Marais de saules à effluent nul pour le traitement d'eau contaminée

Au cours des dernières décennies, un type de marais filtrant, généralement planté de saules, a été développé pour opérer un système de traitement d’eau sans effluent, par évapotranspiration. Ces marais à effluent sont généralement utilisés pour le traitement d’eau usée domestique, mais pourraient aussi présenter une alternative intéressante pour d’autres applications, comme le traitement de lixiviat contaminé. Les guides de conception actuellement disponibles ne permettent toutefois pas de concevoir un système flexible qui permettrait de gérer les grandes variations de volume à traiter liées à la nature même des lixiviats (i.e. résultent entièrement des précipitations) et ne tiennent pas compte de la variation inter et intra-annuelle de l’évapotranspiration (ET) du saule. Cette thèse présente d’abord une revue de littérature de ET du genre Salix et de ses facteurs de variation les plus importants. Il apparaît que les conditions de croissance ont plus d’importance que l’identité spécifique et que la disponibilité en eau, la fertilisation et la contamination sont les principaux facteurs dictant l’ET des saules. L’effet de l’âge, du contexte expérimental, de la densité de plant et du type de sol n’a pas pu être clairement démontrés par cette revue. Ensuite, une étude portant sur le potentiel d’ET de S. miyabeana ‘SX67’ est présentée. Il est démontré qu’un modèle basé sur des paramètres foliaires et sur le déficit de pression de vapeur d’eau dans l’air permet de prédire l’ET de S. miyabeana en condition de marais filtrant. Cette étude permettra entre autres d’améliorer les plans de conception d’un éventuel marais de saules à effluent nul. Pour continuer, la réponse de S. miyabeana ‘SX67’ à différentes concentrations de lixiviat et à différents types de substrats a été étudiée. Ce cultivar s’est montré tolérant aux concentrations du lixiviat brut retrouvées sur un site d’entreposage de poteaux de bois traité. Le type de substrat a influencé la réponse du saule et ses performances écophysiologiques, en plus d’affecter la dynamique des contaminants. Finalement, la modélisation hydrologique d’un système à effluent nul par marais de saules permet de proposer une méthode de dimensionnement des différents compartiments du système pour atteindre un objectif d’effluent nul sur une période de 20 ans, ainsi que de proposer des solutions de conception et d’opération optimale. L’application du modèle au cas spécifique d’un site d’entreposage de poteaux de bois traité a permis d’évaluer la faisabilité, d’un point de vue hydrologique, de cette technologie dans le contexte climatique du sud du Québec. Sur la base de cette étude, la principale limite pour l’application des marais à effluent nul au Québec sont la surface de marais et le volume de stockage requis. Dans le cas où une étape de prétraitement efficace précède le marais de saule, la durée de vie du marais ne devrait pas être limitante et dépendra principalement de la durée de vie des végétaux. Cependant, le destin des contaminants dans le système, qu’il s’agisse de la disposition des contaminants accumulés à l’étape de prétraitement ou d’une éventuelle translocation de contaminants dans les parties aériennes des végétaux, devrait être considéré avant d’établir un système à effluent nul. Les résultats de cette recherche permettent, entre autres, de proposer les marais de saules à effluent nul comme une alternative intéressante pour le traitement d’eau contaminée au Québec.During the last decades, a type of constructed wetlands, usually planted with willows, was developed to operate a water treatment system with zero effluent, by evapotranspiration. These zero liquid discharge wetlands are typically used for domestic wastewater treatment, but could also be an attractive alternative for other applications, such as contaminated leachate treatment. However, the design guidelines currently available do not allow for the design of a flexible system that would manage the large variations of volume to be treated related to the very nature of leachates (i.e. produced entirely from precipitation) and do not take into account inter and intra-annual variation of willows evapotranspiration (ET). This thesis first presents a literature review of ET for the genus Salix and its most important driving factors. It appears that growing conditions are more important than species identity and that water availability, fertilization and contamination are the main factors dictating ET in willow. The effect of age, experimental context, planting density, and soil type could not be clearly demonstrated by this review. Then, a study on the potential ET of S. miyabeana 'SX67' is presented. It is shown that a model based on foliar parameters and on the water vapor pressure deficit in the air makes it possible to predict the ET of S. miyabeana under wetland conditions. This study will, among other things, improve the design plans for a potential zero effluent willow wetland. To continue, the response of S. miyabeana 'SX67' to different leachate concentrations and different types of substrates was studied. This cultivar has been tolerant of raw leachate concentrations found at a treated wood pole storage site. The type of substrate influenced the willow response and ecophysiological performance, and affected the dynamics of the contaminants. Finally, the hydrological modelling of a system with zero effluent by willow bed makes it possible to propose a method of dimensioning for the different compartments of the system in order to reach a zero effluent objective over a period of 20 years, as well as to propose solutions for optimal design and operation. The application of the model to the specific case of a treated wood pole storage site made it possible to assess the hydrological feasibility of this technology in the climate context of southern Quebec. On the basis of this study, the main limit for the application of zero effluent willow bed in Quebec is the wetland area and the storage volume required. In the case where an effective pre-treatment step precedes the willow bed, the life of the wetland should not be limiting and will depend mainly on the lifespan of the plants. However, the fate of the contaminants in the system, be it the disposition of the accumulated contaminants at the pre-treatment stage or a possible translocation of contaminants into the aerial parts of the plants, should be considered before establishing a system with zero effluent. The results of this research make it possible, among other things, to propose zero-effluent willow wetlands as an interesting alternative for the treatment of contaminated water in Quebec

