Combining thermal hydrolysis with advanced thermal conversion processes for micro-contaminant destruction

Abstract: 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 micro-contaminants 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 diverge away from land application. However, these processes are energy intensive as they require drying upstream. This paper looks at how thermal hydrolysis, both with and without digestion can make these processes sustainable by reducing energy demands due to fundamental improvements in dewaterability and presents results from research where the technologies have been combined to optimize destruction of micro-contaminants. KEYWORDS Thermal hydrolysis; pyrolysis; micro-contaminants; energy demand; drying INTRODUCTION Thermal hydrolysis (TH), a popular pre-treatment to digestion with approximately 130 facilities worldwide including several in North America, 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 thermal hydrolysis was initially conceived as a dewatering aid prior to thermal processing. Although thermally-hydrolysed digested cake dewaters well compared to other process trains, previous work has shown that having digestion downstream of thermal hydrolysis actually deteriorates the dewaterability potential of the hydrolysed cake. This can be 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 thermal hydrolysis 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. Driers usually require 900 -- 1,100 kWhr energy per metric tonne water evaporated. Even with extreme energy recovery this is still orders of magnitude higher than the energy needs of aeration which are considered high and account for half of the energy needs of wastewater treatment. The aim of this work is to highlight the influence thermal hydrolysis has on downstream energy requirements of thermal processing systems with respect to reducing the large energy demands of drying prior to thermal destruction. The work will show the impact of hydrolysis both with and also without digestion on these thermal systems and will look at research work into the impacts of hydrolysis on outputs of pyrolysis. This work is based on a variety of aspects. A study was set up to determine the influence of thermal hydrolysis and operating conditions, with and without digestion on dewaterability and ultimately on drying and advanced thermal processing. The study was a combination of theoretical determinations based on site-data, data collection from multiple sites and analysis, investigation of full-scale operating plants which combined thermal hydrolysis with thermal processing, and a review of literature on combining thermal hydrolysis with advanced thermal processing looking at the impacts of thermal hydrolysis on pyrolysis. As an example of a full-scale facility, thermal hydrolysis was installed on half of the produced waste activated sludge at Psyttalia in Athens, Greece as part of a project to become more energy efficient. Prior to thermal hydrolysis installation, sludge was digested and dried in 4 drum driers to 93% dry solids. Biogas produced from the site was diverted to the drying plant to meet its energy demand. Following a combination of dewatering optimization with thermal hydrolysis, dewatering improved from 21% to 31% dry solids. The influence of this on the energy demands of the drying plant is shown in Figure 1. Prior to the improvements, almost 75% of the biogas was consumed by drying, and the energy demand reduced by 40%. In combination with more biogas generated via digestion, almost three times additional energy was available for non-drying purposes such as the generation of renewable energy. In this instance the energy required for steam to run thermal hydrolysis, was available via high-grade heat recovered from co-generation. Figure 2 expands that data to look at different hydrolysis configurations combined with hydrolysis without digestion. It is based on the potential impacts on dewaterability as shown in Figure 3. Surplus energy is shown as the energy left after accommodating the energy needs of both drying and thermal hydrolysis. In this instance, thermal hydrolysis placed after digestion -- to redestroy the extracellular polymers, provides the greatest overall energy benefit. In this example, thermal hydrolysis without digestion is assumed to have no biogas benefit. However, other data, at both full- and lab-scale show separate digestion of the liquid fraction of the dewatered cake. Here, cake is at 50% dry solids or above, and biogas is produced from the liquid. Laboratory work has looked specifically at energy recovery from waste activated sludge (WAS) with/without thermal hydrolysis and compared this with energy recovered from separate liquid and solid fractions immediately following thermal hydrolysis combined with dewatering (i.e. with no downstream digestion of the sludge). The results showed approximately 30% increase in methane production from 11 litres to 15 litres produced/litre WAS processed with standard thermal hydrolysis followed by digestion. However, when the WAS was directly dewatered and the liquid and solid fractions were separated, almost as much biogas was produced from the liquid fraction with a further 40% from the solids. In combination this resulted in over 60% more energy recovered compared to absence of thermal hydrolysis. The work then studied at the solid dewatered fraction with respect to combining it with drying and downstream pyrolysis. This work investigated the influence of various operating parameters on the production of char and its energy content. Whilst thermal hydrolysis has been associated with the production of a high-quality biosolids agricultural product, its influence on sludge reduction, and more importantly dewaterability, make it complementary to thermal processes. Better dewatering fundamentally reduces energy needs of drying, and is dependent on the configuration of thermal hydrolysis. Combining thermal hydrolysis, either with or without digestion, with thermal processing can help with the large energy requirements of advanced thermal processing facilities.