The anaerobic fixed film reactors (AFFR), containing a mixed population of bacteria immobilized on the surfaces of expanded-clay support medium, have been successfully applied in the simulation of high-strength trade effluent treatment. This paper presents residence time distribution (RTD) studies to investigate the hydrodynamic characteristics in the packed bed of this novel type of reactor under the mixing effects of an effluent recycle stream. The results show that the flow pattern is characterized by various extents of back-mixing as predicted by the dispersion model. An effluent recycle at a rate equivalent to replacing the liquid content of the AFFR three times per hour achieved a large amount of dispersion in the packed bed. The AFFR without recycle had an intermediate amount of dispersion. This study shows that treatment performance of AFFRs and the cause of reactor failure are related to the effects of the effluent recycle in diluting and distributing the organic constituents of the trade effluent.

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