Research has shown that the biggest single contaminating component in wood residue mixes is that of preservative treated wood, which estimates suggest can account for over a quarter of all wood residue contaminants (“Study of UK wood flows and recycled waste wood markets”).
Wherever present, it can be a barrier to the recycling of the whole of the wood residue. Furthermore, if the preservative treated wood component exceeds 10% of the total volume, then the whole unit will almost always be rejected for recycling purposes. Identification of treated wood by eye is almost impossible to do accurately and laboratory analysis can be both expensive and time consuming.
In 2004 TRADA Technology completed a project for the Waste and Resources Action Programme (WRAP) entitled “Rapid identification and sorting of preservative treated wood”, which was part-funded by the Timber Research and Development Association (TRADA).
Its main objective was to facilitate the increased recycling of wood by identifying rapid, preferably portable, analytical techniques that could be used to classify and sort waste wood into treated and untreated material. Untreated wood could then be recycled through standard routes, while treated material could be separated and suitably processed or re-used.
Review of existing information
A literature search was carried out as part of the project, gathering together relevant articles/publications. TRADA Technology‘s previous work in recycling was also included in the review and the proceedings of any relevant conferences and journals over the last 25 years were scanned.
In addition, a database of industrial contacts in various relevant fields was created. This included manufacturers of analytical equipment, preservative manufacturers and treaters, recycling companies and laboratory/research organisations worldwide. Contact was established with relevant professional/trade bodies.
This all culminated in an information review document that was used for reference throughout the remainder of the project.
The document was analysed to identify those techniques which potentially had the capability to identify treated wood. Those techniques were then discussed with organisations and individuals in the appropriate fields.
Equipment manufacturers who wished to participate were supplied with treated wood samples prepared by TRADA Technology, so that limited exploratory trials could be carried out. These samples comprised of beech veneers, fully penetrated with various preservatives. Beech was chosen because it contains low levels of extractives – naturally occurring chemicals which could affect the results – and it is very permeable. By using a veneer of less than 1mm thick, full penetration of the preservative treatment could be achieved quickly and easily without using specialised equipment.
The most promising techniques were then assessed by looking at the relative practicalities of using these techniques to identify preservatives in a recycling situation, for example at the wood recycler’s yard or on the building site. A long list was then drawn up.
From this, the commercial viability of the analytical techniques listed was evaluated by analysing their technical capabilities in relation to capital and operating costs. This involved in-depth discussions with recyclers, equipment manufacturers and industry representatives, including an evaluation of the potential for future developments to overcome any existing limitations.
The outcome was a short list of those techniques with greatest potential. This itself was split into three categories, as it was found that some of the techniques were closer to market than others – ‘category one’ techniques are the closest to market, with ‘category three’ the furthest away.
Category one
Category two
Category three
The project made several recommendations for further work:
TRADA Technology is now in discussion with manufacturers about further work in 2005, to bring the most promising techniques closer to market.
