Dielectric monitoring of microwave assisted sludge valorisation: a pathway towards enhanced biogas production

Microwave (MW) irradiation is an intensive sludge pre-treatment process because it promotes floc disruption, and macromolecular hydrolysis, thereby increases the accessibility of substrates for anaerobic digestion (AD). Dielectric parameters reflect the composition, ionic/polar character, and structural changes. In MW sludge processing technologies, these parameters are important in two ways: they determine how the material converts MW energy into heat, and they can serve as non-destructive indicators for monitoring disintegration, and biological degradation.

In the research the biogas production (by batch mesophilic AD tests) of meat-, and dairy processing sludge was investigated using combined (MW assisted alkaline, Fenton process) pre-treatments. The efficiency of pre-treatments and AD was also monitored by the change of soluble chemical oxygen demand (COD) and dielectric parameter measurement (200-2400 MHz frequencies).

The experimental results verified that MW pre-treatment can increase the soluble COD/total COD ratios to above 30%. This improved solubilisation was directly linked to enhanced AD performance, as the biogas yield increased from about 220 mL/g to more than 600 mL/g. Combined microwave–alkaline treatment is proved suitable to increase sCOD/tCOD to above 45% for meat industry sludge. Magnetite nanoparticle-assisted MW treatment can produce more than a 1.5-fold sCOD enhancement, and more than a threefold increase in cumulative biogas yield. Analysis of dielectric behavior verified that MW-Fenton treatment of meat-industry wastewater, >40% COD removal was accompanied by an approximately threefold increase in dielectric loss tangent, showing that dielectric response reflected organic-matter degradation. In AD process, the dielectric constant and loss factor followed the main fermentation phases (progress of organic matter degradation). Very strong quantitative correlations were found between dielectric parameters and treatment/fermentation performance (SCOD–dielectric constant correlation at 500 MHz frequency; R>0.85), and furthermore, between relative permittivity and maximum methane production (R>0.9).

Acknowledgement: The research was financed by National Research, Development and Innovation Office (NKFI) FK 146344 project.