ABSTRACT
Carbon foot printting employs the concept of Life Cycle Assessment (LCA) to estimate the GHG emissions throughout the life cycle, extraction, production, distribution and consumption and dismantling, of a certain process or a product. Following the framework of LCA, the scope of the study needs to be identified first, which is, in this study, is to estimate the GHG emissions from life cycle of wastewater treatment process. In order to fulfill the estimation, system boundary is then needed to be settled to facilitate the analysis of flows of energy, materials and GHG emissions. In this study, only the operational process of WWTPs is considered due to its significant contribution to the life cycle GHG emissions, compared with construction and dismantling process (Corominas et al. 2013). Therefore, the study boundary starts with the arrival of wastewater at WWTPs and ends with the treatment and disposal of biosolids and effluent discharge. GHG estimation covers: 1) direct emissions from liquid treatment at WWTPs, including CO2 emissions from aerobic biological treatment process, and N2O from nitrification and denitrification process in Biological Nutrient Reactors (BNR); 2) GHG emissions from biosolids treatment, such as CO2 emissions from aerobic digestion, CH4 and CO2 emissions from anaerobic digestion and among other; 3) indirect emissions from energy and chemicals consumption required during the liquid and biosolids treatment at WWTPs, such as GHG emissions from outsourcing electricity for driving mechanical devices like pumps and blowers, and emissions from dosing
1 INTRODUCTION
Municipal Wastewater Treatment Plants (WWTPs) has been recognized as a significant energy consumer and source of Greenhouse Gas (GHG) emissions, which is responsible for global climate change. The electricity consumption in WWTPs usually account for 3% of total electricity consumption in the U.S. (Mo. & Zhang 2012) While in China, it is estimated that electricity consumption for wastewater treatment is about 17.5 billion kilowatts hour (kWh) in 2013, accounting for 0.4% of China’s total electricity consumption. With the soaring growth of infrastructure and operation of WWTPs, energy consumption is believed to be increase significantly. Besides, worldwide wastewater is the fifth largest source of anthropogenic methane (CH4) emissions, with India, China, U.S. and Indonesia combined accounting for 49% if the world’s CH4 emissions from wastewater. Also, worldwide wastewater is contributing to 3% of total nitrous oxide (N2O) emissions, as the sixth largest emission source (Gupta & Singh 2012). In 2005, China’s GHG emissions from wastewater treatment are around 114 million metric tons of CO2 equivalents, accounting for 72% of total CH4 emissions, 26% of total N2O emissions (State Council 2012). In pursuit of low carbon development in wastewater treatment industry, it is necessary to understand the carbon footprint of WWTPs and identify the opportunities for emissions reduction. This paper aims to set up an accounting model for GHG emissions including direct emissions from wastewater biological treatment and indirect emissions from energy consumption and materials consumption in WWTPs.
