A two-stage hydrothermal process aimed at improving acetic acid production using municipal biosolids was evaluated against thermal hydrolysis and conventional wet oxidation process in a 600 ml Parr batch reactor. Thermal hydrolysis was conducted at 140 °C, wet oxidation at 220 °C and the two-stage process, which acted as a series combination of thermal hydrolysis and wet oxidation, at 220 °C. Initial pressure of 1 MPa was maintained in all the three processes. The results indicated that the highest acetic acid production of up to 58 mg/g dry solids feed was achieved in the wet oxidation process followed by the two-stage process with 36 mg/g dry solids feed and 1.8 mg/g dry solids feed for thermal hydrolysis. The acetic acid yield obtained by the thermal processes increased from 0.4% in the thermal hydrolysis process to 12% during the single stage wet oxidation, with the two-stage process achieving 8%. The purity of the acetic acid improved from 1% in thermal hydrolysis to 38% in the wet oxidation process. The two-stage process achieved acetic acid purity of 25%. This work demonstrated no enhancement of acetic acid production by the two-stage concept compared with the single stage wet oxidation process. This is in contrast to similar work by other researchers, investigated on carbohydrate biomass and vegetable wastes using hydrogen peroxide as the oxidant. However, the data obtained revealed that substrate specificity, reaction severity or oxidant type is clearly important in promoting reaction mechanisms which support enhanced acetic acid production using municipal biosolids.
Keywords: acetic acid, carbohydrates, municipal biosolids, thermal hydrolysis, volatile fatty acids, wet oxidation