Impact of process design on greenhouse gas (GHG) generation by wastewater treatment plants M. Bani Shahabadi*, L. Yerushalmi, F. Haghighat Department of Building, Civil and Environmental Engineering, Concordia University, 1455 de Maissonnuve Blvd. West, Montreal, QC H3G 1M8, Canada article info Article history: Received 15 October 2008 Received in revised form 9 February 2009 Accepted 20 February 2009 Published online 11 March 2009 Keywords: Greenhouse gas Wastewater treatment On-site emissions Off-site emissions Hybrid process abstract The overall on-site and off-site greenhouse gas emissions by wastewater treatment plants (WWTPs) of food processing industry were estimated by using an elaborate mathematical model. Three different types of treatment processes including aerobic, anaerobic and hybrid anaerobic/aerobic processes were examined in this study. The overall on-site emissions were 1952, 1992, and 2435 kg CO 2 e/d while the off-site emissions were 1313, 4631, and 5205 kg CO 2 e/d for the aerobic, anaerobic and hybrid treatment systems, respectively, when treating a wastewater at 2000 kg BOD/d. The on-site biological processes made the highest contribution to GHG emissions in the aerobic treatment system while the highest emissions in anaerobic and hybrid treatment systems were obtained by off-site GHG emissions, mainly due to on-site material usage. Biogas recovery and reuse as fuel cover the total energy needs of the treatment plants for aeration, heating and electricity for all three types of operations, and considerably reduce GHG emissions by 512, 673, and 988 kg CO 2 e/d from a total of 3265, 6625, and 7640 kg CO 2 e/d for aerobic, anaerobic, and hybrid treatment systems, respectively. Considering the off-site GHG emissions, aerobic treatment is the least GHG producing type of treatment contrary to what has been reported in the literature. ª 2009 Elsevier Ltd. All rights reserved. 1. Introduction Wastewater treatment plants (WWTPs) are recognized as one of the larger minor sources of GHG emissions (U.S. EPA, 1997). These plants produce the three major greenhouse gases (GHGs) CO 2 , CH 4 , and N 2 O during the treatment processes while producing additional amounts of CO 2 and CH 4 from the energy demands of the plant. The estimation of GHG emis- sions of WWTPs has been traditionally based on the on-site emissions only without considering the off-site or upstream emissions associated with the plant operation. On-site GHG emissions are due to liquid and solid treatment processes as well as biogas and fossil fuel combustion for energy generation. Off-site emissions which stem from the produc- tion of electricity for plant, production and transportation of fuel and chemicals for on-site usage, degradation of remain- ing constituents in the effluent, as well as transportation and disposal of solids are traditionally allocated to the energy sector and not to the end-user which is the WWTP. Waste- water treatment plants, especially those related to industrial operations that treat high-strength wastewaters are particu- larly vulnerable to international protocols and regulations such as the Kyoto Protocol which plan to impose taxes and penalties on GHG emissions. Therefore, any attempt to iden- tify the sources of GHG emissions and recommend processes for the mitigation of emissions is vital and timely. * Corresponding author. Tel.: þ1 514 848 2424x7246; fax: þ1 514 848 7965. E-mail addresses: m_banish@encs.concordia.ca (M. Bani Shahabadi), laleh@encs.concordia.ca (L. Yerushalmi), haghi@bcee.concordia.ca (F. Haghighat). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.02.040 water research 43 (2009) 2679–2687