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The treatment of wood leachate using constructed wetlands Frankowski, Kevin Anthony

Abstract

Rainfall percolating through wood chip piles, hog fuel, and log storage areas will leach naturally-occurring chemicals from the wood. This leachate is often characterised by high carbon content, strong colour, and high concentrations of tannin and lignin, resin acids and phenolics. This can be very toxic to aquatic life, and has serious implications from an environmental discharge viewpoint. It can lead to regulatory problems for facilities operators. Research was conducted in a series of phases at a cedar processing site near Mission, British Columbia. It was determined that cedar leachate was amenable to biological treatment. Chemical and ecotoxicological characterization was performed, and combined with the data obtained from treatment screening trials, laboratory-scale constructed wetlands were designed and tested. These constructed wetlands were able to achieve a 90% reduction in toxicity and removal rates of >90% for biochemical oxygen demand (BOD) and 80% for chemical oxygen demand (COD). A pilot-scale program demonstrated that constructed wetlands were able to treat the cedar leachate under field conditions. Initial data obtained during winter operations produced removal rates for toxicity on the order of 50%. BOD and COD removal was slightly less. The system still needs to undergo optimization, but preliminary results indicate several factors which may yield significant improvements in system performance. Thus, it has been demonstrated that a wetland-based biological treatment process is potentially a practical and cost-effective treatment technique for wood leachate. Unit treatment costs are much lower than other technologies; effective physio-chemical treatment techniques are 10 - 30 times more expensive, conventional biological treatments at least 6 times. Also, many of the conventional biological systems have failed to demonstrate effective treatment performance for this type of wastewater. In addition to its cost-effective performance, this constructed wetlands system operates in a passive manner. It requires minimal operational attention and infrastructure. No chemical addition or by-products handling is needed. Possible applications include the collection and treatment of wood leachate from chip piles, wood waste landfills, and hog fuel piles, and the treatment of wood extractives in stormwater runoff from log yards, dryland sorts and staging areas. This system is ideally suited for those applications which require effective, economic treatment of fluctuating runoff loads containing a broad, and often changing, spectrum of contaminants.

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