Calculation of greenhouse gas emissions in Canary Islands wastewater treatment plants

This research analyzes the water-energy nexus contribution to climate change from the integral water cycle perspective. A prior analysis of the sectors involved in global warming showed that clear water/energy interdependence was a crucial factor in increasing greenhouse gas (GHG) emissions. Although the analysis of water consumption in the energy sector has been widely studied, global requirements of energy for water are still poorly understood as is their contribution to climate change. With this in mind, wastewater treatment plants (WWTPs) were chosen for the specific analysis conducted in the present study as they are one of the biggest GHG emitters in the water industry. They are located in the Canary Islands (Spain), an archipelago that, like many others, is characterized by its dependence on external water and energy resources. The methodology employed involved an initial analysis of the WWTPs in the islands and the selection of four full-scale WWTPs of varying capacity for further in-depth analysis. This was followed by consideration of the different protocols and tools that can be used to quantify direct and indirect GHGs emissions. The Intergovernmental Panel on Climate Change protocol was selected due to the great diversity of operational data and the emission factors proposed by experts when the data are not available for a specific region under analysis. The results show that the highest emission source is due to energy consumption (ranging from 165 to 2,716 Tm-eq CO2/y for the smallest and largest size WWTPs, respectively). To solve the problem, the introduction of renewable energies is presented as a feasible and attractive option due to the specific characteristics of the analyzed territory. Likewise, the use of sludge as an internal energy resource (in anaerobic digestion) obviates the need for its transport and management, contributing to the circular technology and reducing emissions.