Ecophysiological Responses of a Willow Cultivar (Salix miyabeana ‘SX67’) Irrigated with Treated Wood Leachate

As wood preservatives leach from exposed treated wood, they contaminate soil and water, creating an environmental problem that needs to be addressed. Treating this contamination is particularly challenging since it includes mixed compounds, such as heavy metals and trace elements, as well as xenobiotic organic pollutants like polychlorinated dibenzo-dioxin/furan congeners (PCDD/Fs) that are very toxic and are under very strict discharge regulations. Cultivating fast-growing willow shrubs, either in soil or in treatment wetlands, offers a flexible and inexpensive treatment option. The main objective of this study was to evaluate the tolerance of a frequently used willow cultivar (Salix miyabeana ‘SX67’) to irrigation with leachate contaminated with pentachlorophenol (PCP) and chromated chromium arsenate (CCA), two important wood preservatives. We designed a mesocosms experiment with willow grown in three different substrates and irrigated over 12 weeks with three different leachate concentrations. Willow proved to be tolerant to irrigation with the raw leachate, with only leaf area decreasing with increasing leachate concentration. However, the type of growing substrate influenced willow ecophysiological responses and overall performance, and seemed to affect contaminant dynamics in the plant-soil system. All contaminants accumulated in willow roots, and Cu and PCDD/Fs were also translocated to aerial parts. Overall, this study suggests that Salix miyabeana ‘SX67’ could be a good candidate for treating water or soil contaminated with wood preservatives

Evapotranspiration of a willow cultivar (Salix miyabeana SX67) grown in a full-scale treatment wetland

Since woody plants like willow are used increasingly in treatment wetlands, there is a growing need to characterize their ecophysiology in these specific growing conditions. For instance, evapotranspiration (ET) can be greatly increased in wetlands, due to factors like high water availability as well as oasis and clothesline effects. Few studies report willow ET rates measured in full-scale constructed wetland conditions, and fewer still in a temperate North-American climate. The objective of this study was to measure and model evapotranspiration of a commonly used willow cultivar, Salix miyabeana (SX67), to provide the ET rates and crop coefficient for this species. During two growing seasons, we studied a 48 m2 horizontal subsurface flow willow wetland located in eastern Canada, irrigated with pretreated wood preservative leachate. Over two seasons, from May to October, we measured a mean monthly evapotranspiration rate of 22.7 mm/day (16.5 mm/d modelled), for a seasonal cumulative ET of 3954 mm (2897 mm modelled) and a mean crop coefficient of 6.4 (4.2 modelled). Both the evapotranspiration results and leaf area index (LAI) were greater than most results reported for open field willow plantations. Maximal stomatal conductance () was higher than that expected for deciduous trees and even for wetland plants, and mean values correlated well with temperature, solar radiation, relative humidity and day of the year. We demonstrated that an ET model using , LAI and water vapor pressure deficit (VPD) as parameters could predict the evapotranspiration rate of our wetland. This simplification of traditional ET models illustrates the absence of evapotranspiration limitations in wetlands. Furthermore, this study also highlights some factors that can enhance ET in treatment wetlands. Our results should both improve the design of treatment wetlands using willows, and provide a simple ET predictive model based on major evapotranspiration drivers in wetlands

Willows for environmental projects: A literature review of results on evapotranspiration rate and its driving factors across the genus Salix

