Optimising thermal hydrolysis for reliable high digester solids loading and performance

Chertsey (Thames Water) Cambi thermal hydrolysis was refurbished during the early part of 2005 and started operation in August 2005 under a separate contract to Cambi operations. The plant has run continuously since that time except for planned shut downs (inspection and Christmas day) with 2 hours of unplanned shut down.

Initially it was envisaged the plant would achieve an average of 22 dry tonnes per day. However when sludge is available the plant treats over 25 dry tonnes per day. The sludge is fed at about 10.5% DS to two diegsters at 1,600 m3. Assuming 90% EDV and average VS of 75% the VS loading is 6.5kg VS per m3 digester capacity per day with an HRT of just over 12 days. The digestion process is extremely stable and an average of 400m3 of biogas is generated per dry tonne at 68% methane. This is well above typical performance despite the high VS loading and short HRT.

The plant acts as a test bed for engineering improvements on process equipment and process control. The aim has been to improve the reliability and the energy efficiency of future plants by testing the limits of equipment and process at Chertsey

Three new hydrolysis plants are in construction or design in the UK. Two for Anglian Water Services for 20,000 dry tonnes each and one for Northumbrian Water Ltd for 40,000 dry tonnes. All these plants will operate at high VS loading.

The first project for AWS at Cotton Valley, Milton Keynes is mechanically complete. The  paper presents the developments from Chertsey and elsewhere that have been applied in this process plant and describes the construction process.