Permittivity of waste-activated sludge by an open-ended coaxial line

TL;DR

This study measures the complex permittivity of waste-activated sludge across a broad frequency range, revealing water-related dispersions and the influence of solid content on dielectric properties, aiding in wastewater treatment analysis.

Contribution

It provides detailed permittivity spectra of waste-activated sludge over 3 MHz to 40 GHz, highlighting biological tissue-like features and solid content effects, which were previously uncharacterized.

Findings

Debye-type dispersion at 19 GHz linked to bulk water

$eta$-dispersion between 10-100 MHz due to cell membrane charging

Bound water effects observed at intermediate frequencies

Abstract

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).