Greenhouse gas emission and denitrification kinetics of woodchip bioreactors treating onsite wastewater.

The accurate evaluation of denitrification rate and greenhouse gas (GHG) emission in field-scale woodchip bioreactors for onsite wastewater treatment are problematic due to inevitably varied environmental conditions and underestimated GHG production with limited analysis of dissolved gas in field samples. To address these problems, batch incubation experiments were conducted with controlled conditions to precisely evaluate the denitrification kinetics and NO and CH emission of both gaseous and dissolved phases in fresh (6 months) and aged (5 years) woodchip bioreactors treating onsite wastewater at high (1-3 mg L) and no (0 mg L) dissolved oxygen (DO) levels. NO removal rate decreased from 37.5-119.0 g NO-N md at no DO to 8.8-16.6 g NO-N md at high DO (1-3 mg L) due to the growth suppression of NO reducing microorganisms (37-55 % lower nirS+nirK abundance). However, the presence of high DO increased NO emission level from 5.6-6.9 mg NON m at no DO to 179.5-273.6 mg NON m) due to the enhanced growth of NO reducing microorganisms (1-7 times higher norB levels) and the decreased abundance of NO reducing microorganisms (53-75 % lower nosZ abundance). On the other hand, increased DO level negatively correlated with CH production (1.0-3.9 g CH-C md) in fresh woodchips, while showed insignificant impact on CH production (0.1-1.4 g CH-C md) in aged woodchips. Woodchip age increase (5 years) negatively impacted the NO removal rate (75-85 % lower than fresh woodchips) and CH production rate (>3 times lower than fresh woodchips), probably due to the reduced biomass density of NO reducing microorganisms (52-58 % lower nirS+nirK abundance) and methanogens (95-98 % lower mcrA levels). The incubation results suggested that long hydraulic retention time (>2-5 days) and anaerobic/anoxic condition are preferred for the optimal NO removal and low NO emission potential of woodchip bioreactors treating onsite wastewater.