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Research article - Peer-reviewed, 2022

Identifying targets for increased biogas production through chemical and organic matter characterization of digestate from full-scale biogas plants: what remains and why?

Ekstrand, Eva-Maria; Bjorn, Annika; Karlsson, Anna; Schnurer, Anna; Kanders, Linda; Yekta, Sepehr Shakeri; Karlsson, Martin; Moestedt, Jan;

Abstract

Background This study examines the destiny of macromolecules in different full-scale biogas processes. From previous studies it is clear that the residual organic matter in outgoing digestates can have significant biogas potential, but the factors dictating the size and composition of this residual fraction and how they correlate with the residual methane potential (RMP) are not fully understood. The aim of this study was to generate additional knowledge of the composition of residual digestate fractions and to understand how they correlate with various operational and chemical parameters. The organic composition of both the substrates and digestates from nine biogas plants operating on food waste, sewage sludge, or agricultural waste was characterized and the residual organic fractions were linked to substrate type, trace metal content, ammonia concentration, operational parameters, RMP, and enzyme activity. Results Carbohydrates represented the largest fraction of the total VS (32-68%) in most substrates. However, in the digestates protein was instead the most abundant residual macromolecule in almost all plants (3-21 g/kg). The degradation efficiency of proteins generally lower (28-79%) compared to carbohydrates (67-94%) and fats (86-91%). High residual protein content was coupled to recalcitrant protein fractions and microbial biomass, either from the substrate or formed in the degradation process. Co-digesting sewage sludge with fat increased the protein degradation efficiency with 18%, possibly through a priming mechanism where addition of easily degradable substrates also triggers the degradation of more complex fractions. In this study, high residual methane production (> 140 L CH4/kg VS) was firstly coupled to operation at unstable process conditions caused mainly by ammonia inhibition (0.74 mg NH3-N/kg) and/or trace element deficiency and, secondly, to short hydraulic retention time (HRT) (55 days) relative to the slow digestion of agricultural waste and manure. Conclusions Operation at unstable conditions was one reason for the high residual macromolecule content and high RMP. The outgoing protein content was relatively high in all digesters and improving the degradation of proteins represents one important way to increase the VS reduction and methane production in biogas plants. Post-treatment or post-digestion of digestates, targeting microbial biomass or recalcitrant protein fractions, is a potential way to achieve increased protein degradation.

Keywords

Anaerobic digestion; Degradation efficiency; Residual methane potential; Macromolecules; Full-scale biogas plants; Trace metals; Ammonia; Enzyme activity; Biogas; Digestate

Published in

Biotechnology for Biofuels and Bioproducts

2022, volume: 15, number: 1, article number: 16
Publisher: BMC

Authors' information

Ekstrand, Eva-Maria
Linköping University
Bjorn, Annika
Linköping University
Karlsson, Anna
Linköping University
Swedish University of Agricultural Sciences, Department of Molecular Sciences
Linköping University
Kanders, Linda
Linkoping University
Yekta, Sepehr Shakeri
Linkoping University
Karlsson, Martin
Linkoping University
Moestedt, Jan
Linkoping University

Associated SLU-program

Food Waste

Sustainable Development Goals

SDG7 Affordable and clean energy
SDG12 Ensure sustainable consumption and production patterns

UKÄ Subject classification

Biochemistry and Molecular Biology
Bioenergy

Publication Identifiers

DOI: https://doi.org/10.1186/s13068-022-02103-3

URI (permanent link to this page)

https://res.slu.se/id/publ/116389