  The complex permittivity of thickened waste activated sludge (WAS) was
measured from 3 MHz to 40 GHz. The solid content of the thickened WAS sample
was varied from 4.5% to 18% by weight. The permittivity spectra exhibit
features typical of biological tissues that have a high water content. At high
frequencies, a Debye-type dispersion is observed with a relaxation rate of 19
GHz characteristic of the bulk water in the sample ($\gamma$-dispersion). At
lower frequencies, the solid content of the samples determines the properties
of the permittivity. The onset of the so-called $\beta$-dispersion, attributed
to the charging of cell membranes, occurs between 10-100 MHz. For samples with
higher solid concentrations, a weak dispersion of the real part of the
permittivity, characteristic of bound water, was observed at intermediate
frequencies ($\delta$-dispersion).
