A method to assess inhibition kinetic data for packed-bed anaerobic reactors is presented and discussed. The method is based on the evaluation of overall substrate utilization rates by immobilized anaerobic sludge subjected to different substrate concentrations. The reaction rates were estimated by using a differential reactor filled with polyurethane cubic matrices containing anaerobic sludge. A synthetic glucose-based substrate was recycled in a closed circuit at liquid superficial velocity of 0.008 cm s^-1. The initial substrate concentration was varied from one experiment to another from 532 to 3,015 mg COD L^-1. A substrate inhibition kinetic model fitted well with the data of overall reaction rates as a function of substrate concentration and the apparent kinetic parameters were estimated. The substrate concentration that provides maximum substrate utilization rate by the biomass was found to be 2,546 mg COD L^-1. A slight decrease of the overall substrate utilization rates was observed for substrate concentrations higher than this value, due to primary or intermediate substrate inhibition.

The application of anaerobic reactors for treatment of wastewaters from industrial processes, agricultural and domestic activities has increased significantly from the 1980s. This advancement can be mainly credited to a better understanding of the anaerobic process fundamentals and the development of new reactor configurations, which provide good performance and operational stability. Anaerobic filters, anaerobic fluidized bed reactors and the upflow anaerobic sludge blanket reactors are examples of units operating successfully. These reactors have their performance closely associated to the biomass immobilization, allowing their operation at very high cellular retention times, even at very short hydraulic detention times.

Although cell immobilization technology improves the anaerobic process due to the attainment of high cellular retention time, mass transfer limitations resulting from this conformation of the biomass must be considered. In heterogeneous systems, the substrate has to be transferred from the bulk liquid to the bioparticle surface and from the surface to the center of the bioparticle. By this process, the overall specific rate of substrate utilization in solid-liquid systems can be reduced due to solid and liquid mass transfer resistances. However, it is possible that mass transfer resistances can act as a protection for the entrapped biomass, avoiding the accumulation of intermediate substrates that cannot be completely metabolized by the biomass. It is well-known that the accumulation of intermediate substrates in methanogenic reactors can lead to process inhibition. This is particularly true for systems containing suspended biomass because the cells are directly subjected to variations occurring in the bulk volume.

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