Abstract: Typical unsteady unsaturated conditions can profoundly affect the hydrodynamics of vertical subsurface flow constructed wetlands (SVF-CW). In this study we analyze the hydrodynamics of a pilot plant with three different saturation conditions (fully saturated, partially saturated and draining). Tracer tests were performed with rhodamine WT, an organic fluorescent molecule, which has several advantages compared to other classical tracers. The performances of this tracer were very good, despite the mass recovery being only partial. Breakthrough curves were analyzed both by means of classical retention time distribution analysis and a numerical plug-flow model with dispersion. We found that the degree of global mixing in the SVF-CW increased as the water content increased, following an increase in the hydraulic retention time. Conversely, the degree of local mixing was inversely affected by water content; the dispersivity was 4.5, 10 and 14 cm for fully saturated, partially saturated and draining conditions, respectively. We explain the dependency of dispersivity on water content in physical terms, but further studies are needed to mathematically include this relationship in the numerical models that describe SVF-CW behaviour.

Abstract: Evapotranspiration is not a negligible term in the hydraulic balance of constructed wetlands. However soil and vegetative conditions in constructed wetlands are very different from agronomic conditions, so that experimental studies of evapotranspiration available for agronomic soils are not representative; dedicated studies are limited, not homogeneous and often contradictory. We experimentally evaluated evapotranspiration losses in horizontal and vertical subsurface flow constructed wetlands planted with Phragmites australis, comparing them with evaporation losses in unplanted vertical flow systems and with pan evaporation. Data were collected in a dedicated lysimetric apparatus expressly built in Pisa (centre Italy: 43°43’N, 10°25’E). Obtained results show that evapotranspiration losses could reach up to 50 % of the inflow for horizontal systems and about 20 % for vertical systems. As a consequence water reuse could be strongly limited. The reduction of the outflow is the main but not the only limitation. Therefore, when evapotranspiration rates too high to allow water reuse are expected, vertical subsurface flow constructed wetlands should be preferred to horizontal subsurface flow and free water flow systems.

Abstract: Constructed wetlands for sludge dewatering have been used for over 30 years. They appear to be more efficient then traditional sludge drying beds, with comparably low construction costs and lower operation costs. An extended literature review revealed that performances of reed bed systems are usually presented according to long-term evaluations of case studies. However, short-term water content analyses could be useful to better understand the dynamics of the dewatering process and hence to rationally describe the system, with the final goal of an overall improvement of the system itself. In this study we performed daily water content analyses in the interval between subsequent sludge loadings in a real scale reed bed system located in Pisa (centre Italy). Water content analyses were performed in different periods of the year to evaluate the varying contribution of evapotranspiration and for both planted and unplanted beds to assess the role of the plants in the short-term period. Afterwards a semi-empirical equation describing the dynamics of the dewatering process was proposed and calibrated. This equation is the basis for the technical and economical optimization of the resting time with reference to the required surface area and the drying efficiency.

Abstract: The paper presents the results of a double approach (pilot-scale tests and numerical evaluations) to evaluate the use of pure oxygen in the composting process compared with air and oxygen-enriched air in conic piles. The experiments on a conic pile demonstrated that pure oxygen allows a faster process with reduced odour emissions, but with some problems of oxygen consumption and temperature control that can be solved with an accurate control of the shape and porosity of piles and of the flow-rate and duration of the oxygen injections.

Abstract: Constructed wetland with vertical subsurface flow are effective natural wastewater treatment systems; they present a great potential, especially for onsite applications. Nevertheless rational design and management criteria are yet lacking; this could limit the diffusion of this kind of systems, while constructed wetland are in general gaining more and more success in Europe. This work thoroughly analysed the conceptual and experimental aspects of the vadose zone in this kind of systems; hydraulic, hydrologic and gas phenomena were studied, even with measurement techniques never tested before in constructed wetland with subsurface flow. We performed a dynamic analysis of water content in different layers of the system for various saturation conditions by means of capacitance technology; the hydraulic analysis was supported with hydrodynamic experiments testing Rodamina WT as a tracer. Evapotranspiration losses were measured and the role of plants and saturation conditions were pointed out. The original use of respirometric techniques led us to study the distribution and the activity of the attached biomass in the system. The experimental study was mainly performed in the pilot plant of Shafdan located in Tel Aviv (Israel) and in a lysimetric apparatus built in the area of CNR, Pisa. The results of this study permitted to improve the development and the calibration of a mathematical model (FITOVERT) for simulating constructed wetland with subsurface vertical flow. The model is the basis and the reference for rational design and effective management of this important kind of constructed wetlands.