Proceedings available at: http://www.extension.org/67645 Anaerobic digestion has many advantages both environmentally and economically. First, it produces renewable energy in the form of methane, a renewable energy source, which leads to a steady increase in the number of anaerobic manure digesters in the United States. According to the report from the World Dairy Expo held in Madison, Wisconsin in 2009 (Expo'09, 2009), the US dairy industry is taking the lead in adopting anaerobic technology because the majority (over 75%) of operating US manure digesters is installed on dairy farms. It is anticipated that this trend will continue as the country has determined to reduce its reliance on ever diminishing fossil-based energy resources.

1. Anaerobic Digestion of Dairy
Manure Influenced by the Wasted
Milk from Milking Operations
X. Wu, C. Dong, W. Yao, J. Zhu
Dept. of Bioproducts and Biosystems
Engineering
University of Minnesota

2. Introduction
• Milking parlor generates milk contaminated
water ending up in manure lagoon
• Milk has high COD (190,000 mg/L)
• Lagoon digesters are the commonly used
type on current dairy farms
• What will be the impact
of milk on digester
function in terms of
biogas production?

3. Objectives and procedures
• Investigate the overall response of co-
digesting dairy manure with milk using lab-
scale batch anaerobic digesters
• Milk was added to manure at 7 different
levels, i.e., 1, 3, 5, 7, 9, and 14%
• Evaluation was based
on total biogas volume
produced, methane
content, and other
chemical variables

4. Objectives and procedures
• Fresh dairy manure was collected from a
dairy farm equipped with a scraped manure
handling system
• Raw milk for the experiment was obtained
from another dairy farm close to
Waseca, MN
• One control digester without milk addition
was run side by side with the milk treated
digesters
• Biogas composition was determined for
each digester every two days using a GC

5. Results
• Increasing milk content increases
cumulative biogas production

6. Results
• Increasing milk content increases
cumulative biogas production
0
2000
4000
6000
8000
10000
12000
0 2 4 6 8 10 12 14 16 18 20
Totalbiogasproduced(mL)
Percent of milk added (%)

9. Results
• Maximum biogas production rate for each
treatment can be obtained by the slopes of the
volume curves.
10
14
18
22
26
0 3 6 9 12 15 18 21
Biogasproductionrate(mL/h)
Percent milk added (%)

12. Results
• Milk effect on COD reduction and TKN
The average values of COD and TKN for the batch digestion process with different milk content1
Milk content COD (mg/L)*
TKN (mg/L) C/N ratio2
Initial Final Reduction Initial Final Reduction Initial Final3
(%) (%)4
0 (Control) 14500 7300 49.7 2794 2742 1.9 5.19 2.665
1% 15561 7700 50.5 2817 2792 0.9 5.52 2.766
3% 17683 7300 58.7 2863 2840 0.8 6.18 2.577
5% 19805 8100 59.1 2909 2794 4.0 6.81 2.898
7% 21927 8300 62.1 2955 2908 1.6 7.42 2.859
9% 24049 7600 68.4 3002 2970 1.0 8.01 2.5610
14% 29354 7800 73.4 3117 3080 1.1 9.42 2.5311
19% 34659 7700 77.8 3232 3206 0.8 10.7 2.4012
13
*COD – chemical oxygen demand; TKN – total Kjeldahl nitrogen14
15

13. Conclusion
The following conclusions can be reached based on
the data from this study.
• Added milk will increase biogas volumetric
production and the maximum biogas production
rate as compared to the control (7 and 18% vs. 1
and 3%).
• Added milk will reduce the methane content in the
biogas produced, thus increasing CO2 production.
To avoid excessive CO2 production, the milk
content in the digester liquid should be ≤ 3%.
• Added milk will increase COD removal efficiency
from 49.7% (no milk) to 77.8% (19% milk)