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Willow constructed wetlands to treat leachate from wood pole yards
J. Vincent
1,2
, J. Brisson
1
, Y. Comeau
2
1
Institut de Recherche en Biologie Végétale, Université de Montréal. 4101 East, Sherbrooke St,
Montreal, Quebec, H1X 2B2 CANADA.
2
Polytechnique Montreal 2900, Édouard-Montpetit boulevard, Montreal, Quebec, H3T 1J4 CANADA
Introduction
Storage yards of wood poles treated by preservatives such as pentachlorophenol (PCP) and
chromated copper arsenate (CCA) are exposed to weather conditions and leach organic and
inorganic pollutants. Of these pollutants, the most critical are highly chlorinated polychlorinated
dibenzo-p-dioxins and furans (D&F), PCP, copper, chromium and arsenic. D&F are present as
impurities of PCP in wood preservatives and are recognized as the most toxic and persistent
chemicals in the environment. As conventional leachate treatment technologies (e.g. advance
oxidation or thermal processes) are energy consuming, expensive and not efficient for metals
removal, there is a need for cost-effective, semi-passive and reliable technologies to remove
wood preservatives pollution. Phytotechnologies, and more specifically constructed wetlands
(CW), have been recently reported to having a promising potential in this respect.
This study is part of a research program that aims at evaluating the treatment capacity of
phytotechnologies to treat such leachate. The treatment train considered consists of a sequence of
2 horizontal subsurface flow constructed wetlands (HSSF CWs); the first aims to treat raw
leachate, its effluent is then directed to the second HSSF CW planted with willows (i.e. willow-
CW) to enhance treatment and evapotranspiration. In the HSFF-CW, pollutants are either
retained (i.e. metals) or degraded (i.e. organics) due to oxygenation conditions (i.e. aerobic and
anaerobic) types of media added to the HSFF-CW (i.e. zero valent iron and steel slag), based on
physical (e.g. adsorption), chemical (e.g. dechlorination) and biological (e.g. biosorption,
biodegradation) reactions. Then, the willow-CW provides a polishing step by further treatment
and evapotranspiration to meet storm sewer discharge criteria. In addition to treating purpose,
willows present an interesting potential for biomass valorization (Gregersen and Brix, 2001).
This work focuses on the evaluation of the performance of the willow-CW considering; (i) the
pollutant (PCP, CCA, D&F) removal efficiencies, (ii) the evapotranspiration ability of willows,
and (iii) the willows growth and health, using pilot-scale CW unit planted with
Salix miyabeana
.
Methods
Experimental site:
The experiment was carried out at a wood pole yard site in Quebec, where
pilot scale units of four HSSF CW were built to assess different design efficiencies for pollutant
removals and one willow-CW was built for polishing and evapotranspiration purposes (Figure 1).
All pilot units were operated since the spring of 2012. The size of the four HSSF CW units varied
from 7 to 20 m
2
, while that of the willow-CW was 48 m
2
(8 m x 6 m) with a depth of 1 m.
Operating conditions:
The four HSSF CW were fed in parallel with the raw leachate (from the
pole yard) at a flow rate of 0.5 m
3
/d. Their effluents were centralized at the point 12 (Figure 1)
