Microbial nitrate removal from mining waters

Mining activities cause high levels of nitrogen in the process water. The nitrogen originates from the blasting procedure, where undetonated explosives in the form of ammonium nitrate dissolve in the water. Part of the water is discharged to receiving water bodies where it may cause environmental problems. To meet current regulations, the nitrate concentration in the discharge water needs to be reduced. Two nitrate removal systems were studied in the thesis, denitrifying bioreactors and pond sediments. The overall aim was to identify and investigate factors affecting microbial nitrate removal in the systems. This was done by analysing the microbial communities in the systems using molecular methods and biochemically by measuring denitrification rates. We reported for the first time a pilot-scale denitrifying bioreactor treating mining water. The reactor efficiently removed nitrate after addition of external carbon. There were indications on preferential flow paths in the reactor, hence probably only a part of the substrate volume contributed to the removal. In a laboratory experiment we tested different reactor substrates; barley straw and Carex rostrata supported higher nitrate removal rates than woodchips did. Initially, there was an increase in bacterial alpha-diversity in all reactor types and when the bacterial community stabilised, it was reflected in more stable nitrate removal rates, most obvious in the woodchip reactors. All three substrates developed distinct bacterial communities. The denitrification rates in pond sediments from the LKAB mining site in Kiruna, Sweden were limited by organic carbon availability. A microcosm experiment showed that treating the sediment with carbon for a period can increase the rates. However, the choice of carbon type impacts the metabolic pathways in the system and adverse effects in the form of methane production and accumulation of ammonium were observed in the treatment with carbon in the form of algae. To conclude, constructed systems where denitrification takes place can successfully remove nitrate from mining waters. Organic carbon quality and availability is a crucial factor for the removal efficiency and for determining the composition of the microbial communities performing the reduction of nitrate