Long term impact of lignin accumulation in cattle manure digesters on biomass activity and chemical post treatment

• Long-term anaerobic biodegradability of lignin-rich cattle manure was 42 %. • The highest solubilization of digested cattle manure was observed at pH 12. • Digested cattle manure solubility does not corelate to biodegradability. • Lignin is refractory to chemical and anaerobic treatment. • Bacteroidota and Firmicutes were dominant in cattle manure and digestate. Animal manures, which are typically rich in lignocellulosic content, pose both significant environmental impacts and opportunities for renewable energy. Lignin is particularly resistant to anaerobic degradation. In this work, the effect of lignin accumulation on anaerobic biomass activity and its potential degradation by chemical post treatment was evaluated. Anaerobic digestion of lignin-rich cattle manure (CM) in a 12-L continuously stirred tank reactor for 224 days (d) under mesophilic conditions, at an average organic loading rate of 2.9 g COD/L/d, and sludge retention time (SRT) of 30 d achieved average steady-state COD, lignin, cellulose, and hemi-cellulose removal efficiencies of 41 %, 11.9 %, 54.5 %, and 55.4 %, respectively. Fluorescence excitation-emission matrix parallel factor (EEM-PARAFAC) analysis for the cattle manure and digestate indicate the presence of aromatic compounds, potentially a lignin hydrolysis by-products, which may be inhibitory. The application of chemical post treatment to enhance the anaerobic biodegradability of lignin-rich digested cattle manure (DCM), achieved additional biodegradability of 15%–24 %, which did not correlate with solubility. This work has demonstrated that despite the accumulation of lignin in the digestate, the activity of the acclimatized lignocellulose-degrading bacteria was enhanced, and thus post treatment technologies should be assessed not only based on their impact with respect to lignin solubilization, but also with respect to how they affect microbial activity.