The effect of milk co-digested with dairy manure on biogas production and COD removal in batch processes

Abstract

Co-digestion of dairy manure with milk for biogas production was investigated in this study using batch experiments. Lab-scale digesters consisting of 500 mL flasks were employed (effective working volume: 300 mL) with temperature controlled at 37°C. A total of eight treatments at different milk additions were examined, i.e., control (without milk), 1, 3, 5, 7, 9, 14, and 19%. The results showed that the cumulative biogas volume produced over the experimental period increased from around 4984 mL for the control to 10,228 mL for the 19% treatment. In parallel, the cumulative CH₄ volume produced increased from 3306 mL to 6515 mL in the same treatment percentages. The high milk chemical oxygen demand (COD) had no negative impact on the final COD removal by the digestion process, evidenced by the observed good efficiencies of COD removal by 49.7, 50.5, 58.7, 49.0, 62.1, 68.4, 73.4, and 77.8% for the control, 1, 3, 5, 7, 9, 14, and 19% milk treatments. This clearly indicated that COD removal was improved with the increasing milk content in the co-digestion process. There was a good linear relationship between the peak biogas production rates and the increasing milk treatments, with a correlation coefficient of 0.9930 (R²= 0.9861), meaning that about 98.6% of the increase in peak biogas production rate could be explained by the milk addition. The study has provided useful information that it is feasible and beneficial to enhance the overall biogas and CH₄ productivities by batch co-digesting dairy manure with milk.

Keywords:

• Co-digestion
• dairy manure
• biogas production rates
• COD removal

Acknowledgement

The authors wish to thank the Minnesota Legislature Rapid Agricultural Response Fund for providing financial support to this project.