Microbial nitrogen transformations in constructed wetlands treating contaminated groundwater

Cottbus, Techn. Univ., Diss., 2015

Groundwater is the main source for potable water and domestic use in numerous countries around the world. However, water quality can be affected by pollution, which influences the natural environment and human health. One of the widespread pollutants in water is ammonia which is toxic to fish and causes eutrophication of lakes and wetlands. Constructed wetlands are promising in situ water treatment methods thanks to enhanced microbial growth within the plants’ rhizospheres, which creates an effective contaminant degradation zone. The ammonia in constructed wetlands can be removed either via total nitrification with further denitrification or partial nitrification coupled with anaerobic ammonium oxidation (anammox). However, so far, the role of anammox in constructed wetlands as well as its correlation with other nitrogen transformations remains unclear. The quantification of nitrogen turnover processes in constructed wetlands is difficult due to the complexity of the wetland systems. Accordingly, the main aim of this research is to investigate pathways of ammonium removal in constructed wetlands treating contaminated groundwater. For this, several approaches were applied: a) physico-chemical parameters measurements; b) investigations of nitrogen stable isotope fractionation; c) stable isotope labelling approach; d) molecular biological methods. Also, seasonal and spatial variations in nitrogen transformations in several types of constructed wetlands (unplanted horizontal subsurface flow, planted horizontal subsurface flow, and floating plant root mat) were investigated. - The application of the stable isotope approach combined with common physico-chemical investigations enabled us to identify key factors influencing efficiency of nitrogen removal in constructed wetlands, which was the plant presence as substrate for attachment and growth of microorganisms. Ammonium removal efficiencies were not different between investigated seasons (spring, summer, and autumn), what could be explained by the fact that throughout all investigated seasons the air temperature remained high (above 10°C). While plant uptake accounted for significant part of ammonium removal during spring and summer in planted constructed wetlands, isotope fractionation patterns revealed that nitrification-denitrification were prevailing processes in planted constructed wetlands throughout the year, occurring in a linear way along the flow path, and not depending on depth in the root zone. - The research results also illustrated that in the planted horizontal subsurface flow constructed wetland, the functional genes of the nitrogen cycle were evenly distributed in a linear way along the flow path with prevalence at the superficial points. The same trend was observed for the nitrification and denitrification turnover rates using the isotope labeling techniques. Significant nitrate consumption under aerobic conditions diminishes nitrification rates and should therefore be taken into account when estimating nitrification turnover rates. This nitrate consumption was due to aerobic denitrification, the rate of which was comparable to that for anaerobic denitrification. Consequently, denitrification should not be considered as an exclusively anaerobic process. Phylogenetic analysis of hydrazine synthase (hzsA) gene clones indicated the presence of Brocadia and Kuenenia anammox species in the constructed wetland. Although anammox bacteria were detected by molecular methods, anammox activity could not be measured and hence this process appears to be of low importance in nitrogen transformations in these freshwater ecosystems. In conclusion, this research demonstrated that combination of physico-chemical measurements with stable isotope and molecular biological approaches is an effective tool for investigation of nitrogen transforming processes in constructed wetlands. Such information is not only valuable for understanding of the processes ongoing inside these wastewater treatment facilities but also necessary for further technological improvement of constructed wetlands.