Sulfur and iron cycles promoted nitrogen and phosphorus removal in electrochemically assisted vertical flow constructed wetland treating wastewater treatment plant effluent with high S/N ratio

Phosphate (PO43--P) and nitrate (NO3--N) in the effluent of wastewater treatment plants are the predominant sources of eutrophication. In this study, a bench-scale electrochemically assisted vertical flow constructed wetland (E-VFCW) was developed, which exhibited favorable PO43--P (89.7-99.4%), NO3--N (82.7-99.6%), and TN (51.9-93.7%) removal efficiency in tertiary wastewater treatment. In addition, little N2O accumulation (0.32-2.19% of Delta NO3--N) was observed. The study further elucidated that PO43--P was removed mainly in the anode chamber by co-precipitation (Fe(n+)OH-PO4) and adsorption (FeOOH-PO4) pathways. Multi-pathway of NO3--N reduction was proposed, with 13.9-30.2% of NO3--N predominantly eliminated in the anode chamber by ferrous-dependent NO3--N reduction bacteria. In the cathode chamber, electrons storage and resupply modes during S cycle exerted crucial roles in NO3--N reduction, which enhanced the resilience capabilities of the E-VFCW to shock loadings. Stoichiometric analysis revealed that 3.3-6.6 mmol e(-)/cycle were stored in the form of S-0, FeS, and FeS2 in the E-VFCW under longer HRT or higher current density. However, the deposited S resupplied 19.6% and 28.3% of electrons for NO3--N reduction under shorter HRT (1 h) or lower current density (0.01 mA cm(-2)). Moreover, ferrous-driven NO3--N-reducing or DNRA bacteria also promoted NO3--N elimination in the cathode chamber. These findings provide new insight into the coupling interactions among S, Fe and H cycles, as well as N and P transformations in electrochemically assisted NO3--N reduction systems. (C) 2018 Elsevier Ltd. All rights reserved.