Abstract
A continuous-flow stirred tank reactor (CSTR) and a soil slurry-sequencing batch reactor (SS-SBR) were maintained in 8 l vessels for 180 days to treat a diesel fuel-contaminated soil. The SS-SBR provided markedly enhanced contaminant degradation relative to the CSTR. Diesel fuel removal efficiency was 96% in the SS-SBR, compared with 75% in the CSTR. Microbial growth was approximately 25% greater in the SS-SBR than the CSTR. However, significant biosurfactant production and foaming occurred in the SS-SBR, whereas none was observed in the CSTR. Surfactants were produced in the SS-SBR at concentrations up to 70 times the critical micelle concentration (CMC), but were biodegraded by the end of the cycle. Reactor operation was reversed after 80 days. The reactor converted from an SS-SBR to a CSTR lost surfactant production and showed reduced diesel fuel degradation. Converting the CSTR to an SS-SBR resulted in surfactant production and enhanced diesel fuel degradation. These results indicate that fill-and-draw operation selected for microbes with a greater ability to produce surfactants and degrade diesel fuel than the CSTR operation. Decreasing the diesel fuel addition rate in the SS-SBR by four times on day 160 reduced the maximum surfactant concentration and foam thickness by more than three times without affecting diesel fuel removal.
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