Woodchip bioreactors bioaugmentation
Woodchip bioreactors (WBRs) are constructed wetlands built to prevent excess nitrogen leaching from agricultural fields to aquatic ecosystems. Nitrate removal is accomplished by biological denitrification, but the process slows down at cold temperatures, where nitrogen removal in WBRs may be less than 20%. We investigated the use of bacterial bioaugmentation in replicated (n = 3) pilot-scale WBRs bioreactors (94 dm3) as a technology to enhance nitrate removal at cold temperatures. Nitrate removal rates increased after the injection of a nitrate-reducing inoculum (Pseudomonas sp. and Klebsiella sp.), but the effect was transient and faded within one week. A notable shift in bacterial community composition after bioaugmentation was observed in the planktonic phase of the woodchip reactors, but not in the solid phase (woodchip matrix). Thus, only in the planktonic phase, Pseudomonas and Klebsiella increased in their relative abundance of 16S rRNA gene sequences. Analysis of metagenomic sequences supported these results with an increase of genes related to nitrate transformation following bacterial injection. After about one week, microbial composition came back to its initial state and indicated that the microbial communities in the woodchip bioreactors are resilient. We conclude that bioaugmentation has the potential to increase nitrate removal at cold temperatures, but an improved inoculation method should be developed to make bioaugmentation a viable technology for practical use.