Sponge　iron　(SI)　can　serve　as　an　indirect　electron　donor　to　provide　Fe(II)　for　the　nitrate-dependent　ferrous　oxidation　(NDFO)　process,　producing　OH-　and　magnetite.　The　SI-NDFO　system　mainly　uses　Fe(OH)(2)　as　an　electron　donor,　achieving　a　TN　reduction　rate　of　0.42　mg-TN/(gVSS.h)　for　a　period　of　at　least　90　days.　The　enrichment　of　iron-oxidizing　bacteria　and　the　competition　of　iron-carbon　micro-electrolysis　for　reaction　sites　on　the　surface　of　SI　are　the　main　reasons　for　the　improvement　of　total　nitrogen　removal　efficiency　(TNRE).　With　an　influent　NO3--N　concentration　of　50　mg/L　and　a　SI　concentration　of　50　g/L　(at　pH　5.0　and　30　degrees　C),　the　TNRE　reached　a　maximum　level　of　38.28%.　In　addition,　reducing　the　pH　environment　was　found　to　improve　the　denitrification　efficiency　of　the　SI-NDFO　system,　although　denitrification　stability　was　also　reduced　as　a　result.　Overall,　the　SI-mediated　NDFO　process　is　a　promising　technique.