THE DRYVAC PROCESS –CAN BIO-SOLIDS DRYING BE HAZARD-FREE AND CHEAPER?
Steve Clay1, Kim Thompson2, Rachel Edgington3
1Consultant, 2TVD(UK) Ltd, 3Severn Trent Water
(free)
Sewage sludge driers have been used for almost a hundred years and the “new generation”
have been installed, with varying success, in the UK since 1992. Many of these have suffered
from poor reliability, safety problems and high costs.
The DryVac process overcomes several of these problems. There is no handling of
dewatered sludge, no contact between the sludge and hot gases, no contact with moving
steel parts and the product does not become dusty. The process was developed in the US
from well-tried filter press technology – once dewatered in the standard way low pressure
steam is passed through the filter plates and a vacuum is applied to the filtrate removal
channels.
The first such plant in the UK has been operating at Severn Trent Water’s Coventry Sewage
Treatment Works since 2005. The plant comprises four presses each containing 185 square
plates measuring two meters by two meters. The approximate rated throughput is 15,000
tonnes per annum dry solids. Over the last two years detailed design improvements have
been implemented which have resulted in a robust plant capable of producing a consistent
product at 70-80% dry solids. TVD(UK) Ltd have been awarded a framework contract to
supply a further three DryVac plants to Severn Trent Water over the next three years.
Although the product is finding a ready market in the farming community the long-term future
of sewage sludge recycling is uncertain. For this reason TVD(UK) Ltd are working with
Severn Trent Water and thermal processing companies to develop an integrated energy from
sludge process. DryVac’s process design makes it particularly suitable for use in such an
integrated system.
KEYWORDS
Sewage sludge, drying, vacuum filtration
