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Biological wastewater reactors are traditionally divided into two groups based on modes of cell growth: suspension and attached (biofilm) growth. Kinetic descriptions of these reactors are based on confining cell growth to solid surfaces or void space. Because suspended cells grow in void space and biofilms grow on surfaces, both forms of microbial growth must in principle occur in a biological reactor, unless the surface is inhabitable by a biofilm. Cell growth and substrate utilization in both modes, suspension and attached, are fully accounted for in the model developed here. Simulations based on this model show that biofilms growing on the walls of a reactor, classified as a suspension culture, can contribute substantially to the total organics removal. Similarly, suspended cells in the voids of a "traditional biofilm" reactor can contribute significantly to degradation of organic substrates. The presence of biofilms can obviate total washout of suspended cells and avert reactor failure. Model simulations enable a comparison of attached and suspended biomass in terms of biomass accumulation, substrate degradation, and effectiveness of substrate utilization and illustrate interactions between the two forms of biomass. The model provides a unified way to analyze and design biological wastewater processes.

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