The past 10-15 years has witnessed the development of a number of new technologies aimed at improving the performance of anaerobic digestion systems. Anaerobic digestion comprises four stages namely, hydrolysis, acidogenesis, acetogenesis and methanogenesis. The hydrolysis step in anaerobic digestion is usually rate limiting because of the slow degradation of waste activated sludge as a result of the low biodegradability cell walls and biopolymers formed in activated sludge.

Most of these technologies have focused on sludge pre-treatment, using thermal, mechanical or enzymic treatment to enhance hydrolysis and breakdown complex solid organic particles. This releases soluble compounds, resulting in enhanced fermentative processes during anaerobic digestion. Sludge from these pre-treatment systems is characteristically high in soluble COD compared to untreated sludge. These technologies have resulted, albeit to different success rates in the following benefits:

• Increase in biogas production and hence bigger payback on CHP
• Increase in volatile solids destruction
• Enhanced digester capacity
• Reduction in cake disposal costs
• High pathogen removal
• Better dewaterability of digested sludge

Our trials have shown that the type of sludge as well as the sludge handling practices and facilities at a site can have a large impact on the performance and therefore suitability of a given process.  Consequently, upfront treatability trials are vital for process verfification.

There has so far been little work carried out combinging advanced treatment technologies with co-digestion. It is anticipated that as legislation and regulatory drivers change, this will become a major focus area to maximise water company assets and reduce greenhouse gas (GHG) emissions.