Valorisation of Anaerobic Digestate: Towards Value-Added Products

Biogas digestate as a sustainable phytosterol source for biotechnological cascade valorization

“…Although anaerobic digestate is generally considered a valuable commodity predominantly used as fertilizer, the sheer volume of its accumulation and the ecological problems accompanying massive fertilization require the implementation of more sustainable and economical utilization routes. For this, anaerobic digestate needs to be established as a resource for additional cascade valorization to facilitate comprehensive biomass utilization throughout the whole biogas production process (Guilayn et al, 2022 ; Kaur et al, 2020 ; Monlau et al, 2015 ; Tumaševičiūtė & Ignatavičius, 2019 ). In this study, we show that valuable sterol lipids are not degraded during anaerobic digestion in typical agricultural and biowaste biogas plants and therefore remain in the final digestates, which provides a novel opportunity for digestate valorization as a sterol resource for biotechnological applications.…”

“…However, the conversion of organic matter into biogas in anaerobic digestion is generally incomplete and produces significant amounts of residual solid and liquid digestate as a by‐product (Guilayn et al, 2020 ; Möller & Müller, 2012 ; Monlau et al, 2015 ). This digestate offers a great potential for cascade valorization because it contains high concentrations of undigested organic compounds, microbial biomass as well as inorganic components such as potassium, nitrogen and phosphorus compounds (Guilayn et al, 2022 ; Kaur et al, 2020 ; Monlau et al, 2015 ). Especially recalcitrant organic compounds like lignin and non‐hydrolysable lipids persist to a large extent or are only marginally transformed into smaller moieties and (pre‐)humic material under typical anaerobic digestor settings (Molinuevo‐Salces et al, 2013 ; Möller, 2015 ; Tambone et al, 2009 ).…”

“…Despite its high potential for cascade valorization, anaerobic digestate is today predominantly used as organic fertilizer in agriculture and for soil improvement and amendment (Guilayn et al, 2022 ; Nkoa, 2014 ; Sheets et al, 2015 ; Tambone et al, 2010 ). However, this practice has raised serious concerns regarding its general sustainability because over‐fertilization, nutrient runoff, eutrophication of surface waters, spreading of pathogens and ammonium and nitrous oxide emission caused by intense digestate land application pose significant environmental risks on soils, water bodies and the atmosphere (Kaur et al, 2020 ; Monlau et al, 2015 ; Nkoa, 2014 ).…”

“…Based on these concerns, restrictive regulations of digestate land application have been enacted by legislative bodies in most countries with significant biogas production capacities (European Parliament, 2019 ; U.S. Environmental Protection Agency, 2021 ; Wang et al, 2020 ). Accordingly, cascade valorization of anaerobic digestate is increasingly used to maximize biomass utilization and minimize its adverse effects (Guilayn et al, 2022 ; Kaur et al, 2020 ; Monlau et al, 2015 ; Tumaševičiūtė & Ignatavičius, 2019 ). Thermo‐chemical valorization, such as pyrolysis, combustion or hydrothermal carbonization, generates additional energy and value‐added products from whole or solid digestate in the form of heat, bio‐char, bio‐oils or syngas (Guilayn et al, 2022 ; Monlau et al, 2015 ; Sheets et al, 2015 ; Szulc et al, 2021 ).…”

“…Accordingly, cascade valorization of anaerobic digestate is increasingly used to maximize biomass utilization and minimize its adverse effects (Guilayn et al, 2022 ; Kaur et al, 2020 ; Monlau et al, 2015 ; Tumaševičiūtė & Ignatavičius, 2019 ). Thermo‐chemical valorization, such as pyrolysis, combustion or hydrothermal carbonization, generates additional energy and value‐added products from whole or solid digestate in the form of heat, bio‐char, bio‐oils or syngas (Guilayn et al, 2022 ; Monlau et al, 2015 ; Sheets et al, 2015 ; Szulc et al, 2021 ). Liquid digestate fractions are typically valorized by nutrient recovery technologies such as ammonia stripping and struvite precipitation to produce concentrated fertilizer solutions (Macura et al, 2019 ; Tampio et al, 2016 ; Törnwall et al, 2017 ).…”