Search

Green cooling towers, or thermal green-walls, recirculate hot water flowing through a living plant and porous material. Through convective and evaporative heat and mass transfer they can actively cool a building or an industrial heat waste. Thus they can provide an alternative cooling technology. Thermal green-walls use wetland plant species in highly constructed wetlands selected for their heat tolerance. In addition, they offer such co-benefits as carbon capture and control of local microclimates. Optimal heat rejection (HR) requires the plant rhizosphere to be completely inundated at temperatures much higher than the ambient air temperature, potentially inducing thermal physiological stress. To study the viability of specific plants within a HR system, we exposed three macrophytes, Iris versicolor, Scirpus cyperinus and Carex lurida, to hydroponic recirculating water at three different temperatures (25 °C, 35 °C, and 40 °C). We assessed the plant responses over a period of eight weeks by measuring photochemical efficiency, leaf temperature, stomatal conductance, G s and biomass output. Plant biomass was highest at 35 °C (particularly in I. versicolor), even higher than ambient water temperature at 25 °C. We observed a general homeothermic-like response in G s over a 5-week time period, but the response was species-specific with respect to temperature. Results suggests a possible leaf-level physiological acclimation mechanism as well as high maintenance of tissue moisture that allows macrophytes to tolerate hydro-thermal stress, and support their suitability to perform in HR systems at input temperature, T win = 35 °C.

Chlorantraniliprole (CAP), an emergent insecticide commonly replacing banned neonicotinoids, is used worldwide despite the risk of contaminating water bodies. Treatment wetlands (TWs) have shown great potential for mitigating various pesticides in agricultural runoff, but little is known about CAP removal. The aim of this study was to determine the effect of adding biochar to subsurface flow treatment wetlands (SSF TWs) and the performance of three macrophyte species (Phragmites australis subsp. americanus, Scirpus cyperinus and Sporobolus michauxianus) in CAP removal. Removal efficiency was monitored over a one-month period in water-saturated SSF mesocosms fed with synthetic agricultural runoff containing CAP. To reflect temporal changes in agricultural runoff dynamics, two CAP concentrations were used in influent: a peak concentration (4 μg/L) for the first week and a trace concentration (0.4 μg/L) for the three subsequent weeks. Results showed that mesocosms with biochar were very effective in removing CAP mass (90 to 99%) and remained so throughout the experimental period. On the other hand, the level of CAP removal achieved in planted mesocosms without biochar was low (less than 13%). Evapotranspiration contributed significantly to volume reduction, but no general pattern in CAP mass removal efficiency was detected among the planted treatments without biochar. However, planted treatments acted as buffer zones, accumulating CAP and reducing its peak mass in effluent. Evapotranspiration rates of Scirpus and Phragmites were higher than that of Sporobolus, resulting in a greater buffering effect. This study suggests that addition of biochar to SSF TW substrate is a promising approach for CAP mitigation in agricultural runoff, but long-term efficiency remains to be assessed.

Metal accumulations in sediments and plants of constructed and natural wetlands were compared in two wetlands constructed by the Tennessee Valley Authority (TVA) for the treatment of acid mine drainage and a natural wetland. Load rates and removal efficiencies of most metals were generally greater in the constructed wetlands than in the natural wetland. There were similar sediment and plant metal concentrations between one constructed wetland and the natural wetland and greater metal concentrations in the sediments and plants in the other constructed wetland compared to the natural wetland. Data indicate that Mn, Zn, Cu, Ni, B, and Cr are being accumulated in the plants at all three wetlands, although accumulation of metals by these plants accounts for only a small percentage of the removal of the annual metal load supplied to each wetland.