Anaerobic digestion of dairy manure influenced by the waste milk from milking operations

It is not uncommon that a significant amount of milk from milking operations is discharged to manure digesters on dairy farms. To understand the effect of milk on the digester performance, experiments using batch digesters (500-mL flasks) were carried out in this study to co-digest milk and dairy manure at different milk levels for biogas production and pollutant reduction, and a total of 8 treatments were examined [i.e., control (without milk) and 1, 3, 5, 7, 9, 14, and 19% milk additions]. The temperature for all digesters was maintained at 37 ± 0.5°C throughout the experimental period, which was 28 d. The results showed that co-digesting milk with dairy manure could increase biogas productivity, with the percent cumulative biogas volume increased by 5.6, 16.3, 26.5, 40.8, 50.2, 79.9, and 103.8%, as compared with the control, for milk addition of 1, 3, 5, 7, 9, 14, and 19% (vol/vol), respectively. However, the CH₄ content in the biogas decreased slightly as the milk content increased (from 66.5% for the control to 63.5% for 19% milk treatment), implying that the added milk could promote CO₂ production. To avoid that, the milk content in the manure should be controlled below 3%. A linear relationship for the total biogas volume produced with the milk content in the manure was revealed, with a correlation coefficient of 0.99. An improved removal efficiency of chemical oxygen demand was observed for milk-treated digesters. Good linear regressions between the total biogas production and the percent chemical oxygen demand decrease and the substrate carbon/nitrogen ratio were also obtained (correlation coefficients: 0.93 and 0.99, respectively). Besides, co-digestion of dairy manure and milk was found to improve substrate solids breakdown, but had little effect on percent volatile fatty acid decrease. In summary, the waste milk co-digested with dairy manure may not cause negative effects on anaerobic digester performance.