Abstract

Production costs of polyhydroxyalkanoate (PHA) are high, therefore, to make their production more competitive, inexpensive carbon sources such as biodiesel liquid waste (BLW), the major waste byproduct of biodiesel production, was considered to be a suitable choice. BLW has a relatively high glycerol content (about 50 wt%) and fatty acid content (about 20 wt%). The objective of this study was to isolate PHA-producing bacteria that could efficiently convert BLW to PHA with a PHA content greater than 60 wt% of the cell dry weight (CDW). An enrichment technique using a mineral salts medium (MSM) supplemented with BLW was used for the isolation of single colonies that were then screened for their PHA content using the Nile Red staining method followed by confirmation using gas chromatography (GC). Three of the potentially best PHA-producing isolates from the original 254 isolates i.e. BLW-2, BLW-4 and BLW-24, were selected for further study. The highest biomass concentration of 2.5 g/L and a PHA content of 46 wt% CDW were produced by isolate BLW-2 when cultivated in MSM containing 0.5 % (v/v) of BLW and 1.0 g/L of ammonium sulphate as the sole carbon source and nitrogen source, respectively. Optimal conditions for PHA production were determined and these allowed for a PHA content of 3.25 g/L and 62 wt% CDW when grown with MSM containing 2.0 % (v/v) of BLW with an initial pH of 7.0 at 37 °C. The PHA produced contained 94 mol% of 3-hydroxybutyrate (3HB), 2 mol% of 3-hydroxyoctanoate (3HO) and 4 mol% of 3-hydroxydecanoate (3HD) as determined by gas chromatography-mass spectrometry (GC-MS). This indicated that there was scope for modifying the PHA composition for special product purposes. In addition to PHA, this isolate also produced under the same culture conditions approximately 1.75 g/L of alginate, which is a member of exopolysaccharide (EPS) consisting of mannuronic and guluronic acid. Liquid chromatography-mass spectrometry (LC-MS) analysis revealed that the produced alginate consisted of 1-deoxy-β-D-mannuronic acid and 1-deoxy-α-L-guluronic acid.