TREATMENT OF STABILIZED CUTTING OIL WASTEWATER BY FENTON PROCESS: EFFECT OF FENTON'S REAGENTS DOSAGE AND FEEDING MODE

The objective of this research is to investigate the treatment of the stable cutting oil wastewater by Fenton process, an advanced oxidation used as a post-treatment process. Hydroxyl radical can be produced by the reactions between the Fenton's reagents including ferrous (Fe2+) and hydrogen peroxide (H2O2). The result showed that highest COD removal efficiencies of 97% can be achieved for treating 100 and 150 mg/l oil concentration with 200 mg Fe2+/l and 1:2.5 of Fe2+/H2O2 ratio, while 98% of 50 mg/l oil concentration can be succeeded at 200 mg Fe2+/l and 1:10 of Fe2+/H2O2 ratio in 1 minute reaction time. However, excess Fenton’s reagents cause lower oxidation of cutting oil due to hydroxyl radical scavenging effect. The multiple step feedings were therefore applied. The result showed that three feedings of 16.67 mg Fe2+/l and 41.67 mg H2O2/l at t = 0, 5, and 10 minutes reaction time provided COD removal efficiency as high as initial full feeding of 200 mg Fe2+/l and 500 mg H2O2/l (97%). After cutting oil was oxidized by Fenton, pH of the treated water was acidic, which cannot be discharged to an environment. Therefore, the pH neutralization by NaOH was conducted. This pH adjustment causes the iron precipitation, which required the sedimentation to remove. The optimum overflow rates of sedimentation tank for 50 mg Fe2+/l and 200 mg Fe2+/l were in range of 0.2 – 0.45 m/h and 0.7 – 1.9 m/h, respectively. In addition, reuse of iron sludge in the Fenton reaction was carried out in order to minimize the sludge production and reduce the disposal cost. The sludge from 50 mg Fe2+/l and 125 mg H2O2/l can be reused to treat 100 mg/l oil concentration without replacement at least 5 recycling times.