Iron-catalyzed wet air oxidation of biomethanated distillery wastewater for enhanced biogas recovery

Abstract

In the present work, catalytic wet air oxidation (WAO) technique was applied to biomethanated spent wash from a local sugar factory. This wash water exhibited high biological oxygen demand (BOD5 = 8100 mg/L) and chemical oxygen demand (COD = 40,000 mg/L). The objectives of oxidative pre-treatment were two-fold, viz. efficient treatment of wash water and enhanced biogas recovery. For the catalytic oxidation process, two iron-based heterogeneous catalysts were employed, viz. Fe2O3 and Fe/C. To synthesize the Fe/C catalyst, activated carbon (AC) support was modified either by thermal treatment or chemical treatment with nitric acid; accordingly, the catalyst was named as Fe/AC-T or Fe/AC-N. In a batch slurry reactor, catalyst performance was investigated at T = 175 °C, PO2 = 0.69 MPa and ω = 33 mg/L (here, T, PO2 and ω denote temperature, oxygen partial pressure and catalyst loading) for 1 h. Based on the conversions of COD and total organic carbon (TOC) and the improvement in biodegradability index (BI), it was found that the activity of the catalysts reduced in the order: Fe/AC-N > Fe/AC-T > Fe2O3. The results were more encouraging (COD conversion = 87%, color reduction = 88% and BI value = 0.71) when carbon adsorption (5% w/v) followed WAO over Fe/AC-N. Clearly, our novel hybrid process for pre-treatment, viz. wet oxidation-carbon adsorption showed potential. Post biomethanation, around 1.2 Nm3 biogas (CH4 72%) was formed per cubic meter of the wastewater; without pre-treatment by catalytic WAO and carbon adsorption, the yield of biogas (CH4 11%) was just 1 Nm3 for every cubic meter of wastewater. After a last aerobic treatment step, 97% COD was removed and BI value was 0.84. Finally, a kinetic model was proposed to describe kinetics of COD reduction. In this way, a promising method was suggested for treating a complex wastewater.