Use of an elemental balance over primary and secondary anaerobic digestion of sewage sludge to determine its carbon impact
This paper details how a mass balance, which was conducted for all of United Utilities’ anaerobic digestion plants, was used to calculate emission factors for various types of anaerobic digestion processes to enable calculation their carbon footprint. The mass balance showed average volatile solids destruction rates of 45% and 11% over primary and secondary digestion respectively. The calculated release of biogas from secondary digestion was within than the emission factor proposed by UKWIR for standard anaerobic digestion. However, in terms of carbon footprint, biogas released from secondary digestion had a more detrimental impact than the beneficial influence of biogas export as electricity from primary digestion, thereby potentially removing the benefits of sludge digestion in carbon terms. However, carbon footprint of digestion was found to decrease significantly when the secondary biogas was collected and flared. Furthermore, advanced anaerobic digestion was also found to appreciably reduce carbon footprint. It is therefore recommended that secondary biogas be flared, or advanced digestion installed as standard to make anaerobic digestion sustainable in the long term with respect to emissions of Green House Gases.