A LIFE-CYCLE APPROACH FOR ESTIMATION OF GREENHOUSE GAS EMISSIONS FROM CANADIAN WASTEWATER TREATMENT

An inventory of emissions of greenhouse gases (GHGs) from Canadian municipal wastewater treatment plants was prepared using a life-cycle approach. The inventory procedure differs from that used by the Inter-governmental Panel on Climate Change (IPCC) in that upstream electricity emissions associated with power consumption on-site at the treatment facilities were included in this inventory, instead of being assigned to the energy sector. Additionally, this inventory calculated carbon dioxide (CO2) emission rates resulting from wastewater treatment, while the IPCC Protocol calls only for estimation of methane (CH4) emissions.The inventory was completed using a generic approach based on sound wastewater engineering fundamentals. The estimation procedures for GHG emissions from conventional activated sludge facilities using mesophilic anaerobic digestion for solids treatment were calibrated with more detailed operating data from 16 facilities in the Provinces of Alberta and Ontario.This inventory revealed that the IPCC Protocol likely leads to over-estimation of methane emissions from municipal wastewater treatment facilities in Canada. Even assuming a generous estimate of 5 % leakage from digester biogas collection and distribution, the estimated CH4 emission rate was only 1,600 Mg/yr compared to a Canadian National estimate of 19,000 Mg/yr. Due to wide implementation of secondary treatment levels or higher, most of the organic carbon entering treatment facilities as BOD5 is converted to CO2. Methane produced in facilities using anaerobic digestion from solids stabilization is virtually all combusted to CO2 for one reason or another (boilers, engines, flaring). Slightly over one-half of the GHG emissions in this inventory were produced by on-site treatment processes, while upstream electricity generation and natural gas production produced approximately one-third of the emissions. The balance was due to on-site fossil fuel consumption for heat and electricity.Ontario, the most populous province in Canada, produced the highest GHG mass emission rates, as expected. When emissions from upstream electricity generation were factored in, however, emissions per volume of wastewater treated (kg CO2 equivalents/m3 wastewater treated) were higher for provinces relying on thermal generation for electricity (e.g., Nova Scotia and Alberta) than for provinces with greater use of hydro-electricity (Quebec) or nuclear power (Ontario). Greater production and use of biogas on-site would lessen the purchase of off-site energy and fossil fuel, which together with conservation measures, would improve the financial and environmental situation in the sector.