPs is less investigated, while it also contributes significantly to the overall plant carbon footprint. The results of full-scale CH4 quantification studies are summarized in this chapter. Emissions of CH4 in WWTPs mainly originate from the influent, anaerobic wastewater treatment and anaerobic sludge handling processes. The amount of CH4 emissions varies greatly with different configurations of WWTPs. For WWTPs without anaerobic sludge handling processes, the CH4 emissions can mainly be traced back to the CH4 dissolved in the influent. When anaerobic treatment is applied in WWTPs for wastewater COD removal, its CH4 emissions might substantially increase the overall plant carbon footprint. GHG monitoring campaigns carried out in WWTPs should include the monitoring of fugitive CH4 emissions. Finally, CH4 and N2 O emissions reported from sewer networks are also summarized in this chapter. The last part of the chapter summarizes some mitigation strategies applied at full-scale to control fugitive CHG emissions from WWTPs and sewers.