Energy and carbon footprints of sewage treatment methods

• Scale and technology choices influence energy and carbon footprint.
• Embodied energy contributes nearly half of the total energy footprint.
• Fugitive emission from biological process causes 87% of the total carbon emission.
• Electrical energy use in STPs in India and UK is 0.14 and 0.46 kWh/m3 respectively.
• Secondary treatment accounts for about 70%–80% of electrical energy used in an STP.

The paper presents energy and carbon footprints of sewage treatment plants (STPs) operating at different scales and using different technology options based on primary data from 50 STPs operating in India and the UK. The study used a combination of fundamental mass-balance approach for energy consumption and the methodology defined by IPCC for the carbon emissions. Small-scale institutional STPs consume twelve times the energy consumed by large-scale municipal STPs, the corresponding energy intensities being 4.87 kWh/m3 and 0.40 kWh/m3 respectively. Embodied energy from construction material and chemicals accounted for 46% and 33% of the total energy intensity of the municipal and institutional STPs respectively. The average carbon footprint of large-scale STPs is 0.78 kgCO2eq/m3 and for small-scale STPs it is 3.04 kgCO2eq/m3. However, fugitive emissions from large-scale STPs constituted 74% of the total carbon emissions whereas the figure was only 0.05% for small-scale STPs. Average electrical energy intensity in STPs in India is much lower (0.14 kWh/m3) than that in the UK (0.46 kWh/m3). This is due to the reason that STPs in India do not have resource recovery processes and use solar heat for sludge drying. The paper offers information and insights for designing low carbon strategies for urban waste infrastructure.