Assessing nitrogen transformation processes in a trickling filter under hydraulic loading rate constraints using nitrogen functional gene abundances

A study was conducted of treatment performance and nitrogen transformation processes in a trickling filter (TF) used to treat micro-polluted source water under variable hydraulic loading rates (HLRs), ranging from 1.0 to 3.0 m3/m2 d. The TF achieved high and stable COD (97.7–99.3%) and NH4+-N (67.3–92.7%) removal efficiencies. Nitrification and anaerobic ammonium oxidation were the dominant nitrogen removal processes in the TF. Path analysis indicated that amoA/anammox and amoA/(narG + napA) were the two key functional gene groups driving the major processes for NH4+-N and NO2−-N, respectively. The analysis also revealed that anammox/amoA and nxrA/(nirK + nirS) were the two key functional gene groups affecting processes associated with the NO3−-N transformation rate. The direct and indirect effect of functional gene groups further confirmed that nitrogen transformation processes are coupled at the molecular level, resulting in a mutual contribution to nitrogen removal in the TF.