Impact of long‐term water and energy consumption in Tokyo on wastewater effluent: implications for the thermal degradation of urban streams

Abstract

AbstractThe heated effluent of urban wastewater results in the thermal degradation of urban streams. To better understand the process of wastewater heating and its thermal impact on streams, a watershed‐scale water and heat balance model which accounts for water and energy supply, consumption, and transport processes within urban areas is proposed to quantify the annual and monthly mean effluent temperatures from treatment plants. The model is then applied to the central Tokyo area and shown to accurately reproduce the increase in the measured effluent temperatures over the past 34 years. The marked increasing trend in effluent temperature is attributed mainly to the increased heating of wastewater that occurred over this time period. Although there was also an increase in supply water temperature during this period, the effect of this on effluent temperature is shown to be relatively small. Of the three energy‐use sectors considered in this publication, the residential sector had the largest increase in estimated drain temperature. This increase is probably due to the increase in the amount of water and energy used for bathing, which is likely to be due to the increasing use of showers and the increasing size of bathtubs. In the commercial sector, the rise in calculated drain temperature is shown to originate from the increasing floor area and the resulting increase in energy consumption. It is shown that energy‐use changes in the residential sector are the primary cause of the rising effluent temperatures in the central Tokyo area. Copyright © 2007 John Wiley & Sons, Ltd.