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Clogging is a well-known phenomenon in the application of a biological trickling filter for both waste gas and wastewater treatment. Nevertheless, no such observations or even significant changes in pressure drop have ever been recorded during the long-term processing of a waste gas containing dichloromethane (DCM) as a sole carbon source. To obtain more information about this phenomenon, a detailed investigation into the carbon balance of this system has been performed. During a period of operation of about 200 days the rate of DCM elimination and the overall rate of CO, production in a continuously operating filter were therefore recorded daily, thus allowing an evaluation of the overall conversion process. Furthermore pseudo-steady-state measurements were carried out on a regular basis. These experiments reveal more detailed information on the actual DCM conversion by Hyphornicrobiurn GJ21 within the biofilm. The combined results of the experiments de-scribed in this article show that on an overall basis a so-called biological equilibrium, i.e., a situation of no net biomass accumulation, is obtained in the course of time. It appeared that the overall rate of CO, production slowly increased until, after some 200 days, it finally counterbalanced the conversion rate of DCM on a molar basis. As opposed to this result, all pseudo-steady-state experiments indicated that about 60% of the eliminated primary carbon source is converted into biomass. This is in good agreement with results from microkinetic experiments. Based on these results and evaluation of the experimental data, it is concluded that interactions between several microbial populations are involved in this biological equilibrium. These interactions include both biomass growth and biomass degradation.
Two problems still hamper the widespread industrial application of biotrickling ®lters (BTFs) for waste gas treatment in practice: clogging of the ®lters at higher carbon loads and a decrease in the elimination of a target compound when more than one compound is present in the waste gas. To investigate these phenomena three identical BTFs removing dichloromethane (DCM) from an arti®cial waste gas were operated counter-current wise for 12 months at a DCM load of 0.94 Cmole-DCM/(m 3 r á h). After ®ve months of operation methylmethacrylate (MMA) was added to the waste gas. Three different MMA loads were applied: 0.5, 1.0 and 1.5 Cmole-MMA/(m 3 r á h). Although the elimination of DCM in all three BTFs decreased after the introduction of MMA to the air stream, it stabilised at a lower steady-state value than before the MMA addition. MMA was completely degraded during the applied standard conditions. In all three ®lters biomass accumulation eventually caused clogging of the packing. In the ®lter with the lowest MMA load the ®rst signs of clogging were observed only after 7 months of stable operation, illustrating the need for long term studies to evaluate process stability. Short term experiments have provided information about the system's dynamics and showed that an accumulation of intermediates and a subsequent adaptation of the biomass in the BTF will occur upon a step increase in MMA load. To evaluate whether a stable BTF operation without clogging is possible, a novel process parameter (the rate of Carbon Conversion per unit void packing Volume) is introduced which possibilities and limitations are discussed.List of symbols EC dcm DCM elimination capacity (g/(m 3 r h)) EC max dcm
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