Characterizing denitrification kinetics at cold temperature using various carbon sources in lab-scale sequencing batch reactors Yalda Mokhayeri, Rumana Riffat, Imre Takacs, Peter Dold, Charles Bott, Jeneva Hinojosa, Walter Bailey and Sudhir Murthy ABSTRACT Yalda Mokhayeri Rumana Riffat Civil and Environmental Engineering Department, George Washington University, Washington, DC 20052, USA E-mail: yalda@gwu.edu; riffat@gwu.edu Imre Takacs Peter Dold EnviroSim Associates, Ltd, 7 Innovation Dr., Flamborough, Ontario L9H7H9, Canada E-mail: imre@envirosim.com; dold@envirosim.com Charles Bott Civil and Environmental Engineering, Virginia Military Institute, Lexington, Virginia, USA E-mail: BottCB@vmi.edu Jeneva Hinojosa Civil and Environmental Engineering Department, George Washington University, Washington, DC 20052, USA E-mail: hinojosa@gwu.edu Walter Bailey Sudhir Murthy DC Water and Sewer Authority, 5000 Overlook Ave, SW, Washington, DC 20032, USA E-mail: wbailey@dcwasa.com; smurthy@dcwasa.com Wastewater treatment plants in the Chesapeake Bay region are becoming more interested in external carbon sources for denitrification. This is in response to the recent regulations to remediate the Chesapeake Bay, which will limit effluent total nitrogen to near 3 mg/L for plants, thus requiring near complete elimination of inorganic nitrogen species. Since sufficient internal carbon is usually not available for complete denitrification, external carbon is needed to supplement internal sources. Of particular interest is the use of an alternate external carbon source to replace the least expensive source methanol. This study focuses on three commonly available external carbon sources: methanol, ethanol and acetate. The aim of this study was to obtain the specific denitrification rate (SDNR) of the substrates under several conditions. Sequencing batch reactors (SBRs) were set up to first grow biomass to the specified substrate while in situ SDNRs were conducted concurrently. Once the biomass was grown with the corresponding substrate, a series of ex situ SDNRs were performed using various biomass/substrate combinations to evaluate response to substrate combinations at 138C. Results from this study indicate that the SDNRs for biomass grown on methanol, ethanol and acetate were 9.2 mg NO 3 -N/g VSS/hr, 30.4 mg NO 3 -N/gVSS/hr and 31.7 mg NO 3 -N/g VSS/hr, respectively, suggesting that acetate and ethanol were equally effective external carbon sources followed by much lower SDNR using methanol. Ethanol could be used with methanol biomass with similar rates as that of methanol. Additionally, methanol was rapidly acclimated to ethanol grown biomass suggesting that the two substrates could be interchanged to grow respective populations with a minimum lag period. Key words | acetate, cold temperature, denitrification, ethanol, methanol, sequencing batch reactor INTRODUCTION Current regulations in the Chesapeake Bay region are calling for more stringent nutrient discharge limits, where in the near future many municipal facilities will be required to meet an effluent total nitrogen concentration near 3 mg/L. In response, many facilities in this region are focusing attention on complete denitrification; the process whereby heterotrophic bacteria utilise organic substrate as the electron donor to convert nearly all of the nitrate, the electron acceptor, to nitrogen gas. Since denitrification is crucial for the removal of nitrogen, maximising this process will result in a decrease in nutrient load to the receiving waters. doi: 10.2166/wst.2008.670 233 Q IWA Publishing 2008 Water Science & Technology—WST | 58.1 | 2008