Life cycle assessment of advanced anaerobic digestion process configurations for sewage sludge – a UK perspective

Over the past 10 years significant development has been made in advanced anaerobic digestion technologies for sewage sludge. These processes are now being implemented at large scale across the UK and within Thames Water. Although there are significant economic benefits to advanced anaerobic digestion (AD) processes, life cycle impact assessments have been limited in depth. This paper attempts to fill this gap in knowledge by comparing several process variants as part of a Life Cycle Assessment (LCA). Using operating data, a process model was created to calculate the economic and environmental impact factors of the options during the life of the operational plant. It was found that advanced AD processes have advantages over conventional AD but the increased energy input requirements of thermal pre-treatment make it less beneficial under some conditions. Cleaning up biogas to enable Gas to Grid (GtG) injection requires significant renewable incentives to be economic and environmental impact can be negative when compared with Combined Heat Power (CHP).