Wood residues: Waste or resource?

The first company is a large sawmill processing British-grown softwood, mainly spruce but with some larch logs, 40% of which arrive pre-cut as pole lengths. A variety of residues are produced, including log off-cuts, butt reducings, bark, wood chips, sawdust, mill off-cuts, dried shavings and rejects. Volumes are shown in Table 3.1.

Table 3.1 Logs consumed and products/ residues produced Note: the volumes suggest a greater output than input because they assume a cylindrical log based on narrow(top end) diameter and residue volumes are estmated from the weight of non-compacted material over the weight bridge, rather than true volumes.

The log off-cuts, butt reducings, bark and dried shavings are sorted on site by a wood residue trader which purchases and processes them for resale. Most are chipped along with off-cuts although some rejects are downgraded to lower value products. Whitewood chips are sent for paper production and larch chips for chipboard manufacture.

The second sawmill is of similar capacity, again processing mostly spruce with some larch logs. The main types of residue produced are wood chips, sawdust, off-cuts, shavings, bark and butt reducings. Estimated volumes are shown in Table 3.2.

Table 3.2 Logs consumed and products/residues produced

All wood residues are graded and sorted prior to sale as are bark residues, which are composted as part of grading. Bark is also delivered to the site from other sawmills in the region as well as a small amount of reclaimed / recycled timber from a local scrap merchant.

Whitewood residues are collected and screened into four main fractions:

The majority of the screened whitewood chips go a paper manufacturer which carries out regular quality inspections and can impose penalties or reject loads. The ideal chip should be fresh and white, best within 2-3 days of cutting, and between 7 and 45 mm. Loads with > 0.5% bark or fines (chips < 5 mm) suffer a price reduction penalty while loads with > 1% bark or fines are rejected.

Larch chips are not screened and go with other unscreened whitewood chips and ‘carry over’ (floor sweepings etc) for chipboard production. Quality control inspections are carried out and a price penalty of up to 15% can be imposed, usually for dust. Dry sawdust also goes for chipboard production.

Any oversized, pin chips, wet sawdust, off-cuts, butt reducings, shavings and other dry sawdust go to a specialist wood fibre recycler for further processing prior to selling on.

Bark is classed as either good or bad. The good bark is sorted into five grades for horticultural, playground floor and other purposes. A portion of the finest of the grade is also composted for horticultural mulch. Delivery is either by bulk loads or pre-bagged.

Some ‘bad’ bark is sent for chipboard production with the other residues, as above. Any remaining bark, residues and ‘waste’ wood, which is rejected and not acceptable to residue users, is collected and burnt on site. The burner has the capacity to generate 5 mW of power and currently provides hot water at 95ºC and 2 bars pressure for heating the drying kilns.

Both companies recognised an increase in the number of poor quality logs in terms of shape (roundness) and straightness and the volume of small round wood on the market. This increases the availability of low value wood fibre to wood chips users causing a reduction in demand and price for sawmill residues.

The potential effect of an increase in the availability of recycled fibre was viewed differently by the two companies interviewed:

The availability of recycled paper, small round wood and forest residues in the medium term are likely to have a negative effect on demand and price of wood chips and other similar residues. Recycled fibre is not expected to have any significant effect on either the demand for or the price of wood residues.

The price of products, both sawn timber and residues, can be highly volatile. Most sawmills are dependant on revenue from residues as much as from sawn timber in order to be profitable, therefore any significant increase in recycled fibre going into paper or panel products could have serious consequences.Fluctuations in the paper industry are a good example of how recycled wood fibre might affect the industry. In 1996, the Canadians effectively dumped vast quantities of paper pulp for newsprint on to the open market with the effect that the demand for whitewood chips dried up virtually overnight. It is thought that the Scandinavians could cause the same effect in the near future because of their drive for total recycling of all paper products.

The advent of wood burning power stations could absorb surplus material, depending upon location and transport costs..

The first company produces a range of panel products from logs, sawmill residues and small amounts of recycled wood fibre.

Currently, chipboard contains up to 20% dry recycled fibre, possibly more in the future.

MDF and paper production account for 50% of chipped sawmill fresh wood residues. MDF can also use some recycled paper such as cardboard and packaging. Paper mills use logs, chips and waste paper.

OSB does not utilise wood residue as it requires fresh round wood saw logs to generate the appropriate type of wood flakes.

40% of chipboard costs are the raw materials; hence availability of the raw material and demand for the product has a big influence on the price paid for residues. So much so that the effect of exchange rates on panel manufacturers sales income has a big knock on effect on residue prices.

The second company produces chipboard panels mainly for flooring and furniture, some of which are laminated with various foils, papers and other overlays. Raw material input includes small round wood, chipped on site, sawmill wood chip residues and small amounts of recycled fibre. Furniture grade panels represent 80% of production and have a very limited use for recycled wood fibre whereas worktop blanks, which could use lower grade raw materials,represent only 1% of production.

There is currently a potential surplus of residues with a growing supply. Most sawmillers and processors do not currently work “hand-in-hand”, therefore market balances are affected.

Shavings and sawdust from the joinery/ furniture industries are largely used for animal bedding. They could be used in panel production, as could broken/ end-of-life pallets, but only if ‘clean’ and in quantities large enough to make transport viable (minimum load 23 tonnes). The use of such residues would also depend on the type of panel being produced. Dark coloured timbers such as tropical hardwoods, oak, etc, are not acceptable for furniture panels and low grade residues are not acceptable for high grade flooring; though hardwoods such as birch, beech and ash could be used for their strength properties. Most producers of such residues do not separate hardwoods from softwoods, which reduces their respective values. Currently there is a surplus of supply over demand for raw materials in the medium term; therefore chipboard producers are not looking to pay a high price for any raw materials.

