Shift from growth to nutrient-limited maintenance kinetics during biofilter acclimation.

During long-term operation of a biofilter, the mandatory absence of net cell growth forces the cells into maintenance metabolism, which is of relatively low rate compared to substrate consumption during the active growth of the acclimation phase. A model based on this shift in metabolism can explain the postacclimation decrease in activity sometimes reported for biofilters. The cessation of growth can be caused by nutrient depletion in the bed. Postacclimation nutrient addition increases activity primarily by allowing a return to the high substrate consumption rate of active growth, and only secondarily helps raise bed activity because of the ultimately higher amount of biomass in the bed. Simulations incorporating the acclimation period and the role of maintenance metabolism predict about 4 logarithms of growth during acclimation of a hexane biofilter, which was confirmed experimentally. Changes in a biofilter's biomass during the acclimation phase can be estimated from substrate conversion data using two approximate methods. The first follows the cumulative amount of substrate converted and uses the estimated yield of cells from substrate during active growth to estimate the total biomass created. The second method follows a rate constant for conversion of substrate in the bed.