Greenhouse Gas Emissions from Biological Nutrient Removal During Wastewater Treatment

Abstract

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 (CO2), 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, CO2, 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 N2O analyzer. The results of the study indicated that the detection limit of