Published: March 09, 2011 r2011 American Chemical Society 2734 dx.doi.org/10.1021/es103534g | Environ. Sci. Technol. 2011, 45, 2734–2740 ARTICLE pubs.acs.org/est Comparison of Partial and Full Nitrification Processes Applied for Treating High-Strength Nitrogen Wastewaters: Microbial Ecology through Nitrous Oxide Production Joon Ho Ahn, Tiffany Kwan, and Kartik Chandran* Department of Earth and Environmental Engineering, Columbia University, 500 West 120th Street, New York, New York 10027, United States b S Supporting Information ’ INTRODUCTION The increasing regulatory demands to achieve greater nutrient removal from wastewater treatment plant effluents, while mini- mizing infrastructure investments and operating costs, has re- sulted in the development of several innovative biological nitrogen removal (BNR) processes. Partial nitrification based processes such as the single reactor system for high ammonium removal over nitrite (SHARON 1 ) and its variants are attractive for treating high-strength nitrogen waste streams such as anae- robic digestion reject water or centrate, owing to their reduced consumption of energy (for aeration) and organic carbon (for denitrification). Indeed, separate treatment of centrate via partial nitrification is one of the options for limiting nitrogen discharges to Jamaica Bay in New York City 2 and is part of PlaNYC, a sustainability plan for New York City targeted for 2030. The energy and carbon savings of partial nitrification pro- cesses for nitrogen removal are by virtue of restricting ammonia oxidation to nitrite rather than to nitrate. On the other hand, nitrite is a known trigger for nitrous oxide (N 2 O) and nitric oxide (NO) production via nitrification 3,4 and denitrification 5,6 path- ways. Full-scale measurements also point to nitrite as a factor in N 2 O production. 7,8 Low (but not zero) dissolved oxygen con- centrations were initially implicated as a significant factor for N 2 O and NO emissions from nitrification. 9-11 However, the production of N 2 O by nitrifying bacteria under aerobic condi- tions has also been shown. 3,4,12,13 Recent reports suggest that N 2 O and NO emissions by ammonia oxidizing bacteria are related to imbalances in their metabolism and gene-expression patterns. 14,15 Given that the greenhouse impact of N 2 O is about three hundred times that of carbon dioxide 16 and both N 2 O and NO contribute to ozone layer depletion, 17 it needs to be determined whether N-removal processes based on transient nitrite accumu- lation are systematically greater contributors of N 2 O and NO than full nitrification based processes. The mechanisms of such differential N 2 O production from partial and full-nitrification systems at the microbial level also need to be understood. Therefore, the overarching goal of this study was to compare the microbial ecology, gene expression, biokinetics, and N 2 O emissions from a lab-scale bioreactor operated sequentially in full-nitrification and partial-nitrification modes. It was hypothe- sized that operation in partial nitrification mode would result in higher N 2 O and NO emissions than operation in full nitrification mode. It was additionally hypothesized that the high emissions of the gases would parallel the sustained elevated expression of the genes coding for their production. Received: October 19, 2010 Accepted: February 23, 2011 Revised: February 14, 2011 ABSTRACT: The goal of this study was to compare the microbial ecology, gene expression, biokinetics, and N 2 O emissions from a lab-scale bioreactor operated sequentially in full-nitrification and partial-nitrification modes. Based on se- quencing of 16S rRNA and ammonia monooxygenase subunit A (amoA) genes, ammonia oxidizing bacteria (AOB) populations during full- and partial-nitrification modes were distinct from one another. The concentrations of AOB (X AOB ) and their respiration rates during full- and partial-nitrification modes were statistically similar, whereas the concentrations of nitrite oxidizing bacteria (X NOB ) and their respiration rates declined significantly after the switch from full- to partial-nitrification. The transition from full- nitrification to partial nitrification resulted in a protracted transient spike of nitrous oxide (N 2 O) and nitric oxide (NO) emissions, which later stabilized. The trends in N 2 O and NO emissions correlated well with trends in the expression of nirK and norB genes that code for the production of these gases in AOB. Both the transient and stabilized N 2 O and NO emissions during partial nitrification were statistically higher than those during steady-state full-nitrification. Based on these results, partial nitrification strategies for biological nitrogen removal, although attractive for their reduced operating costs and energy demand, may need to be optimized against the higher carbon foot-print attributed to their N 2 O emissions.