Summary and conclusions

Nyttiggørelse af trykimprægneret træ

In Denmark, the main environmental problem associated with used impregnated wood is considered to be end disposal, since only small amounts of impregnated wood waste are generated during the manufacturing process. During its lifetime, impregnated wood will leach some heavy metals and organic solvents used in the impregnation process into soil and ground water but most of the heavy metals will remain in the wood until end disposal.

Impregnated wood waste contains a number of heavy metals. The heavy metals most widely used are copper, chromium and arsenic. The concentration of these heavy metals may vary significantly in the impregnated wood collected.

In the national Danish waste strategy ”Affaldsstrategi 2005-2008” (The Danish government 2003) it is assessed that since the 1960’s about 4 million tonnes of impregnated wood has accumulated in Denmark and that these amounts must be disposed of over the next 40 years. The amount of impregnated wood was estimated at about 50,000 tonnes in 2004 and is expected to rise to about 100,000 tonnes/year around 2010.

The goals in the national Danish waste strategy ”Affaldsstrategi 2005-2008” aim at utilizing the raw material and energy resources in the impregnated wood waste.

The main purpose of this project is to investigate whether the Watechs extraction process can be used to treat impregnated wood waste by selectively removing heavy metals from the impregnated wood waste.

Different wood waste fractions have been investigated for their metal content. The wood waste fractions investigated are:

new CCA (Copper, Chromium, Arsenic) impregnated wood waste
collected CCA impregnated wood waste from collection schemes
wood waste to be recycled in chipboard
wood waste from demolition of buildings
return wood chips for energy utilization (fuel)
wood fuel

Goals have been set for the quality of the treated wood waste from the Watech extraction process using the metal contents found in the different wood waste fractions. Two scenarios for utilization of the treated wood waste have been investigated:

Scenario 1: Reuse of the treated wood waste for chipboard production

Scenario 2: Utilization of the treated wood waste by energy recovery in a biomass fired combined heat and power plant

The expected metal content in collected wood waste from municipal collection schemes is shown in the table below. Also shown in this table are goals for metal contents in the treated wood waste from the extraction process in the two scenarios.

Substance	Metal content In collected wood waste (from ref.3)	Metal content Goal treated wood Scenario 1 Recycling	Metal content Goal treated wood Scenario 2 Energy recovery
	mg/kg DM^*	mg/kg DM^*	mg/kg DM^*
Copper	1.200	40	5
Chromium	850	40	5
Arsenic	750	20	5

Expected metal content in collected wood waste from municipal collection schemes and goals for metal contents in the treated wood waste for the two scenarios. Scenario 1 is reuse of treated wood chips in chipboard production. Scenario 2 includes spreading the ashes from energy utilization on agricultural soil. ^*DM=Dry Matter.

It has been investigated in detail both in laboratory scale and in pilot scale experiments whether the Watech extraction process (developed for the treatment of coke from WAPRO pyrolysis of PVC waste) could be further developed to treat wood waste impregnated with heavy metals.

Extraction of heavy metals Copper, Chromium and Arsenic from collected untreated impregnated wood waste was investigated. Also impregnated wood waste that had been pre-treated by heat treatment was investigated. The heat treatment of the wood waste gave no advantage in the extraction process.

It was found that the extraction of heavy metals could not be accelerated with wood waste that had been saturated with water prior to the extraction process and furthermore the use of ultra sound did not increase the extraction efficiency significantly.

It has been shown that by using a suitable combination of temperature, pH and residence time then the Watech extraction process can be used to treat CCA impregnated wood waste. The heavy metals extracted are concentrated in a heavy metal product. It is possible to extract copper selectively from the extraction liquid by ion exchange and hereafter precipitate only Chromium and Arsenic.

A number of extraction process principles have been investigated in detail and the best results were obtained by using a “stirred” process.

