Microbial communities and nitrogen cycling functions in barrier systems for treatment of nitrogen polluted water

Water is an increasingly threatened resource. Anthropogenic input of nitrogen (N) to surface waters causes eutrophication, and N in the groundwater can be a threat to human health, hence management measures are needed. The aim of this thesis was to evaluate two types of systems for remediating N-polluted water using permeable reactive barriers by determining N removal efficiency, possible adverse effects, and the role of the microorganisms in the systems. Four projects evaluated denitrifying barriers, DBRs, for treatment of nitratepolluted water originating from the use of explosives in mining industry. Woodchip DBRs were found to be suitable for remediation of this type of water. Initial leakage of nitrite, ammonium and organic carbon was observed, the emission of nitrous oxide was small. Another study used barriers based on compost or woodchips in combination with aquifer recharge to replenish groundwater using ammonium-rich treated wastewater. One out of four barriers significantly removed ammonium from the wastewater, and instead nitrate was produced. All compost-based barriers released organic carbon, replacing one pollutant with another. The carbonaceous material in the permeable reactive barriers determined the composition of the microbial communities in the barriers; significantly different microbiomes developed depending on substrate type. Besides microorganisms capable of a range of N-transforming reactions, all barriers harboured groups of bacteria known for degrading macromolecules like cellulose. There were temporal and spatial patterns in the distribution the microorganisms, and the abundances of specific bacterial groups correlated to the chemistry in the water. Future studies should focus on how different types of carbon affect the Ntransforming processes, to maximise N-removal and minimise adverse effects.