Iron-carbon galvanic cells strengthened anaerobic/anoxic/oxic process (Fe/C-A2O) for high-nitrogen/phosphorus and low-carbon sewage treatment

The treatment of sewage with high-nitrogen/-phosphorus and low-carbon remains a challenge. A novel iron-carbon galvanic cells strengthened anaerobic/anoxic/oxic process (Fe/C-A2O) was developed for high-nitrogen/-phosphorus and low-carbon sewage treatment. The cost-effective iron-scraps (ISs) was recycled as Fe(0)-source under the mediation of Fe/C galvanic cell reaction to develop effective Fe(0)-oxidizing autotrophic-denitrification and -dephosphorization. Utilizing practical high-nitrogen/-phosphorus and low-carbon sewage as target wastewater, the performance, impact factors, contribution of Fe/C galvanic cell reactions, microbial characteristics, strengthening mechanisms, and application potential of Fe/C-A2O process were investigated. The Fe/C-A2O process achieved high TN and TP removal efficiencies of 92.0 ± 1.3% and 97.2 ± 0.9% with removal loads of 0.176 ± 0.002 kg TN/(m3·d) and 0.017 ± 0.002 kg TP/(m3·d), respectively. Optimal HRT of 12 h, DO of 4.0–4.5 mg/L, and reflux-ratio of 4:1 were obtained, and no sludge-reflux was required. Autotrophic-denitrification and -dephosphorization supported by the Fe/C galvanic cell reactions contributed 63.1% and 75.3% of TN and TP removal, respectively. Microbial characterization revealed the dominance of autotrophic denitrifiers (e.g., Thiobacillus), AOB (e.g., Nitrosomonas), NOB (e.g., Nitrospira), and heterotrophic denitrifiers (e.g., Zoogloea). The mechanism analysis demonstrated that Fe/C galvanic cells strengthened nitrogen removal by raising Fe2+/H2-supported autotrophic denitrification; and strengthened dephosphorization by introducing Fe3+-based PO43−-precipitation and enhancing the denitrifying phosphate-accumulation by denitrifying phosphate-accumulating organisms (DPAOs). Based on the efficiency and cost evaluation, the ISs-based Fe/C-A2O process showed significant application potential as an upgrade strategy for traditional A2O process in advanced high-nitrogen/phosphorus and low-carbon sewage treatment.