Statistical optimization for fatty acid reduction in waste cooking oil using a biological method and the continuous process for polyhydroxyalkanoate and biodiesel production

Waste cooking oil (WCO), an alternative feedstock for biodiesel, contains a high content of free fatty acid (FFA) which is unsuitable for biodiesel production. This study focused on the reduction of FFA in WCO by converting into polyhydroxyalkanoate (PHA). For this purpose, a polyhydroxyalkanoate (PHA)-accumulating strain of Bacillus thermoamylovorans was used. The ability of B. thermoamylovorans for FFA reduction and PHA production were determined. Moreover, the number of experiments was reduced owing to the response surface methodology (RSM). Moreover, treated WCO were continuously subjected for biodiesel production. The predicted operating condition for the biological treatment step consisted of a WCO concentration of 50 g/L, a temperature of 45 °C and agitation speed of 200 rpm giving the lowest %FFA at 0.99% (R² = 0.9893) with maximum PHA concentration at 3.47 g/L (R² = 0.9798). Similar values of FFA and PHA were obtained while culturing B. thermoamylovorans in a 5-L bioreactor under the optimum values. The PHA was characterized and identified to be poly(3-hydroxybutyrate-co-3-hydroxyvalerate) P(3HB-co-3HV) with 80 mol% of 3HB and 20 mol% of 3HV. Moreover, biodiesel from treated WCO was obtained at 96.5%, evaluating with Thai and international (ASTM and EN) standards biodiesel quality had of good characteristics. This is the first report using native Bacillus thermoamylovorans in pre-treatment process of high FFA containing WCO using statistical optimization. The result suggested that the biological treatment with a continuous process of PHA and biodiesel by B. thermoamylovorans had a good potential for scale up and apply in an industrial scale.