On the use of Constructed Wetlands in mountain regions: innovative tools and configurations

The use of Constructed Wetlands (CWs) has been increasing over the last twenty years for decentralized wastewater treatment projects (e.g. rural communities, isolated houses, etc.) because of the low maintenance requirements and operational costs, efficiency in terms of organic matter, nitrogen and suspended solid removal. Nevertheless, the application of these systems in mountain areas is faced with some issues related to the specific characteristics of these areas, namely: the complex morphology with steep slopes and limited extensions of flat land, low temperatures and, in tourist contexts, population variations throughout the year. Limited availability of suitable land is a key issue for the application of a technology requiring considerable surfaces to produce effluents of good quality. Land area requirements constitute a well-known problem of CWs that is related to a lack of knowledge on the biological reactions occurring inside the bed. In fact, usually CWs are designed by considering simple first order decay models and specific surface area requirements, while the real requirements are not taken into account, leading most of the times to an overestimation of the area required. The limited knowledge on the processes and relative efficiencies of CW leads to overdesign of CW, mainly in low temperatures contexts and where there is a fluctuation on the resident population. Despite the efficiency that could be achieved through overestimation, those systems would be underutilized for a large part of the year. Ultimately, overestimated CWs consume more land than needed, eventually leading to the decision of switching to other systems. This research aims to identify approaches and configurations that may improve the applicability of CWs for wastewater treatment of mountain communities. These approaches try to overcome the cross-cutting issue of land area requirement, as well as those related to the variation of temperature and population through the year. This was done by exploring the use of respirometric techniques for the estimation of kinetic and stoichiometric reactions inside the bed and by testing, in a pilot plant, the influence of the tourist presence and low temperatures on the efficiency of innovative CW configurations. The research was developed at both the lab and the field scale. At the lab scale, two different tests were used in order to estimate the oxygen consumption in CW filter material: liquid respirometry and the off-gas technique. Liquid respirometry proved to be a reliable method when used to measure kinetic and stoichiometric parameters of the CW’s biomass. The off-gas technique was applied at the lab scale showing promising results, though further research is needed to improve the applicability of the method to CWs. Along with that, at the lab scale, a modified AUR method was applied on the CW material to quantify the nitrification rate of real systems at different temperatures and therefore to predict the removal efficiency throughout the year. At the field scale, several tests were performed in a pilot plant composed by two hybrid CWs (VSSF+HSSF). Among these: operation under continuous and discontinuous winter conditions, operation with overload during the summer (to simulate the presence of tourists) and the application of innovative configurations (Recirculated and Aerated VSSF). All these tests were designed with the purpose of dealing with the trade-off between the reduction of a CW’s land area requirement and the enhancement of its efficiency. Two innovative configurations were tested in the pilot plant: Recirculated VSSF CW and Aerated VSSF CW. Both configurations can provide saturated and unsaturated conditions, which allow the nitrification/denitrification inside the bed. During the period when experimental configurations were tested, the traditional VSSF CW was operated with an average specific surface area to 3.5 m2/PE, the Recirculated VSSF of 1.5 m2/PE and the Aerated VSSF of 1.9 m2/PE on average. The results showed that the CW’s surface can be considerably reduced without a significant reduction in the removal efficiency. The extra investment needed to equip VSSF CWs with aeration/recirculation would be compensated by a lower area requirement. This study explored some of the problems associated with the application of traditional CWs under the physical and social conditions that characterize mountain contexts, providing important information for future research and application. First of all, a reliable tool, the respirometric technique, was explored for the estimation of kinetic and stoichiometric parameters that will allow a more precise estimation of the land area required for these systems. Moreover, two innovative configurations (the use of recirculation and aeration in CWs) were proposed to be used where traditional configurations, though well designed, are still too large to be applied. Such configurations can also be used as a temporary solution to increase the treatment capacity during tourist peak seasons, while a traditional configuration is kept over the rest of the year. While this research focused on mountain environments, the configurations and results contained therein could be applied to a wide variety of settings where shortage of land or difficult climate conditions would exclude CWs from the list of wastewater treatment options available.