USE OF AN ELEMENTAL BALANCE OVER PRIMARY AND SECONDARY ANAEROBIC DIGESTION OF SEWAGE SLUDGE TO DETERMINE ITS CARBON IMPACT
Dr William P F Barber PhD DIC BEng(Hons) MIChemE MCIWEM CEng CEnv, United Utilities
(free)
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.
KEY WORDS
Advanced anaerobic digestion; carbon footprint; GHG emissions; renewable energy; sludge
treatment.