Willows are increasingly used for a wide range of environmental projects, including biomass production, leachate treatment, riparian buffers and treatment wetlands. Evapotranspiration (ET), assumed to be high for most willow species used in environmental projects, affects hydrological cycles and is of key interest for project managers working with willows. Here, we present a comprehensive review of ET rates provided in the literature for the genus Salix. We aim to summarize current knowledge of willow ET and analyze its variability depending on context. We compiled and analyzed data from 57 studies, covering 16 countries, 19 willow species and dozens of cultivars. We found a mean reported ET rate of 4.6 ± 4.2 mm/d, with minimum and maximum values of 0.7 and 22.7 mm/d respectively. Although results reported here varied significantly between some species, overall interspecific standard deviation (±3.6 mm/d) was similar to intraspecific variation (±3.3 mm/d) calculated for S. viminalis, suggesting a greater influence of the growing context on ET than species identity. In terms of environmental and management variables, water supply, fertilization and contamination were identified as driving factors of ET across willow species. Effects of root age, experimental context, planting density and soil type were more nuanced. Our findings provide synthetic data regarding willow ET. We encourage practitioners who use ET data from the literature to be aware of the main drivers of ET and to consider the influence of the experimental aspects of a study in order to interpret data accurately and improve project planning

Establishment and potential use of woody species in treatment wetlands

International audiencePlant species selection is an important criterion for improving treatment wetland performance. The aim of this work was to evaluate removal efficiency and potential uses of woody species in treatment wetlands during the establishment year. Plant development, removal efficiency and evapotranspiration rate of five woody species (Salix interior, Salix miyabeana, Sambucus canadensis, Myrica gale, Acer saccharinum) and four herbaceous taxa typically used in treatment wetlands (Typha angustifolia, Phragmites australis australis, Phragmites australis americanus, Phalaris arundinacea) were compared in a mesocosm-scale study during one growing season. Woody species showed significantly slower growth, but displayed several characteristics of interest for treatment wetland applications: good adaptation to wetlands conditions; high organic matter removal (76-88%); high nutrient accumulation in tissues and high evapotranspiration capacity. During the establishment year, herbaceous species showed greater biomass development (above- and belowground parts), higher evapotranspiration rate (>3.84 L m(-2) d(-1) compared to <3.23 L m(-2) d(-1) for woody species) and overall pollutant removal efficiency. These characteristics confirm the high efficiency of treatment wetlands planted with herbaceous species even in the first growing season. However, given their greater potential biomass development, woody species could represent an excellent alternative for improving treatment wetlands long-term performance

Optimization of the wastewater treatment capacity of a short rotation willow coppice vegetation filter

The objective of this study was to determine the conditions to optimize the wastewater treatment efficiency of a short rotation willow coppice (SRWC) plantation (Salix miyabeana ‘SX67’) used as a vegetation filter to treat small municipal primary effluents (with less than 800 population equivalent). With the aim of maximizing the annual amount of wastewater treated, the effect of adjusting the hydraulic loading rate (HLR) according to the estimated evapotranspiration was tested at demonstration scale under humid continental climate conditions. We proposed a new method to calculate the evapotranspiration rate from plant physiological data, introducing an α factor based on direct transpiration measurements. This method increased the accuracy of the water balance, with a prediction of the crop coefficient (kc) based on either an seasonal approach (R2 of 0.88) or a monthly approach (R2 of 0.94). This led to a more precise estimation of the pollutant loading reaching the groundwater and could be used after plantation establishment as a fine-tuning tool. Adjusting the HLR to that of evapotranspiration between May and October led to an annual increase of 2 mm/d (around 0.35 m3/m2 per growing season) in HLR, while maintaining a pollutant loading removal efficiency of at least 96% for organic matter, 99% for total phosphorus and 93% for total nitrogen. A high HLR at the end of the season caused nitrogen leaching into groundwater, indicating that the HLR should be decreased in October, when willow growth is greatly reduced

Macrophyte potential to treat leachate contaminated with wood preservatives: plant tolerance and bioaccumulation capacity

Pentachlorophenol and chromated copper arsenate (CCA) have been used worldwide as wood preservatives, but these compounds can toxify ecosystems when they leach into the soil and water. This study aimed to evaluate the capacity of four treatment wetland macrophytes, Phalaris arundinacea, Typha angustifolia, and two subspecies of Phragmites australis, to tolerate and treat leachates containing wood preservatives. The experiment was conducted using 96 plant pots in 12 tanks filled with three leachate concentrations compared to uncontaminated water. Biomass production and bioaccumulation were measured after 35 and 70 days of exposure. There were no significant effects of leachate contamination concentration on plant biomass for any species. No contaminants were detected in aboveground parts of the macrophytes, precluding their use for phytoextraction within the tested contamination levels. However, all species accumulated As and chlorinated phenols in belowground parts, and this accumulation was more prevalent under a more concentrated leachate. Up to 0.5 mg pentachlorophenol/kg (from 81 mu g/L in the leachate) and 50 mg As/kg (from 330 mu g/L in the leachate) were accumulated in the belowground biomass. Given their high productivity and tolerance to the contaminants, the tested macrophytes showed phytostabilization potential and could enhance the degradation of phenols from leachates contaminated with wood preservatives in treatment wetlands