Microelectrodes for ammonium, oxygen, nitrate, and pH were used to study nitrifying aggregates grown in a fluidized-bed reactor. Local reactant fluxes and distribution of microbial activity could be determined from the microprofiles. The interfacial fluxes of the reactants closely reflected the stoichiometry of bacterial nitrification. Both ammonium consumption and nitrate production were localized in the outer shells, with a thickness of approximately 100 to 120 ,m, of the aggregates. Under conditions in which ammonium and oxygen

The mobility of water in intact biofilms was measured with pulsed field gradient nuclear magnetic resonance (PFG‐NMR) and used to characterise their diffusive properties. The results obtained with several well‐defined systems, viz. pure water, agar, and agar containing inert particles or active bacteria were compared to glucose diffusion coefficients measured with micro‐electrodes and those calculated utilising theoretical diffusion models. A good correspondence was observed indicating that PFG‐NMR should also enable the measurement of diffusion coefficients in heterogeneous biological systems. Diffusion coefficients of several types of natural biofilms were measured as well and these results were related to the physical biofilm characteristics. The values had a high accuracy and reflected the properties of a sample of ca. 100 biofilms, while non‐uniformity or non‐geometrical shapes did not negatively influence the results. The monitored PFG‐NMR signal contains supplementary information on e.g. cell fraction or spatial organisation but quantitative analysis was not yet possible. © 1998 John Wiley & Sons, Inc. Biotechnol Bioeng 60: 283–291, 1998.

Allatragt--Due to the conversion of acetic acid into the weaker carbonic acid and methane, the pH inside methanogani¢ aggregates is higher than in the bulk liquid. The pH profiles in aggregates were measured with pH microelectrodes. These profiles strongly determine the macro-kinetics of the aggregate, by their influence on the values of the growth parameters/(1 and ~. Acetate transport resistances were not limiting for the conversion rate in methanogenic aggregates. Nevertheless, the effectiveness factor v/did not approach unity, but amounted to 0.57-0.62 in the acetate concentration range relevant for most methanoge.nic reactors. The value of ¢ is determined almost entirely by the pH profiles inside the aggregates. It was concluded that for the physical/mathematical description of the conversion in methanogenic aggregates, information on the pH gradients and the pH dependency of the growth pa~rs is indispensable. Batch experiments showed that acetate uptake by aggregates was not coupled directly to methanogenesis. Consumed acetate was not converted instantaneously to methane, suggesting the conversion to proceed via a pool of acetate or reserve material.

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