Two-Stage Reactor Technology Integrating Biofilm Denitrification and Iron Electrocoagulation for Hydroponic Wastewater Treatment

A two-stage system combining biofilm denitrification and iron electrocoagulation was evaluated for treatment of real hydroponic wastewater from soilless tomato cultivation. Biofilm reactors received sodium acetate (train A) or acetic acid (train B) at a C/N ratio of 1.5, followed by electrocoagulation with iron electrodes at 1.25 A/m2; hydraulic retention time was 12 h per stage. The biological stage removed 76.9–77.5% of total nitrogen (TN) and 94.3–95.7% of total phosphorus (TP), with elevated pH promoting phosphate precipitation with naturally occurring calcium and magnesium. After electrocoagulation, overall TN removal reached 83.6–86.7% and nitrogen speciation shifted from nitrite to nitrate dominance, although residual nitrite remained 21.9–22.1 mg N/L. Residual TN remained 96.7–126.8 mg N/L, above the 30 mg N/L discharge limit. Final TP decreased to 1.5–2.5 mg P/L, below the discharge limit. Overall TOC removal reached 96.3–97.0%. The cumulative sludge had a low volatile fraction (20.0–24.2% VSS) and was enriched in phosphorus and potassium, indicating potential for nutrient recovery. Thus, the sequential system achieved phosphorus compliance and substantial nitrogen reduction, but additional nitrogen treatment is required before discharge.