Biological surfactant production in a biological slurry reactor treating diesel fuel contaminated soil.

A sandy loam with aged diesel fuel contamination was treated for 90 days in an 8-L soil slurry-sequencing batch reactor (SS-SBR). The purpose was to investigate biological surfactant production and its effect on slurry properties and reactor performance. The SS-SBR was operated with a 10-day retention time and a 5-day cycle. Track studies were performed to monitor the fluctuation in slurry properties during a single cycle. Surfactants were produced faster than they were degraded or sorbed during the first 1.5 days of each cycle, resulting in increasing concentrations from less than the critical micelle concentration (CMC) to 60 times the CMC and an increase in aqueous diesel fuel concentration from 0 to 1.2 g/L. Only after the concentration of surfactants and emulsified diesel fuel began to decrease through biodegradation (after 1.5 days) was foaming observed. Foam thickness increased from 0 mm at 1.5 days to 48 mm on day 3, and then decreased to 0 mm again by the end of the cycle. Surfactants were completely degraded by the end of each cycle. Coinciding profiles of foam thickness and emulsification capacity (i.e., ability to emulsify spiked hydrocarbon) indicate that foaming resulted from the temporary accumulation of free surfactant molecules. Biological surfactant production occurred without cell multiplication during the first day, but was later growth associated. The ratio of oxygen to diesel fuel consumed was 1.7, and the microorganism yield on a chemical oxygen demand basis (YCOD) was 0.43.