The global significance of abiotic factors affecting nitrate removal in woodchip bioreactors.

Woodchip bioreactor (WBR) is one of the green infrastructures in the agriculture system to reduce nitrate from agricultural drainage and stormwater. A lot of abiotic factors have been reported that affect nitrate removal lacking a comprehensive understanding. In this study, we studied eight important abiotic factors, including media age, hydraulic retention time (HRT), influent nitrate concentration (Cin), temperature, dissolved organic carbon (DOC), dissolved oxygen (DO), pH, and effective porosity (ρe) of WBR-filling materials. Based on a database that included 10,179 sets of data from 63 peer-reviewed articles, the nitrate removal rate (NRR) and nitrate removal efficiency (NRE) corresponding to the eight abiotic factors by different categories were comprehensively reported. According to this database, this study found the optimal range of abiotic factors for NRR and NRE in WBR were different. Regarding NRR, the optimal range of media age, HRT, Cin, temperature, effluent DOC, DO, pH, and ρe were in the first year, 0-5 h, 10-20 mg L-1, 20-25 °C, 0-5 mg L-1, 0-0.5 mg L-1, 7-8, and 0.6-0.7, respectively. For NRE, the optimal range of media age, HRT, Cin, temperature, effluent DOC, DO, pH, and ρe were in the first year, 500-3000 h, 0-10 mg L-1, 10-15 °C, >50 mg L-1, 0-0.5 mg L-1, 4-5, and 0.4-0.5, respectively. Moreover, the principal component analysis (PCA) indicated the field studies' principal components were different from laboratory studies. Furthermore, the structural equation modeling (SEM) also revealed the causal relationship of the eight abiotic factors on NRR and NRE is totally different. Lessons from this study can be incorporated into DNBR designs, especially improving nitrate removal rates by optimizing different abiotic factors. It also provides insights regarding the contributions of different abiotic factors for NRR and NRE independently and comprehensively.