Abstract
Denitrifying woodchip bioreactors are potential low-cost technologies for the removal of nitrate (NO3 - ) in water through denitrification. However, if environmental conditions do not support microbial communities performing complete denitrification, other N transformation processes will occur, resulting in the export of nitrite (NO2 - ), nitrous oxide (N2 O), or ammonium (NH4 + ). To identify the factors controlling the relative accumulation of NO2 - , N2 O, and/or NH4 + in denitrifying woodchip bioreactors, porewater samples were collected over two operational years from a denitrifying woodchip bioreactor designed for removing NO3 - from mine water. Woodchip samples were collected at the end of the operational period. Changes in the abundances of functional genes involved in denitrification, N2 O reduction, and dissimilatory NO3 - reduction to NH4 + were correlated with porewater chemistry and temperature. Temporal changes in the abundance of the denitrification gene nirS were significantly correlated with increases in porewater N2 O concentrations and indicated the preferential selection of incomplete denitrifying pathways ending with N2 O. Temperature and the total organic carbon/NO3 - ratio were strongly correlated with NH4 + concentrations and inversely correlated with the ratio between denitrification genes and the genes indicative of ammonification (Σnir/nrfA), suggesting an environmental control on NO3 - transformations. Overall, our results for a denitrifying woodchip bioreactor operated at hydraulic residence times of 1.0-2.6d demonstrate the temporal development in the microbial community and indicate an increased potential for N2 O emissions with time from the denitrifying woodchip bioreactor.
Highlights
IntroductionDenitrifying fixed-bed bioreactors are low-cost technologies for the removal of nitrate (NO3−) from water, which passes
Denitrifying fixed-bed bioreactors are low-cost technologies for the removal of nitrate (NO3−) from water, which passesAbbreviations: DNRA, dissimilatory NO3− reduction to ammonium; DWB, denitrifying woodchip bioreactor; HRT, hydraulic residence time; Nonmetric multidimensional scaling (NMDS), nonmetric multidimensional scaling; PCR, polymerase chain reaction; qPCR, quantitative polymerase chain reaction; TOC, total organic carbon.through an organic porous material, supplying electrons for the reduction of NO3− to nitrogen gas (N2) (Schipper, Robertson, Gold, Jaynes, & Cameron, 2010)
Differences in the abundances of all functional genes and the 16S rRNA genes between porewater, inlet water, woodchips, and sewage sludge were tested using Dunn’s test, which is appropriate for groups with unequal numbers of observations (Zar, 2010), in R package ‘FSA’ (Dinno, 2017), which performs a Kruskal–Wallis test followed by pairwise comparisons
Summary
Denitrifying fixed-bed bioreactors are low-cost technologies for the removal of nitrate (NO3−) from water, which passes. Through an organic porous material, supplying electrons for the reduction of NO3− to nitrogen gas (N2) (Schipper, Robertson, Gold, Jaynes, & Cameron, 2010). Woodchips are typically used due to their high permeability, moderate reactivity, and capability of providing a carbon (C) and energy source for denitrification (Cameron & Schipper, 2010; Robertson, 2010; Schipper et al, 2010). The release of other nitrogen (N) species (nitrite [NO2−], nitric oxide [NO], and the greenhouse gas nitrous oxide [N2O]) from intermediate steps during denitrification is a potential drawback of.
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