A thermodynamic framework has been provided for the description of maintenance requirements of microorganisms. The central parameter is the biomass specific Gibbs energy consumption for maintenance, m(E) (kJ/C-mol biomass . h). A large set of data has been used including (i) a large range of different organisms (bacteria, yeasts, plant cells), (ii) mixed cultures, (iii) heterotrophic and autotrophic growth, (iv) growth under aerobic and anaerobic conditions, and (v) a large temperature range (5-75 degrees C). It appears that only the temperature has a major influence, with an energy of activation of 69 kJ/mol. Different electron donors or electron acceptors only show a very minor influence on m(E). On the basis of the data set, temperature correlations of m(E) have been derived for aerobic and anaerobic growth. The generalized concept for maintenance Gibbs energy is used to establish a correlation which allows the estimation of the biomass yield on electron donor as a function of C-source, electron donor, electron acceptor, N source, growth rate, and temperature. The advantage of using the m(E) parameter over other maintenance-related parameters (like mu(e), m(O2), m(D), gamma(D)m(D)) is discussed.

The influences of reactor conditions (substrate loading rate and shear) and microbial characteristics (yield and growth rate) on the structure of biofilms is discussed. Based on research on the formation of biofilms in Biofilm Airlift Suspension (BAS) reactors a hypothesis is postulated that the ratio between biofilm surface loading and shear rate determines the biofilm structure. When shear forces are relatively high only a patchy biofilm will develop, whereas at low shear rates the biofilm becomes highly heterogeneous with many pores and protuberances. In case of a right balance smooth and stable biofilms can be obtained. A hypothesis for the evolution of biofilm structures as a function of process conditions is formulated.

In this article, the conditions for aerobic biofilm formation on suspended particles, the dynamics of biofilm formation, and the biomass production during the start-up of a Biofilm Airlift Suspension reactor (BAS reactor) have been studied. The dynamics of biofilm formation during start up in the biofilm airlift suspension reactor follows three consecutive stages: bare carrier, microcolonies or patchy biofilms on the carrier, and biofilms completely covering the carrier. The effect of hydraulic retention time and of substrate loading rate on the formation of biofilms were investigated. To obtain in a BAS reactor a high biomass concentration and predominantly continuous biofilms, which completely surround the carrier, the hydraulic retention time must be shorter than the inverse of the maximum growth rate of the suspended bacteria. At longer hydraulic retention times, a low amount of attached biomass can be present on the carrier material as patchy biofilms. During the start-up at short hydraulic retention times the bare carrier concentration decreases, the amount of biomass per biofilm particle remains constant, and biomass increase in the reactor is due to increasing numbers of biofilm particles. The substrate surface loading rate has effect only on the amount of biomass on the biofilm particle. A higher surface load leads to a thicker biofilm.A strong nonlinear increase of the concentration of attached biomass in time was observed. This can be explained by a decreased abrasion of the biofilm particles due to the decreasing concentration of bare carriers. The detachment rate per biofilm area during the start-up is independent of the substrate loading rate, but depends strongly upon the bare carrier concentration.The Pirt-maintenance concept is applicable to BAS reactors. Surplus biomass production is diminished at high biomass concentrations. The average maximal yield of biomass on substrate during the experiments presented in this article was 0.44 +/- 0.08 C-mol/C-mol, the maintenance value 0.019 +/- 0.012 C-mol/(C-mol h). The lowest actual biomass yield measured in this study was 0.15 C-mol/C-mol.

In three-phase internal loop airlift reactors, the detachment of biomass from suspended biofilm pellets in the presence of bare carrier particles was investigated under nongrowth conditions. The detachment rate was dominated by collisions between bare carrier particles and biofilm pellets. The concentration of bare carrier particles and the carrier roughness strongly influenced the detachment rate. A change in flow regime from bubbling to slug flow considerably increased the detachment rate. Otherwise, the superficial gas velocity did not directly affect the detachment rate. The influence of particle size was not clear. The bottom clearance did not affect the detachment rate within the tested range. Other aspects of reactor geometry might be important. The main detachment processes were abrasion and breakage of biofilm pellets. During the detachment process, two phases could be distinguished. In the first phase the detachment was relatively high, and both breakage and abrasion of biofilm pellets occurred. During the second phase, breakage dominated and the detachment rate was lower. The two-phase behavior is explained by differences in strength between the inner and outer biofilm layers, possibly caused by variations in local growth rates during biofilm formation. Differences in growth history might also explain the various detachment rates observed with different biofilm batches. (c) 1995 John Wiley & Sons, Inc.

On the basis of the estimated Gibbs energy dissipation per C-mol biomass produced and a convenient black box description of microbial growth, a general equation for the calculation of the yield of biomass on electron donor has been obtained. This black box model defines four formal electron donating reactions for biomass, carbon source, electron donor, and electron acceptor. The proposed description leads to a simple equation which gives the biomass yield on electron donor for chemotrophic growth systems under carbon and energy limitation for which biomass is the only anabolic product. The variables involved are the degrees of reduction and the Gibbs energy characteristics of the four compounds, and the required Gibbs energy dissipation per C-mol produced of biomass. It appears that biomass yields on electron donor for auto- and heterotrophic growth under aerobic, denitrifying, and fermentative conditions can be estimated with 10-15% error in a range of Y(DX)-values of 0.01-0.80 C-mol/(C)-mol electron donor. Also, simple regularities in the Gibbs energy and enthalpy of organic substrates are found. Furthermore, simple relations are derived to calculate the thermodynamic maximal biomass yield, conditions required for growth to occur, heat production, biomass yield on electron acceptor, and anaerobic product yield. Finally a new definition of thermodynamic efficiency is derived.