Role of Microbial Hydrolysis in Anaerobic Digestion

Abstract

There is a growing need of substrate flexibility for biobased production of energy and value-added products that allows the application of variable biodegradable residues within a circular economy. It can be used to balance fluctuating energy provision of other renewable sources. Hydrolysis presents one of the biggest limitations during anaerobic digestion. Methods to improve it will result in broader process applicability and improved integration into regional material cycles. Recently, one focus of anaerobic digestion research has been directed to systems with a separate hydrolysis–acidogenesis stage as it might be promised to improve process performance. Conditions can be adjusted to each class of microorganisms individually without harming methanogenic microorganisms. Extensive research of separate biomass pretreatment via biological, chemical, physical or mixed methods has been conducted. Nevertheless, several methods lack economic efficiency, have a high environmental impact or focus on specific substrates. Pretreatment via a separate hydrolysis stage as cell-driven biotransformation in a suspension might be an alternative that enables high yields, flexible feeding and production, and a better process control. In this review, we summarize existing technologies for microbial hydrolytic biotransformation in a separate reactor stage and the impacts of substrate, operational parameters, combined methods and process design as well as remaining challenges.

Phase Separation in Anaerobic Digestion: A Potential for Easier Process Combination?

“…The advantages and disadvantages of phase separation in AD have been described in several research papers and reviews e.g. from Chatterjee and Mazumder (2019), Van et al (2020) or Menzel et al (2020). The present work aims to complement previous research on the topic, by highlighting the value-adding possibilities which can result from coupling the hydrolytic/ acidogenic with the methanogenic stage of AD.…”

“…Most of these facultative anaerobic or strictly anaerobic hydrolytic microorganisms are very resilient against varying chemical and physical conditions. A recent review focused on the hydrolytic process stage in depth (Menzel et al, 2020) among other previous reports.…”

“…Depending on the available substances, different ratios of acetic acid, carbon dioxide and hydrogen are formed by acetogenic bacteria like Acetobacterium or Clostridium species (Borja, 2011). The hydrolytic/acidogenic stage, eventually exhibiting acetogenic conversions, is also called dark fermentation (DF) or acid fermentation and typically operated with HRT of several days only (Rosgaard et al, 2007;Menzel et al, 2020; Van et al, 2020). A pH-value of about 5.0 up to 6.0 is optimal, while a lower pH-value can lead to increased retention time; alkaline pH values may totally inhibit the process (Zhang et al, 2005;Moestedt et al, 2016).…”

“…When the first stage of an AD application is not dependent on the viability of methanogens due to separated phases, the whole process can gain more stability against a varying organic loading rate (OLR) and chemical inhibitors like oxygen, ammonia, and toxins, that are brought into the process by feedstock feeding. While different fermentation variants exist, like acetic-, butyric-, ethanol-or mixed-hydrolysis, Menzel et al (2020) consider acetic and butyric fermentation as most suitable for multi-stage AD applications. This type of fermentation is favored under thermophilic conditions at pH-values of between 5.0 and 6.0.…”

“…In fact, this preserves nutrients and microorganisms in the system, allowing for the use of higher OLR, e.g. discussed by Menzel et al (2020). By recirculating the liquid fraction of the AD effluent after solidliquid separation, a wash out of fluidized microorganisms to the next stage will decrease.…”