Exploring the upper limits of thermal hydrolysis at Chertsey STW

Chertsey THP plant was the UK’s first Thermal Hydrolysis plant (THP) and over recent years the throughput and performance has exceeded known limits. The plant was originally designed in 1997 for 22 dry tonnes per day in 3,200m3 of digestion capacity equating to a hydraulic retention time (HRT) of 15 days and a VS loading of 5kg VS/m3/day.

The plant was retrofitted in 2005 and further modified in late 2010 to give better steam disintegration and the results reported at the 2011 conference. This paper examines the impact of effective digester working volume and sludge dilution on site THP Anaerobic Digestion performance, and quantifies the change, including VS destruction and gas production at Chertsey using a variety of methods.

Using results from an intensive sample programme, a mass balance has been calculated for the site that showed THP can be pushed very hard and achieve loading rates of >10kgVS/m3/d without digester failure, but at the detriment to energy recovery as the HRT nears 5 days that is the theoretical doubling time of methanogens. The specific energy loading rate (SELR) concept has shown that an SELR 0.45 g COD/g VS is a safe upper limit for loading rate – equivalent to about 8kg VS/m3 of effective digester volume in a Cambi-fed digester.