Nitrous Oxide Emissions from Wastewater Treatment Processes and Process Parameters Affecting Emissions

Abstract

Most wastewater treatment plants in the United States must be upgraded to reduce nutrient discharges. These improvements are needed to meet upcoming permit requirements and to prevent negative environmental impacts like eutrophication of receiving water bodies. However, one of the potential side-effects of increased nitrogen removal in wastewater treatment processes is increased nitrous oxide (N2O) emissions from biological nitrification and denitrification. N2O is a greenhouse gas (GHG) approximately 300 times stronger than an equivalent amount of CO2; thus, it is imperative to (1) establish comprehensive methodologies to accurately quantify N2O emissions from full-scale treatment processes and (2) understand the relevant process parameters and microbiology involved in emissions from wastewater treatment. This serves to ultimately minimize emissions (and therefore the carbon footprint) of wastewater treatment plants (WWTPs). To address this, emissions from two wastewater treatment processes were quantified, and relevant process parameters and microbial emission pathways were investigated. While the two treatment systems differ in terms of scale and processes utilized, both are innovative wastewater treatment technologies designed for efficient use of space and nutrient removal. (1) At Brightwater Treatment Plant (Woodinville, WA), aqueous and gaseous N2O monitoring techniques were employed at a full-scale membrane bioreactor (MBR) for 5.5 months. To the knowledge of the investigators, this campaign was the most comprehensive study of a fully covered MBR to date. Emission estimates from both aqueous and gaseous analyzers were compared to determine their reliability, and the average emission factor (using data from both analyzers) was 0.58% of plant influent total Kjeldahl nitrogen (TKN) emitted as N2O-N. Emissions were positively correlated with influent pH, nitrification efficiency, and aeration basin/effluent NH4+ and NO3-. They correlated negatively with primary effluent COD:N ratio and effluent pH, signifying that nitrification was likely the dominant N2O-production pathway. (2) A laboratory-scale, aerobic granular sludge (AGS) sequencing batch reactor (SBR) was operated for 11 months to measure N2O emissions with full phosphate removal and simultaneous nitrification-denitrification (SND). The reactor was operated at varying dissolved oxygen (DO) concentrations and with nitrite (NO2-) spikes to investigate the impact of these process parameters on emissions. Increased DO and NO2- concentrations were associated with increased emissions. N2O was minimized at a dissolved oxygen concentration of 1 mg O2 L-1, with an emission factor of 0.18% of oxidized NH3-N emitted as N2O-N. This emission factor is lower than many previously reported factors from AGS reactors. Molecular analyses revealed a population of microbes capable of shortcut nitrogen removal, which is advantageous for wastewater treatment because of decreased oxygen and carbon requirements.