Successful and stable operation of anaerobic thermophilic co-digestion of sun-dried sugar beet pulp and cow manure under short hydraulic retention time

Xiomara Gómez-Quiroga, Kaoutar Aboudi, Carlos José Álvarez-Gallego, Luis Isidoro Romero-García

Department of Chemical Engineering and Food Technology, Faculty of Science, University of Cádiz - International Campus of Excellence (ceiA3), P.O. Box No 40, 11510, Puerto Real, Cádiz, Spain.

This work consists of a long-term (621 days) experimental study about biogas production from sun dried sugar beet pulp and cow manure. Thermophilic (55 °C) anaerobic co-digestion was performed in semi-continuous reactors, testing ten hydraulic retention times (30-3 days) (HRTs) and organic loading rates (2–24 gVS/Lreactor∙d) (OLRs). Results showed that the best global system performance (regarding stability, biogas production, and organic matter removal) was achieved at an HRT as short as 5 days (OLR of 12.47 gVS/Lreactor∙d) with a biogas yield of 315 mL/gVSadded. The gradual OLR increase allowed system control and time-appropriate intervention, avoiding irreversible process disturbances and maintaining admissible acidity/alkalinity ratios (<0.8) for HRTs ranging from 30 to 4 days. The accumulation of acetic acid was the main cause of the process disturbance observed at short HRTs. It was deduced that for the HRT of 3 days, the methane productivity was mainly owing to the hydrogen-utilizing methanogens pathway. This research clearly shows how an adequate combination of agro-industrial wastes and livestock manure could be processed by anaerobic co-digestion in short HRTs with great efficiency and stability and deepens in the understanding of the start-up, stability and optimization of the co-digestion.

Keywords: Sun dried sugar beet pulp, Cow manure, Anaerobic co-digestion, Thermophilic, Semi-continuous reactors.