Unraveling the impacts of high solids content on hydrolysis and methane production of complex substrates through experimental and modeling approaches

Total solids (TS) content disproportionately affects the anaerobic digestion (AD) of macromolecules, yet the underlying mechanism remains unclear. This study dissected complex substrates into cellulose, hemicellulose, lipid, and protein to investigate the effects of the high (20%) and low (6%) TS contents on their degradations in AD. The results showed that methane production occurred in two phases at 6% TS content, but three phases at 20% TS. High TS content accelerated lipid and protein hydrolysis but inhibited lignocellulose degradation. In an AD with complext feedstock, hydrolysis was identified as the rate-determining step at 6% TS, while propionate acetogenesis was determined as the rate-determining step at 20%TS, which also reduced the cumulative methane generation by 20.9%. In addition, a multi-phase Gompertz model was developed and used to characterize the complex AD dynamics and calculate kinetic parameters for each phase. With this model, peaks of methane production in BMP test illustrated the synchronicity of complex substrate degradation, and therefore, can be used to optimize feedstocks for industrial biogas plants.