It has been found that when the Watech extraction process is performed as a four-stage counter current extraction including water leaching, then a treated wood waste product with CCA contents similar to the CCA content of wood waste categories “return wood chips” and “wood fuel” can be obtained. The treated wood waste has a lower CCA content than the “waste wood used in chipboard today. This is shown in the table below.

Substance	CCA content Watech process (treated wood)	CCA content Wood waste for chipboard production, ref.4	CCA content Wood waste from demolition (Burman 2005)	CCA content Return wood chips (fuel) (Burman 2005)
	mg/kg DM^*	mg/kg DM^*	mg/kg DM^*	mg/kg DM^*
Copper	20 ± 5	69	73	17
Chromium	31 ± 5	84	45	18
Arsenic	n.d. (<3 ?)	48	9	6
Chlorine	429 ± 4	-	-	1000

Concentrations of heavy metals in treated wood waste (Watech process) compared with wood waste for chipboard production, wood waste from demolition and return wood chips (fuel). ^*DM=Dry Matter.

A technical economic analysis has shown that a Watech extraction treatment plant is profitable and Scenario 1 ”reuse of treated wood in chipboard” is more attractive financially than Scenario 2 ”utilization of the treated wood waste by energy recovery in a biomass-fired combined heat and power plant”.

A financial overview showing income and expenses for a 25,000 tonnes/yr Danish Watech extraction plant in Scenario 1 ”reuse of treated wood in chipboard” is summarized in the table below.

Extraction treatment Financial overview (25.000 ton/yr) Scenario 1: Reuse in particle board	DKK/tonne
Income	1.235
Expenses excl. interest and depreciation	628
Profit before interest and depreciation	607
Profit before interest	478
Profit before tax	394

Financial overview. Extraction treatment 25,000 tonnes/yr. Scenario reuse in chipboard.

From this financial overview it can be calculated that the payback time for a 25,000 tonnes/yr Watech treatment plant in the Scenario 1 “reuse as chipboard” is about 4 years, which may be considered financially attractive. In this Scenario 1 profit before tax in % of the investment costs is about 28 %.

A financial overview showing income and expenses for a 25,000 tonnes/yr Danish Watech extraction plant in Scenario 2 ”energy recovery” is shown in the next table below.

Extraction treatment Financial overview (25,000 tonnes/yr) Scenario 2: Energy recovery	DKK/tonne
Income	1,235
Expenses excl. interest and depreciation	823
Profit before interest and depreciation	412
Profit before interest	283
Profit before tax	199

Financial overview. Extraction treatment 25,000 tonnes/yr. ”Scenario energy recovery”

From this financial overview it can be calculated that the payback time for a 25,000 tonnes/yr Watech treatment plant in the Scenario 2 “energy recovery” is about 7 years, which may be considered financially feasible. In this Scenario 1 profit before tax in % of the investment costs is about 14.2 %.

A significant improvement in the Watech extraction treatment plant economics can be obtained if recycled or used hydrochloric acid replaces the new technical hydrochloric acid used in the process. Recycled or used hydrochloric acid is produced in large quantities in the WAPRO process from treatment of PVC, shredder, or WEEE waste plastics. There are many synergy possibilities between the WAPRO process for halogenated plastic waste treatment and treatment of impregnated wood waste using the Watechs extraction process.

From an environmental analysis it can be concluded that treated wood waste from a Watech extraction treatment plant will have improved environmental characteristics in the form of less impact on the environment than the wood waste used today for both energy recovery and recycling in chipboard production.

The assumption that the Watech-treated wood waste can be utilized either by recycling in chipboard production or in energy utilization in a biomass-fired combined heat and power plant can therefore be confirmed.

In order to further improve the environmental characteristics of the Watech extraction treatment it may be considered to establish 3-4 smaller plants in different locations in Denmark. Distances to the treatment plants can be reduced by approx. 60-75%. This means less transport and a significant improvement in the environmental profile of the Watech treatment plant due to less CO₂ emission to the atmosphere.