Optimization of biomass concentration in a fluidised bed reactor
Fluidized sand-bed, fixed-film denitrifying reactors were tested for the treatment of high strength waters and for the optimization and control of biofilm thickness. Two reactors with sand (0.63 - 0.8 mm) as the carrier particle were operated. Ethanol and propionic acid were used as carbon sources. Nitrate concentrations were in the range of 200-900 mg NO3- -N/l. Tests showed no substrate inhibition (NO3- or ethanol) at these concentrations (Fig. 3). The nitrate removal capacity of the reactors reached 10 kg NO3- -N/m3 d, which corresponded practically to 100 % nitrate removal efficiency (Fig. 2). Nitrite formation was only observable where other conditions (e.g., unfavourable pH) hindered nitrate removal (Fig. 4). Since biofilm growth is a parameter of major influence on reactor performance and mechanical/hydrodynamic functioning, its control is indispensable. A method was developed for simplified determination and optimization of biofilm coverage. This method is based on the expansion coefficient (E) and specific particle volume (ɛ0) parameters. The former is defined as the slope of the bed height-fluidization rate plot (eq.3), and the specific particle volume can be calculated from the intercept (eq.4). The particle content (Cp) (Fig. 1, eqs. 2 and 6) ties these parameters to the pressure gradient measured along a fluidized bed, as introduced in a previous study (Csikor et al. 1995). This method was simplified to replace biofilm thickness with the gravimetric biofilm coverage (G), which is easy to determine gravimetrically. For the determination of fluidization and biofilm parameters, samples were taken from different points of the fluidized bed with differing biofilm thickness (Fig.6) and tested in a small fluidized bed reactor. It was found that G is linearly correlated to E and ɛ0 (Figs. 8 and 9). The reliability of the linear relationship was controlled by transforming biofilm coverage data to biofilm thickness and comparing with previous results. It was shown that differences in microbial cultures cause negligible differences in the hydrodynamics of fluidization (Fig. 5). Volumetric biomass concentration (X), which is directly related to G (Fig. 7), can thus be determined using simple hydrostatic pressure tests. It was demonstrated that X has an optimal value (Figs. 7 and 11) and can reach 19 - 20 g VS/l under normal operating conditions. This corresponds to a G between 80 - 100 mg VS/g support. Increased biofilm thickness does not improve X but increases the diffusion limitation. The sensitivity of the Cp -based biofilm measurement is greater with thin biofilms. However the real volumetric biomass concentration is less sensitive to changes with thick biofilms, which counterbalances this effect (Fig. 10).