Methodology for the assessment of the greenhouse gas impact of wastewater sludge treatment

Nitrous oxide (N₂O) and methane (CH₄) emissions from the wastewater sector represent the second largest source of greenhouse gas (GHG) emissions of the waste sector. A protocol has been developed to measure GHG from sewage sludge treatment. GHG measurements were performed on two industrial sewage sludge composting plants. Measurement campaigns showed GHG emissions range between 0.18 to 3.40 kgCH₄\t dry sludge and 0.74 to 0.48 kg N₂O\t dry sludge. Converted to CO₂ equivalent to assess climate change impact, emission factors of CH₄ range between 4.5 and 85 kg eq.CO₂\t dry sludge and N₂O range between 143 and 221 kg eq.CO₂\t dry sludge when calculated with global warming potentials (GWP) of 25 and 298, respectively. In terms of climate change impact, N₂O emissions contribute much more than CH₄ because of its higher GWP. N₂O is linked to the treatment of nitrogen and produced during the nitrification and denitrification biological processes. Process emissions measured on the two sites were compared with indirect emissions linked to operational energy use.