Abstract
Constructed wetlands are commonly used for sewage treatment. However, as the natural processes operate, these artificial ecosystems can also be used to enhance the equalization of water features to those of the receiving environments, thus reducing the impacts of the treated water on the natural systems. Here, we studied, by a year-round survey, the simultaneous and separated operation of two subsurface wetlands that were used as a tertiary treatment to enhance the naturalization of wastewaters that had already been treated in a waste water treatment plant (WWTP). These wetlands were operating serially, with the first wetland being covered by the riparian plant Helosciadum nodiflorum, which has not been described so far as being used in treatment wetland, whereas the second was covered by Typha latifolia. The changes in the concentrations and transformation among the different types of pollutants and other physical and chemical parameters, as well as in the bacterial abundance and activity, were studied under different operational conditions of serial co-operation or of separately-operating wetlands. Both wetlands were differentially efficient in the reduction and transformation of the remaining pollutants, with very active nitrification and denitrification processes, which reduced the ammonium concentrations by more than 65%, although they changed according to the operational status of each wetland. They also reduced the already low organic matter contents by around 30% and promoted slight shifts in the dominant types of dissolved organic matter to less labile compounds. To a certain extent, the Typha-covered wetland also contributed to phosphorus removal, by up to 35%. Noticeably, both of the wetlands contributed greatly to the reduction of bacterial abundance, which was even 50% lower after the wetland transit, although the resulting community increased its activity, thus keeping the capacity for pollutant removal and transformation. Overall, the wetlands’ operation increased the similarity between the poured waters and those of the receiving stream, thus diminishing its environmental impact.
Highlights
Non-naturalized waters that originated from wastewater discharges, despite being treated in waste water treatment plants (WWTP) achieving the legislated quality standards, show quite different features to those of the natural environments where they are poured
Even when these pollutants could have been reduced on a conventional wastewater treatment plant (WWTP), the physical, chemical, and biological features of these waters might differ very much to those from the receiving aquatic ecosystems [22,23], which strengthens the need to consider the role of constructed wetlands, as systems helping to reduce the amount of pollutants, and, when these had already been reduced to acceptable levels, as driving these water features to others that are more similar to those of the receiving natural environment
Our work was conducted in a small constructed wetland that is annexed to a wastewater treatment plant (WWTP), the latter being efficient for pollutant removal. , the conventional WWTP pours water of very different characteristics to those that circulate through the environment, in this case, a Mediterranean stream where the natural flows are very low and, the effect of the treated waters on the environmental features could be strong
Summary
Non-naturalized waters that originated from wastewater discharges, despite being treated in waste water treatment plants (WWTP) achieving the legislated quality standards, show quite different features to those of the natural environments where they are poured This can cause a drop in the concentration of dissolved oxygen in the receiving environment, as well as promote eutrophication, the loss of biodiversity, and the release pathogenic microorganisms [1,2]. Limits in the concentrations of the pollutants that have been fixed by European Directive 91/271/EEC for WWTP effluents may be excessively high for some deficit basins, such as those in the Mediterranean, where the discharge may constitute an important proportion of the total volume of the receiving water bodies This problem, which has already been addressed by the European Commission, encumbers the achievement of the good ecological status of European water bodies that is required by the EU
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