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

Abstract

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 △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 S0, 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.