Evaluation of thermal, alkaline, thermal – alkaline pretreatments on polyhydroxybutyrate to enhance anaerobic methane production

Abstract

Polyhydroxybutyrate (PHB) has been widely applied in packaging and disposable products to replace conventional plastics because of its higher biodegradability. Anaerobic digestion (AD) is considered a promising biological treatment process to reduce PHB wastes and simultaneously produce methane gas; hydrolysis of PHB is a rate-limiting. Therefore, this study evaluates the effects of temperature (37°C, 55°C, 73°C, and 91°C), pH (7, 10, and 13), and reaction time (24 h, 72 h, and 120 h) on hydrolysis of PHB with pretreatment tests of a full-factorial design and their effects on methane yields, and methane production rate of PHB in anaerobic digestion with BMP test of a full-factorial design. All tests were performed in two different particle sizes of PHB: 5 mm and 600 μm. The higher hydrolysis efficiency was achieved at higher temperatures (55°C, 73°C, and 91°C), with pH 13 for 120 h with 5 mm PHB (91.7 - 99.6 %) and 600 µm PHB (98.5 - 99.95 %). At lower temperatures and pH, e.g., 37°C and pH 7–10, hydrolysis efficiency was significantly lower, < 7%. SEM analysis indicated that the structure surface area of PHB was damaged by thermal-alkaline pretreatment. Thermal-alkaline pretreatments (55°C, 73°C, and 91°C at pH 13) were found to improve the methane yields and methane production rates during 40 d with 5 mm PHB (methane yields of 160 – 366 mL CH4/g COD and rates of 20 - 52 mL CH4/g VSS) and 600 µm PHB (methane production rates of 53 - 84.5 mL CH4/g VSS). The highest solubilization efficiencies, methane yields, and methane production rates were achieved at 91ºC with pH 13 in both particle sizes. The solubilization efficiency and methane production rate were significantly higher with PHB of smaller particle size (600 µm) when compared to PHB of larger particle size (5 mm). The effects of temperature, pH, and reaction time on solubilization, methane yields, and methane production rates were successfully estimated by quadratic models. This study highly recommends using a thermal-alkaline to improve the bioplastics methane potential.