Quantitative response of nitrogen dynamic processes to functional gene abundances in a pond-ditch circulation system for rural wastewater treatment

Abstract

The pond-ditch circulation system (PDCS) is a promising solution for the restoration of rural wastewater. However, the underlying nitrogen removal mechanisms and roles of functional genes in nitrogen transformation processes have not yet been quantified at the molecular level. In this study, PDCSs have been found to have high removal efficiencies when the chemical oxygen demand (COD) and total nitrogen (TN) was high (>70%; 62.7–93.9%, respectively). Meanwhile, the presence of simultaneous nitrification, anaerobic ammonium oxidation (anammox), and denitrification processes were the primary nitrogen removal mechanisms in the PDCSs. According to the stepwise regression analysis and path analysis, two key functional gene groups, nosZ/(nirS + nirK) and (nxrA/amoA), were driving these major processes for TN and ammonia nitrogen (NH4+-N), respectively. Moreover, the key functional gene group narG/nxrA was affecting processes related to nitrate-nitrogen (NO3−-N) and nitrite-nitrogen (NO2−-N) conversion. Furthermore, results from the 15N isotope pairing technique suggested that for PDCSs, anammox was mainly responsible for nitrogen removal due to its high contribution to total N2 production (50.3%) in the ditch; denitrification, on the other hand, was primarily attributed to nitrogen removal, rather than the anammox process, in two ponds (41.3–43.1%).