Treatment of oily wastewater by chitosan immobilized bacteria

Abstract

Removal of oily wastewater by a natural sorbent integrated with biodegradation technique was explored in this laboratory-scale study. Chitosan is a cationic biopolymer produced by the extensive deacetylation of chitin obtained from shrimp shell wastes. To simulate wastewater from gas station, oil-in-water emulsion was prepared by mixing lubricating oil with distilled water and a nonionic emulsifier. The final concentration of oil was equivalent to 200 mg/L. From preliminary study, the maximum oil sorption capacity of powder chitosan, flake shrimp shell chitosan, and flake squid pen chitosan in a pH 7.0 buffer solution were 0.48, 0.19, and 0.24 g oil/g chitosan respectively. The amount of powder chitosan and mixing time were later varied to find the optimum conditions for maximum oil removal efficiency. At the optimum treatment conditions (dosage: 0.5 g/l and mixing time: 60 min), the oil and turbidity removal efficiency were around 80%. In the biodegradation experiment, oil-degrading bacteria were isolated from five soil samples. Two bacteria (Ch2 and Ch4) provided the highest lubricating oil degradation efficiency and specifically degraded the aromatic fraction of lubricant oil. Then, bacteria Ch2 and Ch4 were examined with oil-in-water emulsion 200 mg/L. Bacteria Ch2 obtained 80% of oil-in-water emulsion degradation; whereas, bacteria Ch4 could degrade only 65%. Hence, bacteria Ch2 was selected and immobilized on three chitosan forms for the treatment of high oil-in-water emulsion concentration. There was found that the treatment of oil-in-water emulsion by flake chitosan-immobilized cells achieved the highest oil removal efficiency. Finally, this research investigated and compared the effectiveness of three treatment types: sorption by chitosan, degradation by the isolated bacteria, and sorption and degradation by chitosan-immobilized bacteria treatment. The oil-in-water emulsion removal efficiency was determined from the remaining oil concentration in water. When the concentration of oil-in-water emulsion was increased, the oil-in-water emulsion removal efficiency of chitosan-immobilized cells was the highest (91%) and remained constant throughout the study. These result suggested that sorption and degradation by chitosan-immobilized bacteria was the most appropriate process to treat oily wastewater.