The existence of a biological equilibrium in a trickling filter for waste gas purification

Clogging is well‐known phenomenon in the application of a biological tricking 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 CO2 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 Hyphomicrobium GJ21 within the biofilm. The combined results of the experiments described 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 CO2 production slowly increased until, after some 200 days, it finally counter‐balanced 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 agreements 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. © 1994 John Wiley & Sons, Inc.