This study explored free and immobilised microorganisms to degrade heavy oil. Two oil-degrading bacterial strains (W-1 and W-2) were isolated from heavy oil wastewater samples collected from Shengli Oil Field in China. W-1 and W-2, identified as Rhosococcus sp. and Bacillus cereus sp., respectively, were tested for their growth behaviour and optimal growth conditions in the laboratory. The obtained results showed that the optimal growth conditions for W-1 and W-2 were identified as pH of 8, temperature of 40°C, and salinity of 2% and 4%, respectively. The environmental conditions affecting oil-degrading efficiency by W-1 and W-2 were optimised in the media containing 0.3% heavy oil. The results showed that the optimal degradation and optimal growth conditions were similar, and the oil degradation rates of W-1 and W-2 were about 34.6% and 45.3%, respectively after 5 days.
W-1 and W-2 capable of degrading oil was immobilised in calcium alginate gel beads containing active carbon and used for degradation of heavy oil. The heavy oil biodegradability of immobilised bacteria improved dramatically, compared with that of the free ones. The heavy oil biodegradation rates of immobilised W-2 were found to be maximal at the same optimal growth conditions of pH, temperature, and salinity as the free ones. The best biodegradation rate of immobilised W-2 reached above 78%, which is 33% than that of the free W-2. © 2011 Canadian Society for Chemical Engineering