Hydrothermal treatment of biogas digestate as a pretreatment to reduce fouling in membrane filtration

Stringent regulations on the application and storage of digestate challenge the viability of biogas plants. Current alternatives for reducing the volume of the liquid fraction of digestate include evaporation and membrane processes. For the latter, previous flocculation of solids is required to reduce fouling. In this study, hydrothermal treatment of the separated liquid fraction of digestate was investigated to improve filterability without using flocculants. Temperatures between 120 and 220 °C and times up to 9 h were tested. After treatment, two phases form, a sludge, and a low viscosity liquid. The results show a clear effect of the hydrothermal treatment on viscosity of the liquid phase. Higher treatment intensities resulted in better sludge dewaterability under centrifugation, while the separated liquid phase contained less solids. Excellent filterability for a subsequent ultrafiltration stage can be obtained at 190 °C for 3 h or more. Larger scale tests are recommended. © 2021 Elsevier Ltd