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

SLU Authors

• Schnürer, Anna

  • Department of Molecular Sciences, Swedish University of Agricultural Sciences
    
  • Linköping University

Associated SLU-program

Food Waste


Sustainable Development Goals

Ensure access to affordable, reliable, sustainable and modern energy for all
Ensure sustainable consumption and production patterns


UKÄ Subject classification

Biochemistry and Molecular Biology
Bioenergy


Publication identifier

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

Permanent link to this page (URI)

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