Biological activated carbon: the relative role of metabolism and cometabolism in extending service life and improving process performance
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Granular activated carbon (GAC) is commonly used to remove synthetic organic chemicals (SOCs) from contaminated water. Replacement and subsequent disposal of spent GAC is expensive. By increasing the service life of the GAC, costs can be decreased. Encouragement of biodegradation (metabolism and cometabolism) where one or more of the SOCs are biodegradable can lengthen the GAC service life for some SOC mixtures. The service life increases because a biofilm that forms on the GAC can biodegrade SOCs, thereby reducing competition for GAC adsorption sites and allowing any remaining SOCs to adsorb onto the GAC to a greater extent than in the absence of biodegradation. SOCs in both the aqueous phase and adsorbed on the GAC are available to the microorganisms. Biodegradation of adsorbed SOCs (termed bioregeneration) renews the GAC’s capacity for SOC adsorption, while aqueous phase biodegradation slows the rate of GAC exhaustion, thereby lengthening the GAC service life and decreasing operation and maintenance costs. Adsorption isotherms and biological kinetic studies were performed to describe GAC column performance. Continuous-flow GAC bioregeneration experiments (preequilibrated and virgin) were conducted using a mixture of biodegradable (toluene) and either nonbiodegradable (perchloroethylene, PCE) or traditionally nonbiodegradable (trichloroethylene, TCE) SOCs. In the pre-equilibrated experiments, the GAC was saturated with respect to toluene and PCE or TCE to observe the biggest effect on bioregeneration performance. If no dissolved oxygen limitations occurred, the biodegradable SOC effluent concentration decreased over time and remained low, after which the nonbiodegradable or traditionally nonbiodegradable SOC effluent concentration also decreased because of the increased availability of adsorption sites on the GAC as well as the cometabolism of TCE, if present, by enzymes produced via toluene metabolism. Virgin column experiments were also run and allowed for direct measurement of the service life increase due to biodegradation. Toluene-and TCE-based bioregeneration ranged from 26 - 53% and 2.2 - 7.4%, respectively, of the initial loading after 11 to 20 days. Pre and post-experimental GAC loadings showed a decrease in the biodegradable SOC loading as well as an increase in the nonbiodegradable SOC loading. Greater degrees of bioregeneration were found for higher SOC concentrations and longer EBCTs.