Effects of enzyme addition and pH on viscosity and soluble molecules during incubation of organic fraction of municipal solid waste at thermophilic conditions

Organic fraction of municipal solid waste (OFMSW) is a valuable feedstock within a circular economy. At well-defined process conditions, enzymatic hydrolysis followed by microbial conversion of hydrolysis products can lead to valuable carboxylic acids. The present study assesses enzymatic hydrolysis of OFMSW at 15% dry matter and thermophilic conditions (55°C). Torque measurements from a four-vessel parallel bioreactor system were made to monitor changes in viscosity of the OFMSW slurry. Additionally, the influence of minimum pH setpoints at pH 5.0 and pH 6.0 on enzymatic hydrolysis as well as on the formation of organic acids was evaluated. Adding enzymes, which are tailored to the substrate, led to a rapid drop in torque at the beginning of the incubation. This reduction in viscosity by liquefaction of the OFMSW slurry can improve the processability of the slurry as well as reduce the energy consumption during stirring. It was possible to correlate the release of glucose as well as the changes in torque to the enzyme addition. A minimum pH setpoint of 6.0 appears to favour organic acid formation from indigenous microorganisms. This resulted in an 26.6% increase of lactic acid to 27.9 ± 0.49 g L -1 and a metabolisation into butyric acid of up to 15.4 ± 1.8 g L -1 . The presented results provide valuable information about the processing of OFMSW for future valorisation within biorefinery processes. • Enzyme addition led to rapid reduction in torque and viscosity. • Liquefaction and reduced disturbances during stirring were achieved. • Substantial concentrations of lactic acid and butyric acid were obtained.