Nathalie Korboulewsky

Purification processes involved in sludge treatment by a vertical flow wetland system: Focus on the role of the substrate and plants on N and P removal

Vertical-flow wetland systems were tested for treatment of liquid waste activated sludge with high content of organic compounds from a soft drink factory. A mesocosm experiment was carried out on planted and unplanted systems to understand the relative importance of substrate and plants in purification processes and to compare three species: Phragmites australis Cav., Typha latifolia L., or Iris pseudacorus L. All planted mesocosms performed better than unplanted mesocosms and Phragmites showed the highest efficiencies, both in volumes and loads, closely followed by Typha. Removal efficiencies were very high in all cases, and physical filtration by the organic substrate was identified as the main processes for nutrient removal (>50%). We showed that plants played direct and indirect roles such as nutrients uptake (up to 23% of the N for Phragmites), evapotranspiration reducing outflow volumes; or improvement of filtration by the root systems and stimulation of microbial activities (respiration rate was double compared to unplanted mesocosms).

Influence of plants on microbial activity in a vertical-downflow wetland system treating waste activated sludge with high organic matter concentrations

The rhizosphere is a key zone for pollutant removal in treatment wetlands; therefore, studies on microbial activity may provide helpful information for a better understanding of purification processes. We studied microbial activity in a vertical-downflow constructed wetland system treating waste activated sludge with high organic matter concentrations, under Mediterranean climate. The aims of the work were to study the influence of (i) the presence of plants, (ii) the plant species (Phragmites australis Cav., Typha latifolia L., Iris pseudacorus L.), and (iii) the plant growth stage (plant senescence and plant fast growing stage) on total respiration rate and phosphatase activity in the substrate (intented here as the solid support on which the plants grow). The presence of plants had a positive influence on microbial activity, since substrate respiration and both acid and alkaline phosphatase activity were always higher in planted than in unplanted mesocosms. Among the three tested species, Phragmites was the one that most stimulated both substrate respiration rate and phosphatase activity, followed by Typha and Iris. These differences of microbial activity between mesocosms were corresponding to differences of removal efficiency. Substrate respiration and phosphatase activity were of similar magnitude at the two growth stages, while the stimulating effect of plants seemed to have been delayed and microbial activity showed higher fluctuations at plant fast growing stage than at plant senescence.

Feasibility of using an organic substrate in a wetland system treating sewage sludge: Impact of plant species.

A vertical-flow wetland system was tested for treatment of liquid sludge with high organic concentrations using an organic substrate (peat/crushed pine bark, 1/1) as growing medium. Mesocosms (1 m(3)) were planted with either Phragmites australis Cav., Typha latifolia L., or Iris pseudacorus L. The aim of the work was to determine the feasibility of using an organic substrate in treatment wetlands, through the study of its temporal patterns and of its impact on the water output quality. Results confirmed that the organic substrate can be used in such wetlands treating highly organic sludge, without clogging phenomena for the experimental period. The organic substrate released soluble organic matter but few mineral elements. Over the experimental period, substrate TOC concentration did not change while N concentration increased. Plants showed positive impact on substrate temporal patterns and also on the outflow water quality. Overall, Phragmites seemed to be more beneficial than Typha and Iris.

Depollution potential of three macrophytes: exudated, wall-bound and intracellular peroxidase activities plus intracellular phenol concentrations.

The aim of this study was to investigate the potential role of three macrophyte species (Iris pseudacorus, Typha latifolia and Phragmites australis) for detoxication of xenobiotics, and to study their variations with seasons or concentrations of sewage sludge from the food industry. For this purpose, some aspects of the green liver concept were explored through peroxidase measurements in three compartments in roots: intracellular, cell wall and extracellular. In addition, phenol concentrations were also measured in order to assess heavy metal detoxication potential. Enzyme activities and phenol concentrations were overall lower in winter according to the phenological stages and some sludge effects occurred. Results show that P. australis roots exuded and contained more peroxidase in all seasons: 17 U/g (1373 U/g protein), 0.8 U/g (613 U/g protein) and 4.8 U/g (1329 U/g protein) in intracellular compartments, cell wall and exudates, respectively. In contrast, the highest phenol concentration was found in I. pseudacorus roots: 3.58 mg eq. [corrected] gallic acid/g. Hence, in constructed wetlands, P. australis is suitable for organic waste water treatment, while I. pseudacorus should be used in the case of waters highly charged with heavy metals.

The role of pebbles in the water dynamics of a stony soil cultivated with young poplars

Background and aimsStony soils are widespread and often support plant production; nevertheless, their physical properties remain poorly understood, particularly regarding the role of rock fragments in plant water uptake. Understanding the hydric interactions between rock fragments and fine earth in stony soils remains central to water management in the context of climate change.MethodsA soil water retention curve of stony soil was developed based on a water retention curve for both rock fragments and fine earth. Water transfer between fine earth, rock fragment and plant roots was monitored during experiments under controlled evaporation conditions.ResultsDuring desiccation, the water retention curves for fine earth and rock fragments exhibited water movement from the rock fragment to the fine earth. During our evaporation experiment, fine earth water content decreased as soon as desiccation started, whereas the rock fragments released water after several days. We demonstrated that rock fragments are a water reservoir for plants; plants can uptake water directly from pebbles or after it is transferred into surrounding fine earth. However, this transfer could not be measured due to the non-equilibrium state between the two phases. Furthermore, we exhibited how the water potential dynamics in fine earth containers cultivated with poplar was not influenced by the rock fragments content.ConclusionWater in rock fragments can be directly released to plants or released through fine earth, thereby reducing plant water stress during moderate drought periods. Therefore, the assumption that rock fragments constitute an inert phase inhibiting ecological soil functions must be completely reconsidered.

Experimental assessment of the water quality influence on the phosphorus uptake of an invasive aquatic plant: biological responses throughout its phenological stage.

Understanding how an invasive plant can colonize a large range of environments is still a great challenge in freshwater ecology. For the first time, we assessed the relative importance of four factors on the phosphorus uptake and growth of an invasive macrophyte Elodea nuttallii (Planch.) St. John. This study provided data on its phenotypic plasticity, which is frequently suggested as an important mechanism but remains poorly investigated. The phosphorus uptake of two Elodea nuttallii subpopulations was experimentally studied under contrasting environmental conditions. Plants were sampled in the Rhine floodplain and in the Northern Vosges mountains, and then maintained in aquaria in hard (Rhine) or soft (Vosges) water. Under these conditions, we tested the influence of two trophic states (eutrophic state, 100 μg.l−1 P-PO4 3− and hypertrophic state, 300 μg.l−1 P-PO4 3−) on the P metabolism of plant subpopulations collected at three seasons (winter, spring and summer). Elodea nuttallii was able to absorb high levels of phosphorus through its shoots and enhance its phosphorus uptake, continually, after an increase of the resource availability (hypertrophic > eutrophic). The lowest efficiency in nutrient use was observed in winter, whereas the highest was recorded in spring, what revealed thus a storage strategy which can be beneficial to new shoots. This experiment provided evidence that generally, the water trophic state is the main factor governing P uptake, and the mineral status (softwater > hardwater) of the stream water is the second main factor. The phenological stage appeared to be a confounding factor to P level in water. Nonetheless, phenology played a role in P turnover in the plant. Finally, phenotypic plasticity allows both subpopulations to adapt to a changing environment.