Combining Thermal Hydrolysis With Advanced Thermal Conversion Processes for Microcontaminant Destruction

Barber, W.P.F.

Joint Conference: Residuals & Biosolids and Innovations in Treatment Technology, 2025

Introduction

There is growing concern related to the potential health impacts of continued application of biosolids, produced during wastewater treatment, to land. These relate to a variety of microcontaminants such as perfluorinated compounds (PFAS etc), microplastics, and other xenobiotic compounds. This has led to a trend in the use of advanced thermal processes such as pyrolysis and gasification, and renewed interest in incineration in a bid to divert away from land application. Thermal hydrolysis (TH), a popular pre-treatment to digestion with approximately 130 facilities worldwide including sites in Canada and United States has typically been associated with producing a high quality biosolids product for land application. In combination with improved digestion performance, elevated dewatering (typically over 30% dry solids) results in less than half the biosolids cake compared to standard digestion. However, less well known is that TH was initially conceived as a dewatering aid prior to thermal processing. Although TH-digested cake dewaters well compared to other process trains, previous work has shown that having digestion downstream of THs actually deteriorates the dewaterability potential of the hydrolysed cake by as much as 20% points. This has been shown to be due to the reproduction of extracellular polymers as a consequence of biological metabolism. The dewaterability improvements afforded by TH become of interest with advanced thermal processing. Systems such as pyrolysis and gasification are limited as they are fundamentally dependent on requiring a dried feedstock, typically over 85% solids. Drying of sludge is extremely energy (and subsequently carbon) intensive, and even with exceptional energy recovery need several orders of magnitude more energy than the energy needs of aeration which are considered high and account for half of the energy needs of wastewater treatment.

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