Enhanced denitrification efficiency and utilization characteristics of released carbon from low-lignin woody biomass.

Woody biomass is a popular natural carbon source for eliminating nitrate when used in denitrifying bioreactors, exhibiting a slow denitrification rate due to its high lignin content and complexity of released carbon products. Herein, woodchips with lower lignin contents were placed in denitrification columns for 485 days, followed by sampling to simulate the carbon-release and denitrification utilization steps. The results showed that low-lignin woodchips could maintain a high denitrification efficiency for 485 days. The average rate of nitrate removal from low-lignin woodchips increased 2.5-6.0 times compared with that of natural woodchips, where the rate was 11.6 mg N/(d·L) during days 297-351 at 20-25 °C. Dissolved organic carbon (DOC), nitrite, and ammonium content in the effluent increased with gradual decrease in lignin content of woodchips. Low-lignin woodchips showed significant decrease in extractives, hemicellulose, and volume density. The lifespan of low-lignin woodchips was less than half that of natural woodchip. Denitrification of low-lignin woodchips led to weakening of specific infrared peaks, including peaks attributed to C=O, C-O, and C=C bonds. Around 36.8 %-38.0 % of released DOC from low-lignin woodchips was consumed during denitrification, while only 24.5 % of released DOC from natural woodchips was consumed. Both the volatile fatty acid and reducing sugar contents from the woodchips were exhausted, while most total phenols and humic acid-like substances remained after denitrification. For application, removal of lignin from woodchips could markedly improve the nitrate removal efficiency when used in a bioreactor, and control of lignin removal could be used to decrease the concentrations of by-products such as residual DOC and ammonium. The residual DOC could be further treated as well with chemical coagulation, absorption, or biological oxidation. The excess nitrite produced with the low-lignin woodchips would be eliminated through the extended hydraulic retention time in the woodchip bioreactor.