Hydraulic fracturing flow-back fluid (HFFBF) was treated by a zero-valent iron (ZVI)/H 2 O 2 process. The initial HFFBF chemical oxygen demand (COD) was 3,440 mg/L and the 5-d biochemical oxygen demand (BOD 5)/COD ratio was 0.259. Under the optimal 500/6,884 mg/L of ZVI/H 2 O 2 reagent doses, and a 120-min process time, COD was decreased to 680 mg/L (80.2% removal). Additionally, as a result of pollutants' chemical oxidation, the wastewater's susceptibility for biodegradation was essentially increased to BOD 5 /COD 0.971. To assess the maximum share of coagulation in the total ZVI/H 2 O 2 process treatment effect, a coagulation process was additionally employed. The coagulation for an optimal 1.5mL/L of iron-based PIX 111 coagulant dose obtained 1,200 mg/L of COD, 65.1% removal. The use of coagulation only allowed an increase in BOD 5 /COD to 0.625. As a result of the oxidation step during ZVI/H 2 O 2 process, the persistent compounds were transformed to more vulnerable ones.