How the subsurface wastewater infiltration systems with iron-carbon micro-electrolytic substrate achieve high pollutants removal: A metabolomic view

Being an ecological engineering water treatment technology, subsurface wastewater infiltration system (SWIS) combines ecological principles with anaerobic and aerobic processes. Iron-carbon micro-electrolysis (ICME) is a rapidly developing SWIS substrate type in recent years. In this study, microbial metabolomics was introduced into SWIS aiming to reveal the "black box" of enhanced nitrogen removal in SWIS assisted by ICME (namely ICMESWIS). Ultra performance liquid chromatography-mass spectrometry (UPLC-MS), principal component analysis (PCA), partial least squares discriminant analysis (PLS-DA) and univariate analysis were carried out to classify the samples from SWIS and ICMESWIS. The results showed that compared with SWIS, 28.5 +/- 2.6 %, 33.7 +/- 1.3 % and 19.3 +/- 0.4 % improvements were found for NO3--N, NH4+-N and NO2--N removals in ICMESWIS. According to the model's variable importance factor and fold change >2, metabolites with large differences were screened out. Metabolites, such as 1-(4-nitrophenyl)piperidine (ESI-) and 2-amino-1,3,4-octadecanetriol (ESI-) presented higher abundance in ICMESWIS, in consistent with the higher degradation of pollutants. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that nitrification, nitrate reduction, dissimilatory nitrate reduction, denitrification, anaerobic ammonia oxidation (anammox) are the main nitrogen removal pathways in ICMESWIS.