Conflicting carbon footprint and energy saving in a side-stream Anammox Process

•Impact of side-stream anammox treatment analyzed through LCA and experimental data.•N2O emissions from main WWTP and pilot-scale anammox were 0.37% and 1.3% of KJN.•GWP of reject sludge-water rises from 96 to 98 kgCO2eq with installing an Anammox.•Uncontrolled Anammox N2O emissions expose the WWTP to carbon footprint unbalances.•LCA of the treated reject sludge-waters halves when installing an Anammox process.