Wood residues: Waste or resource?

In many wood processing operations a significant portion of the timber input as raw material does not emerge as ‘product’ but as ‘waste’ or ‘residue’. Depending on the form, consistency and purity of the wood residue, disposal to incineration or landfill can incur a cost, or be a valuable by-product for board manufacture or fuel for energy, thus enhancing the profitability of the processor.

Much of the timber produced by sawmills goes through several further processing stages before reaching its final end use; each stage producing its own volume of wood residues. General conversion factors used for estimating the product volume output from round logs illustrate just how significant a portion of the input materials is actually output as wood residue.

Thus, on average, less than half the raw material ends up in the primary product; the remainder is residue comprising bark, sawdust, chips, off-cuts, sander dust etc. For imported timber and panel products, much of this residue remains in the country of origin.

However, for British-grown timber these residues constitute a valuable fibre resource with the potential for recycling in a variety of different ways. Today, the perception of wood “waste” as an item for disposal should be outdated. Wood residues should be viewed as a by-product for recycling or reuse, especially in the light of environmental and conservation pressures which demand more effective use of all resources, including timber.

For maximum technical and economic use to be made of this resource, it is necessary to have data on the types, quantities, qualities, and location of wood residues of all forms. This information has not been gathered formally in the UK for over 20 years, during which time processing technology has changed substantially and British-grown timber has become a significant component of the UK timber market.

The only regular data compiled for the UK are the annual figures complied for levy purposes by the Forest Industries Council of Great Britain (FCIGB) for those residues generated during sawmilling or suitable for wood-based panel manufacture. In 1994, these were predominantly softwood residues, which amounted to 1.5 million m³ or about half the total raw material intake of UK board manufacturing plants. In the same year, 7.6 million m³ of British grown timber was produced, giving rise to 4.5 million m³ of primary wood residues based on the above product / residue ratios. Thus a large proportion of even the British-grown residue resource is not accounted for.

Residues created in the UK from imported timber and wood products are also important in view of the high proportion of timber imports. These wood residues are effectively ‘bought in’ at finished timber / board material product prices while they are disposed of at a cost, including landfill fees, landfill tax and transport.

To assist the timber industry to maximise revenue from its by-products, this research concentrated on quantifying the types, grades, locations and uses for residues created in the UK to identify best practices in their current utilisation or disposal. Establishing such ‘benchmarks’ provides an effective management tool against which companies can assess their current performance and devise strategies for future improvement.

Generally, wood residues decrease in value and degenerate into ‘waste’ materials, the smaller the particles and the greater the destruction of the fibrous nature of the wood. Sawdust is generally less valuable than chips suitable for chipboard or paper pulp manufacture. Further, mixed or wet residues and those contaminated with coatings, metal fixings, preservative treatments, such as anti-sapstain treatments, are often of low or negative value. The way that residues are handled, segregated, stored, etc also affects their value and can make the difference between a profit contributing asset and a costly disposal problem, a point increasingly emphasised by the cost of landfill tax in the UK.