Alternative approach to estimate the hydrolysis rate constant of particulate material from batch data

• An alternative to the commonly used first-order approach is presented.
• A relationship between kh and the 1% criterion of the VDI 4630 is deduced.
• Equation is proposed to directly calculate kh without the need for data fitting.
• Hydrolysis constant kh can then easily be read-off from a table.

As anaerobic batch tests are easy to conduct, they are commonly used to assess the effects of different operational factors on the anaerobic digestion process. Hydrolysis of particulate material is often assumed to be the rate limiting step in anaerobic digestion. Its velocity is often estimated by data fitting from batch tests. In this study, a Monod-type alternative to the commonly used first-order approach is presented. The approach was adapted from balancing a continuously stirred-tank reactor and better accommodates the fact that even after a long incubation time, some of the methane potential of the substrate remains untapped in the digestate. In addition, an equation is proposed to directly calculate the hydrolysis constant from the time when the daily gas production is less than 1% of the total gas production. The hydrolysis constant can then easily be read-off from a table when the batch test duration is known.