ABSTRACT This paper presents the application of a continuous-flow, mesophilic (35°C), plug-flow digester to cometabolic degradation of toluene by an anaerobic microbial consortium in the presence of conventional pollutants serving as the major carbon and energy sources. In contrast to conventional practice, digester contents were not mixed. Reactor effluents were recirculated to the influent end at a rate of 20 vol. percent of the influent flow rate. The anaerobic digester was charged with 50 mg/l of toluene. The total COD (chemical oxygen demand) concentration of the mixed, non-hazardous carbon sources was 11,000 mg/l. Experimental results showed that anaerobic digestion occurring in a plug-flow reactor can simultaneously stabilize and detoxify conventional and hazardous wastes with the production of a methane-rich (up to 80 mol% methane) fuel gas. The observed gas yield of 0.36 ± 0.01 SCM/kg VS added was higher than that of conventional, complete-mix digesters under similar operating conditions of hydraulic retention time (HRT) and organic loading rate (OLR). Toluene did not inhibit anaerobic biogasification of non-hazardous substrates. A toluene degradation efficiency of 95% and a total COD reduction of 96% were observed at an HRT of 13 days and an OLR of 0.81 kgCOD/m3-day. Digester effluents contained about 2 mg/l of toluene under these operating conditions. Toluene released to the reactor head gas was about 1.4% of the toluene introduced into the anaerobic reactor. The small concentration of toluene in the gas-phase would not be a problem if the digester gas were subjected to high-temperature combustion. A simple model based on the Monod growth-kinetic equation was developed to describe cometabolic toluene-degradation profiles in the plug-flow digester. The kinetic parameters of half-velocity constant (Ks) and the maximum toluene utilization rate per unit culture volume (C) were estimated by nonlinear regression analysis of steady-state toluene concentration profiles in the reactor liquid phase. Values of Ks and C were 12.2 mg/l and 6.2 mg/l-day, respectively. This research showed that a simple, plug-flow anaerobic digester gasified solid and liquid wastes with simultaneous destruction of toluene at efficiencies higher than 95%. There was no evidence of inhibition of the anaerobic digestion process by 50 mg/l of toluene. The reactor produced up to a 80-mol% methane-content product gas.
KEYWORDS: anaerobic, biodegradation, kinetics, plug-flow, toluene
This paper is from the Proceedings of the HSRC/WERC Joint Conference on the Environment, May 1996, published in hard copy and on the Web by the Great Plains/Rocky Mountain Hazardous Substance Research Center.
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