DYNAMICS OF ARTIFICIALLY IMMOBILIZED NITROSOMONAS-EUROPAEA - EFFECT OF BIOMASS DECAY

The dynamics of growth and death of immobilized Nitrosomonas europaea were studied. For this, the death rate of suspended cells was determin ed in the absence of ammonium or oxygen by following the loss of respi ration activity and by fluorescein-diacetate (FDA)/lissamine-green sta ining techniques. The death rates obtained (1.06 x 10(-6) s(-1) or 4.9 7 x 10(-6) s(-1) in the absence of oxygen or ammonium, respectively) w ere incorporated in a dynamic growth model and the effects on the perf ormance of the immobilized-cell process illustrated by model simulatio ns. These model simulations and experimental validation show that if d ecay of biomass occurs the biomass concentration in the center of the bead decreases. As a result, the systems react slower to changes in su bstrate concentrations than if all cells remain viable. To show that c ells in the center of the bead died, the FDA and lissamine-green stain ing techniques were adapted for immobilized cells. It was shown that b iomass decay occurred, especially in the center of the bead; the amoun t of cells decreased there, and the remaining cells were all stained w ith lissamine green indicating cell death. After the substrate availab ility was decreased, also cells near the surface of the bead lost thei r viability. The number of viable cells increased again after increasi ng the substrate concentration as the result of cell multiplication. A t low substrate concentrations and low hydraulic retention times, as f or example in the treatment of domestic wastewater, the death rate of cells is thus an important parameter for the performance of the immobi lized-cell system. (C) 1997 John Wiley & Sons, Inc.