Salinity as a driver for microbial community structure in reactors for nitrification and anammox
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The removal of nitrogen from wastewater has become an important part of the overall treatment process due to the significant impact of nitrogen compounds (NH4–N, NOx–N) on the environment and the more stringent legislation on wastewater discharges. Wastewater streams with high nitrogen concentration are commonly produced by human activity. In addition of organic matter, those streams can also contain highly variable salt concentrations. Conventional microbial nitrogen removal generally involves aerobic nitrification and the subsequent anoxic denitrification. However, the anammox process may be more advantageous than the nitrification – denitrification process for nitrogen removal as it uses less oxygen, addition of external organic carbon is not required and less sludge is produced. To meet wastewater discharge criteria, biological wastewater treatment processes should function over a wide range of salt, which makes the wastewater treatment particularly challenging. Salinity is known to have toxic effects on bacteria and exerts selective pressures on the microbial community. Studies documenting the effects of salt on the nitrification and anammox bacteria are needed in order to optimize the nitrifying and anammox processes under elevated salinity and when salinity is variable. In this study, nitrification at different salinities was investigated in three moving bed biofilm reactors (MBBRs) inoculated with different nitrifying cultures originally from freshwater, brackish water (20‰) and seawater (32‰). Differences in the community structure of these microbial communities were documented by denaturing gradient gel electrophoresis (DGGE) and 454 pyrosequencing data, and statistical analysis. The study resulted in a comprehensive insight into the composition of the ammonia-oxidizing (AOB) and nitrite-oxidizing bacteria (NOB) in the three nitrifying communities. In a second set of nitrifying experiments, the freshwater and the seawater cultures used in our previous study were exposed to cross-transfer in salinity. Both long and short term effects on nitrifying activity were monitored and the community structure and salinity induced successions were evaluated by DGGE and deep sequencing of 16S rRNA gene amplicons, and statistical analysis. The results indicated that the NOB guild was more affected than the AOB guild when the freshwater culture was cross-transferred to a hypertonic (oceanic seawater) and the seawater culture to a hypotonic environment (freshwater). Rapid increase in salt concentration was highly destructive and caused poor nitrifying activity in the freshwater nitrifying biomass. However, the seawater culture showed a high resistance to low-salt stress, with large changes in species composition after the drop in salinity. This SR culture may be a suitable inoculum for N-removal from industrial wastewaters with variable salinity. A third study investigated the effects of low and high salinity on a freshwater anammox consortium in two up-flow anaerobic sludge bed (UASB) reactors. First the functionality of the unadapted consortium was tested with stepwise increase in salinity from zero to 3 gNaCl /L , and then with a further adaptation to high salinity (from 3 to 30 g/L). Salinity induced successions of the anammox communities were evaluated by 454-pyrosequencing of 16S rRNA gene amplicons and statistical analysis. The results indicated that the anammox activity was strongly inhibited at low salinity. Approx. 40 days were needed for adapting the preadapted anammox biomass from 3 to 30 g/L. A major change in the anammox community succession took place at 3 g/L, where the dominating population shifted from Ca. Brocadia fulgida to Ca. Kuenenia stuttgartiensis. The latter dominated at marine salinity and seemed to be essential to achieve a high N-removal efficiency. In this research we achieved knowledge to develop a fundamental understanding of microbial community dynamics in nitrifying and anammox reactors with variable salinity. This may be valuable information to understand and predict future changes in the community structure of nitrifying and anammox reactors. In addition, to provide important guidance in design, optimizing and stable operation for the nitrogen removal of saline wastewaters.