Rheological properties are of crucial importance due to their effect on hydrodynamic conditions near the membrane. The rheological behavior of microbial suspensions has been described in the literature as non-Newtonian pseudoplastic fluids (Rosenberger et al., 2002b). When air is dispersed in a solid-liquid suspension a change can be seen in its rheological behavior due to the change in suspension structure: with increasing shear, the structure opens and biological aggregates are reorganized resulting in a decrease in viscosity. In addition, it is accepted that the microbial suspensions have a thixotropic nature, which means that the viscosity decreases with shear rate when samples are subject to shear stress. Rheology can be described by the Bingham model, the Ostwald model and the Herschel-Bulkley model represented by Eq. (3)-(5):

(3)

( dv

^ — ml dr

In these models is the apparent viscosity, dv/dr is the shear rate and To, m and n are the model parameters. From the models we may deduce that the apparent viscosity can be described as a shear rate function.

Figure 8 shows one example of apparent viscosity reduction with the shear intensity. It decreases down to 75% when the shear varied from 13 to 130 s-1. Additionally, plotting is shown according to the Bingham, Ostwald and Herschel-Bulkley models. In general, both the Ostwald model as well as the Herschel-Bulkley model fits quite well into the experimental data, while the Ostwald was selected because of its simplicity. From the equation of the curve (Figure 8) the parameter values for Ostwald model can be obtained:

n = 0.41
m = 122 mPa s

where n is the flow behavior index and m is the consistency index.

Furthermore, as shown in Figure 8, apparent viscosity (pa)limit can be perceived for higher values (> 130 s-1 ). It does not decrease substantially with an increasing velocity gradient. Therefore, the effect of particle concentration on the viscosity can be evaluated by fitting the (pa)limit to the sludge concentration, measured as MLSS concentration (Figure 9). As expected, microbial suspension viscosity also increased with the MLSS concentration. This behaviour is commonly accepted in the literature (e. g. Pollice et al., 2007).

Therefore, the following equation (Eq. (6)) can estimate the limit apparent viscosity as a function of the MLSS concentration.

ua = 1.1-10-6-SSLM17

r alimrt