CO2 Emission Factor for Rainwater and Reclaimed Water Used in Buildings in Japan

From the standpoint of the preservation of water resources, rainwater and reclaimed water have been widely used in buildings in many countries. However, the CO2 emission factors of these two waters—factors that determine their environmental impacts—have not been calculated. In a previous study, the CO2 emission factor of water for waterworks and sewer systems was determined. In this paper, we evaluate the emission factors of rainwater and reclaimed water in the same manner. First, the emission factor for pumping water in buildings is determined using published values for operating performances. About half of the residential dwellings in Japan are multistory apartments, and these apartments use pumps for the delivery of water. The emission factor of pumping is calculated as 0.69 kg CO2/m3, which adds 16% to the emission factor of waterworks and sewer systems. Next, the CO2 emission factors of rainwater and reclaimed water are calculated for different water delivery cases in buildings. As a result, it is found that the use of reclaimed water increases CO2 emissions by 62%, compared to the use of ordinary water.