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04 August 2009

Waste treatment world better

Autoclave waste treatment adds value to the waste stream and dramatically reduces landfill disposal. Mott MacDonald is project managing delivery of the world’s largest facility.

European laws have imposed a pressing need for the UK to find new ways of managing its waste. Measured against 1995 levels, the UK must achieve a 25% reduction in the amount of biodegradable municipal waste sent to landfill by 2010.

Further cuts to 50% below 1995 levels must be achieved by 2013 and 65% by 2020. Small wonder that a treatment technology capable of diverting all but 10% of waste from landfill is exciting interest.

Acting as technical advisor and project manager for client Graphite Resources, Mott MacDonald is helping deliver the UK’s first – and the world’s largest – steam autoclave waste treatment facility. It is being built at the Derwenthaugh EcoParc, Gateshead. Waste will be loaded into four huge rotating vessels, where it will be subjected to 160°C heat and 6 bar of pressure. After a two hour treatment cycle the waste will be transformed, says Mott MacDonald project manager Richard Goddard.

“One of the beauties of the autoclave system is that it will accept unsorted commercial and municipal waste,” Richard Goddard explains.
For the local authority, this dramatically simplifies waste collection. Instead of requiring householders to segregate their waste streams – metals, glass, plastics, paper, kitchen and garden waste – the whole lot can be picked up in a single bin. The heat and pressure treatment cleans and shrinks plastics and, crucially, brings about a huge reduction in the volume of the biodegradable fraction, rendering it into a homogenous, sterile, cellulose fibre.

“When it comes out of the pressure vessel, 60-70% of the waste stream will consist of cellulose fibre. A series of processes will then remove the recyclable elements,” Richard says. Plastics will be extracted by jets of air triggered by optical sensors. Ferrous materials will be removed by magnets. Eddy currents will trap non-ferrous metals. Screening and grading equipment will sift glass and grit.

“The segregation equipment is leading edge, but it’s proven and has become well established in the industry,” reassures Graphite Resources director Richard Mair. “In fact, although we’re combining equipment in a highly innovative way, there is no part of the process that’s not proven and robust. The autoclave process employs steam generation and water treatment technologies that have been around for decades. The risk in each element is very small.”

As well as simplifying the process of retrieving conventional recyclates from the waste stream, Graphite Resources has been seeking uses for the cellulose fibre. Construction materials manufacturers are looking to use it as an aggregate or binder in bricks and blocks.

It can be used as a soil improver for rehabilitating industrial or quarried landscapes, for example. And there is potential in future to use the material to generate energy: it can either be burned directly or subjected to anaerobic digestion to produce methane. “We’re looking at generating our own power on a second autoclave project,” says Mr Mair.

One of the key challenges in guiding the project towards completion in late 2009 has been coordination between all of the different parties involved, says Richard. To produce a well integrated and buildable design, close liaison has been needed between Graphite’s operating tea, process designer Enpure, main contractor Clugston Construction, steam and vessel manufacturers, suppliers of the equipment for materials separation and the project’s lenders.

“Risk at the interfaces between different technologies and suppliers has been minimised by achieving really strong communication to ensure that everybody knows what the others are doing and what’s expected all round.”
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