Proceedings

OPTIMISING THERMAL HYDROLYSIS FOR RELIABLE HIGH DIGESTER SOLIDS LOADING AND PERFORMANCE

Paul Walley
Cambi UK

(free)

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.

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