Greenhouse Gas Emissions From Biological Nutrient Removal During Wastewater Treatment
Since the 1900s, humans have been altering the global nitrogen (N) cycle by industrially fixing N for fertilizer production. This reactive N is often released back to coastal environments through many mechanisms, including wastewater treatment, where it can lead to numerous consequences such as fish kills and algae blooms.^ In many locations, wastewater treatment effluent is one of the largest sources of excess N to coastal environments. Although regulations limiting N loads in wastewater effluent in the U.S. were first developed in the 1970s, stricter regulations started to emerge in many states in the 2000s. In order to meet new discharge requirements, many centralized wastewater treatment plants (WWTPs) and onsite wastewater systems (OWTS) have been upgraded to include biological nitrogen removal (BNR) systems. These BNR systems make use of nitrifying and denitrifying bacteria to convert reactive forms of N (ammonium and nitrate) to nitrogen gas. Current BNR systems can reduce effluent total N loads to below 5 mg/L. However, nitrous oxide (N2O), a greenhouse gas (GHG) over 200 times more potent than carbon dioxide (CO 2), may be produced along with or instead of nitrogen gas. Further, organisms that respire CO2 and produce methane (CH4) have been documented in BNR systems, making these systems potential sources of these additional potent GHGs. The BNR systems at WWTPs and OWTS can vary in many ways including the order and number of the different zones or compartments (aerated, anoxic, and anaerobic) and recycling arrangements. Therefore, although BNR systems at both WWTPs and OWTS may reduce N loads to coastal ecosystems, they may release GHGs that contribute to climate change.^ The central objective of this research was to examine the magnitude, variability, and potential production mechanisms of GHG emissions from a BNR system at a WWTP and advanced OWTS. This research is timely as BNR systems are increasingly used at both WWTPs and OWTS, but differences in the systems can result in different GHG emissions and N removal efficiency.^ Greenhouse gas emissions were measured using a cavity ring down spectroscopy (CRDS) analyzer (Picarro G2508) capable of measuring N2O, CO 2, and CH4 nearly simultaneously in real time. To first evaluate this new technology, a comparison study was conducted (Chapter 1) to test the CRDS (Picarro G2508) relative to two alternative methods for measuring GHG emissions, Gas Chromatograph (Shimadzu GC 2014) and Los Gatos N2 O analyzer. The results of the study indicated that the detection limit of the Picarro was an order of magnitude lower than that of the Gas Chromatograph, but an order of magnitude higher than that of the Los Gatos N2O analyzer. Although both the Picarro and Los Gatos analyzers offer efficient and precise alternatives to GC-based methods, the Picarro has the unique capability of measuring all three GHGs (N2O, CO2, and CH4 ) simultaneously. Therefore, the Picarro was deemed suitable for use in the WWTP and OWTS studies.^ Two major studies examining GHG emissions from a WWTP and OWTS were performed. The first was a yearlong study to determine the temporal (bi-monthly across annual cycle) and spatial (4 major zones: pre-anoxic, aerated IFAS, post-anoxic, and re-aeration) variability of GHG (N2O, CO2, and CH 4) emissions from an Integrated Fixed Film Activated Sludge (IFAS) BNR system at the Field’s Point WWTP in Providence, RI (Chapter 2). In addition, to understand environmental controls on the GHG emissions, potential relationships between the GHG emissions and water and tank parameters were examined. Finally, the emissions of all three GHGs were used to evaluate the importance of the BNR system to the overall GHG budget of the WWTP. The results of this study indicated that emissions of all 3 GHGs were highest from the aerated IFAS zone and all 3 GHGs varied by season (hourly variation was examined in Appendix 1). The N2O emissions were related to both ammonium and nitrate. When considering the emissions of all 3 GHGs in terms of CO2 equivalence, BNR is responsible for approximately 12% of the total GHG emissions for the Field’s Point WWTP (including emissions from: electricity, natural gas, liquid fuel, sludge disposal, and supplemental carbon). Generally, the BNR tank had higher emissions of all three GHGs than other parts of the treatment train (grit chambers, primary clarifiers, final clarifiers) (Appendix 2). However, the N2O emissions from the BNR tank represented only 0.01 – 0.34% of the influent N. Appendix 3 investigated the use of isotopomers to determine the mechanisms of N2O production from the BNR tank.^ The second major study compared N2O emissions from the BNR system at the Field’s Point WWTP to those from three common types of advanced OWTS used in RI to remove N (Advantex, SeptiTech, and FAST) (Chapter 3) (CH4 and CO2 emission measurements are reported in Appendix 4). The emissions were compared in terms of normalized per capita emissions and emission factors (% of N removed released as N2O). In addition, the specific abundance of a nitrification gene (ammonium monooxygenase, amoA) and denitrification gene (nitrous oxide reductase, nosZ) were quantified in order to determine the abundance of microorganisms that may be producing N2O in these systems. The results of this study (Chapter 3) indicated that in general N2O emissions from N removal during wastewater treatment were <1% of the N removed, except for one SeptiTech system (4%) and one Advantex system (21%). In general, N2O emissions (on a mole/area basis) from the WWTP were larger than those from OWTS and the OWTS with the largest N2O emissions was Advantex. However, when N2O emissions were normalized per capita and surface area of the treatment tank, they were similar between the WWTP and OWTS. Although there was no linear relationship between N2 O emissions and amoA or nosZ abundances, amoA and nosZ abundances did differ between the WWTP and OWTS.^ The results of this dissertation allow us to focus future research efforts on the zones (aerated IFAS at WWTP) and systems (WWTP and Advantex OWTS) that produced higher emissions. In addition, future studies should try to develop a better understanding of the large temporal and spatial variability observed in these systems. The results of this research determined that N2O emissions were related to both ammonium and nitrate, indicating that both nitrification and denitrification likely play a role in N2O emissions. However, preliminary isotopomer results indicate that nitrification may be responsible for the N2O emissions. With additional studies on the mechanisms of production, suggestions to operators can be made so that emissions can be lowered while maintaining N removal.^
Elizabeth Quinn Brannon,
"Greenhouse Gas Emissions From Biological Nutrient Removal During Wastewater Treatment"
Dissertations and Master's Theses (Campus Access).