Dual limitation of biofilm kinetics

Dual limitation of biofilm kinetics occurs when one substrate is rate limiting near the water/biofilm interface, while a second substrate is rate limiting near the attachment surface. Each of the two substrates (1 and 2) has a unique role determined by its kinetic parameters: for example, substrate 1 is defined to have the flatter concentration gradient, which allows it to be rate limiting near the water/biofilm interface, but non-limiting closer to the attaching surface. A mathematic model was developed to generate concentration profiles within a biofilm that had single or dual limitation. From the results of modeling experiments, the boundaries of the dual-limitation region were identified. The boundaries are expressed as ratios of the substrate concentrations at the water/biofilm interface. These boundary ratios are also adjusted to take into account liquid-layer mass transport. An important result is that an algorithm for calculating the substate fluxes in the dual-limitation region is presented. The modeling results are applied to the processes of aerobic removal of soluble BOD, nitrification and denitrification. The analysis suggests the dual limitation occurs frequently for soluble-BOD removal and nitrification, but denitrification is typically limited by the organic substrate.