Treating timber waste in a constructive way

Chemically-treated timber waste is typically buried or landfilled, but studies into safer disposal methods have been carried out with positive results. Anurag Garg and Ibtisam Tothill explain

An alternative option for the safe disposal of treated timber waste from the construction and demolition industry has been identified by Cranfield University and the Centre for Resource Management (CRM).

Waste arising from the construction and demolition industry accounts for around one-third of the total waste produced in the UK. It is comprised of bricks, cement, stones and timber wastes. Timber waste contains biodegradable material along with chemical preservatives that are used to enhance wood durability and to increase its life expectancy - examples include creosote and chromated copper arsenate.

Current disposal practices carried out for chemically treated timbers include burial and landfilling, but the leaching of chemicals from the timber can deteriorate the soil quality, groundwater quality or surface water quality. One alternative is composting and biomediation processes - the study carried out by Cranfield and the CRM considered the use of these techniques to reduce the weight and toxicity of timber waste.

Scientific approach
The study was conducted at laboratory scale with creosote-treated timber. A 20 year-old shredded creosote-treated Scots Pine telegraph pole was used, mixed with shredded untreated Scots Pine. A sample of the untreated timber was also kept for composting and bioremediation. The study identified the typical temperature profile of a composting process. During this process, the temperature reaches mesophilic phase and continues to increase, resulting in a thermophilic phase. After stabilisation, it starts cooling and reaches the maturing phase.

The results showed that untreated timber attained a much higher temperature (70 °C) than creosote-treated wood (45 °C). This may be attributed to the smaller microbial population activities present in creosote-treated timber. The study also revealed that there was a reduction in the creosote level in all the vessels ranging from approximately 90% to 75%. One of the reasons for variability in the results may be the amount of creosote in each sample. This may be due to the heterogeneous distribution of creosote in wood sample.

Conclusions drawn
Some of the reduction in creosote could be attributed to volatilisation of creosote compounds from active aeration under the experimental conditions. However, this needs to be examined in future experiments by analysing air emissions. Characteristic temperature profile of composting indicates that the process is taking place. From temperature profiles, it can be concluded that untreated timber mix was hot composting while the creosote mix was a cold composting process.

There is also evidence of reduction in creosote content but how much of this is due to microbial degradation needs further analysis. However, the outlook for developing a process to treat the contaminated creosote wood waste is positive. Looking ahead, work is needed to examine the role of micro-organisms in the creosote degradation process. There is also a need to optimise the quantity of creosote-treated wood that can be used in the system to achieve optimal degradation.

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