Using wood residues as a raw material can be a problem for MDF production, although wet residues/ refined fibre and possibly other waste, eg waste paper, plastics and flax could possibly go into MDF in the future.

Landfill tax, introduced at £7/ tonne and rising annually, will have an increasing influence on the availability and economic viability of using recycled wood fibre for chipboard. There is also the potential of a big future for recycled wood fibre from pallets and packaging and from construction and development waste because of the forthcoming packaging waste regulations.

A problem in using recycled fibre is where it includes recycled panel products. This is because the resins originally used do not normally come away from the wood fibre so as to leave clean chips and therefore reduce the bonding quality of the ‘new’ woodchips. As a result, only small quantities of recycled panels can be included as raw material and then the proportion would depend upon the quality of panel being produced.

Conversion of contaminated wood residues into board products could be an area for development in the future. Contaminants caused by recently introduced treatments should have full safety data information available.

Recycled wood fibre can be cleaned by separating fibres on a weight basis and using magnets to separate out both ferrous and non-ferrous metals. The waste gases are scrubbed and condensed to a sludge that is incinerated. However, wastewater can contain high levels of heavy metals.

Increasing use of residues could lead to more diversity in supply with the potential for secondary processor residues to be used. Most primary processor residue production is already accounted for.

It may be more appropriate to find alternative uses for low value wood residues than to try to add value by sorting or cleaning residues that panel manufacturers were reluctant to take anyway ie ‘don’t try to drag poor quality materials into high quality products’. A good alternative to investigate might be the use of low quality wood residues / recycled wood fibre as wood fuel in energy generation systems although most wood burning plants currently being developed can’t take such low value wood fibre.

Biofuel companies are increasingly providing outlets for use of wood residues as fuel for energy.

The first company is a specialist wood recycler. The company is thought to have been the first in the UK to process recycled wood into wood chips for particleboard production. The company runs its own vehicles for material collection within a 75 mile radius. Alternatively, recyclable material can be delivered to site by disposers.

The company does not pay for any recyclable wood fibre but charges for the service of removing it.

A load can be rejected if the unacceptable material content is above 10% and a warning is given that contaminant disposal charges will be made if offending continues. The rejection rate can be high with new clients until they gain experience.

The processing of wood residues takes the form of manual grading of input materials to remove plastics, paper, etc. followed by extraction of ferrous and non-ferrous metals. Chipping the wood residue to 6 - 7 cm or less then completes the process. Floatation tanks are not used because the main markets are for dry products and the input materials are invariably dry.

All output goes for chipboard production, or for equestrian uses / animal litter. It is weighed out over the on-site weighbridge and is normally transported by the user. Quality checks are conducted from time to time.

The second company provides demolition, landfill/ transfer, recycling of wood residues and green waste disposal. Most operations are conducted within a 12-15 mile radius of their base, except for some Local Authority contracts which are within a 15-20 mile radius.

Demolition of buildings involves sorting and crushing brick, block, concrete, etc. and sorting lengths of timber on site, often with sale direct from the demolition site. Residual material, waste and smaller quantities are brought back to the landfill/ transfer station for sorting. The problem with most demolition is that high-speed mechanical demolition leads to contaminated timber but manual deconstruction requires scaffolding which is expensive. Therefore timbers only come out whole if the roofing material eg slates, is of sufficient value to warrant the cost of the scaffolding.

Landfill site / transfer station wood residues can include any wood fibre, builders waste, pallet wood, railway sleepers (3,000-5,000 year). Collection of this material is expensive if in small mixed quantities, £65 per 6 yard skip. Economies of scale mean that large skips, even of mixed material, are cheaper, £35 per 40-50 yard skip. The incentive though is for large volumes of sorted clean wood waste, £15 per 40-50 yard skip. Landfill tax, £2 / tonne for inert waste, £7 / tonne for biodegradable waste (now increased to £10 / tonne), is not surcharged but is carried within the above charges. However, the majority of input material is sorted and recycled thus leaving little residual material to incur landfill tax.

Current wood residue throughput is 100 tonnes per week with a target to steadily increase this to 250 tonnes a week, all of which is expected to come from within the present 15/ 20 mile radius. All construction and demolition timber, pallet wood (mainly from a disposal contract), railway sleepers (the only item bought) and other wood fibre (excluding panel products) is sorted by hand to remove unwanted material to produce:

Sorted wood fibre is crushed and chipped to a specified size and metal removed by magnet. The chipped fibre is stored short term and goes for use in chipboard production.

Green waste disposal is a service provided for two Local Authorities for the recycling of trimmings, chippings, leaves, etc from their landfill / transfer stations. Small quantities of green waste also come from the company’s own demolition and landfill/ transfer operations. All material is chipped or shredded at the source landfill/ transfer station and recycled as a mulch for use by the two Local Authorities involved.

The advent of, and increase in, landfill tax is increasing the volume of recycled wood residues which in turn is pushing down the price. At the same time, however, demand from particleboard producers is increasing (clean dry recycled fibre is better than clean wet virgin fibre) which is holding prices up. In the short term, demand is stronger than supply. The future is likely to see tighter specifications regarding acceptable material rather than a decrease in price as volumes available increase.

A greater/ faster development of new power plants is required to fill the potential gap in market demand.

In addition to landfill tax is the actual disposal cost which can vary from £8 to £20 / tonne according to the region of the country. Less compact materials and leachates or items containing leachable material eg preservative treated wood, are charged at higher rates.

Residue producer companies grouping together (networking) to collect and sort residues could be of some value but there are two problems: