Keith Panter, Ebcor Ltd
Paul Fountain, Thames Water
Dr. Achame Shana, Thames Water
Davy Ringoot, Cambi AS(free)
The Thermal Hydrolysis Process (THP) as a pre-treatment for anaerobic digestion was first presented at the European Biosolids conference in 1997. Since then THP has become a standard process in UK and Ireland with about 25% of all municipal sludge being treated this way. These projects are in the 300,000 to 3 million PE range.
In 2010 a second generation operating logic was trialled at Chertsey STW and proved to be successful. The changed philosophy led to a new generation of much smaller and productive systems – the B6 and B2 systems.
The first B6 system was installed at Crawley STW in 2014 and the digestion system ramped up in 2015. The plant is run as a regional sludge centre with import of cake, with a design capacity of 31 tds raw sludge per day.
The plant was assembled over 3-day period in the early part of 2014 – being comprised of 9 iso-container skid units that come ready assembled with pumps, valves, handrails and wiring. The whole process of assembly wiring and battery limit connection took two weeks before the plant was ready to operate on water for testing. Apart from some early problems of vibration and over-vigorous blow down the system has performed well mechanically.
The digesters were seeded 100% with THP digested sludge (6.0% dry solids) from another existing THP digestion plant during October 2014. The digesters did not need to acclimate to THP sludge as the feed sludge was already acclimated and so the plant was operated at 100% feed immediately. The increase in feed rate was initially hampered by lack of raw cake sludge import but the THP/digestion system hit the target of 31 tds per day by March 2015. The digesters have performed at high VS loading without stress.
The integration of ancillaries has been very good. The CHP and boiler integration meets the STW plants power target and the THP steam demand well with no use of supplemental fuel and very little flaring of biogas – the energy balance is described.
The operating costs are low and the plant has been mechanically very reliable; steam consumption is low and polymer consumption is also low (3-4kg/tds) in pre-dewatering belt press and for final dewatering (8kg/tds) with the final cake product at 33-34% DS from the existing belt presses.
Raw sludge throughput has nearly doubled with the exiting digesters, biogas has more than doubled while cake out the gate quantity has remained the same and the haulage distance reduced.
The plant is a good example of how to build and operate a THP sludge centre.
Anaerobic Digestion, Dewatering, Thermal Hydrolysis, CHP, steam explosion