Full proceedings at: http://www.extension.org/pages/72955 Mixing is an important parameter for anaerobic digesters for both design and operation. This is especially true for digesters that treat diary manure, which is a mixture of feces, urine, blood, food wastes, and bedding. Many of the solids fed to the digester have no or low degradability, and some of the large fibers can clog pumps or pipes in the transfer system. Mixing also plays an important role in maintaining a uniform environment for biological processes. However, the energy input for operating mechanical mixers requires a large part of the total energy for a biogas plant. Previous studies have suggested that optimum biogas production does not require continuous active mixing. It is essential to evaluate the mixing duration in order to balance energy inputs and biogas production rates.

1. Effects of Mixing Duration on Biogas
Production and Methanogen Distribution
in Dairy Manure Anaerobic Digesters
Prepared by: Hui Wang, Rebecca A. Larson
March 31, 2015 1

2. Mixing and its Role in AD
Mechanical Mixer
Gas Recirculation
 Start: Heukelekian, 1931, Shaking
 Contact: Biomass and substrate
 Particle size
 Diffusion limitation
 Stratification
 Temperature
 Sedimentation
 Scum
 Inhibitory substance
Source: Chapman, 1989; Parkin and Owen, 1986; Lema et al., 1991, Stenstrom et al., 1983 2
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3. Why Study Mixing Duration?
 Intensity
 Duration
 Location
 Diameter
Energy consumption =
~ 54% to run the biogas plants
5-10% of the energy produced from biogas
Source: Dachs Rehm 2006, FAL 2005
Less energy consumption
More biogas production
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4. Previous Studies on Mixing Duration
 No decrease in gas production with discontinued mixing,
1979
 Continuously mixing produced 8-10% higher CH4 than
2h/d mixing,1982
 2h/3h and 1h/28h mixing produced ~20% more biogas
than continuously mixing, 2013
Coppinger, 1979, Hruska, 1982, Kowalczyk 2013 4
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5. Hypothesis and Objective
 Loading rate: Mixing duration is not critical at low TS
< 10%, Karim 2005
 Retention time (RT): Discontinued mixing causes solids
accumulation which impacts RT
 Microbial community structure
How does mixing duration impact biogas production,
solids and methanogens distribution？
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6. Experimental Design (completely random)
 I fixed factor (Mixing)
 3 levels
30 min/hour (CON): 2*15 min
15 min/hour (INT): 1* 15 min
0 min/hour (NO): 0
 Carried out in triplicates
• Test:
o Gas quantity, Gas quality
o Temperature, pH, TS,VS, VFA, COD, C/N, methanogens
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7. Fig.1 Design of the pilot-scale digester
Feed tank
Effluent
Pump
F
Sample ports
Heating band
Daily Feeding
Mixing
Temperature
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8. Operation Parameters
• Manure source: Local diary farm (Madison, WI)
• Total Solid content: 5.5 ± 0.5 %
• Feed amount: 8 kg of manure per day
• Retention time: 21 days
• Temperature: 33.5 ±0.8 °C
• Mixing rate: 400RPM (safe rate for feeding while mixing)
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9. Cumulative Biogas Production
0
2000
4000
6000
8000
10000
12000
14000
16000
1 2 3 4 5 6 7 8 9 101112131415161718192021
BiogasProductionperkgTSfeed
(L)
Time (Days)
NO CON INT
9
80
Fig. 2 Cumulative Biogas Production with 3 Mixing Durations

10. Biogas Production
b
a
a
b
a
ab
0
200
400
600
800
1000
1200
NO INT CON
Biogasproduction(L)
Mixing Duration
L biogas per kg VS in digester L biogas per kg TS in digester
10
Fig. 3 Biogas production per kg of VS and TS in digester

11. Specific Biogas Yield
a
a
b
0.0
0.5
1.0
1.5
2.0
2.5
NO INT CON
Specificgasproduction(LMethane
perkgVSdestroyed)
Mixing Duration
L Methane per kg VS destroyed
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Fig. 4 L methane per kg of VS destroyed with different mixing duration

12. Solids Sedimentation over Time
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
1T-15 1M-15 1B-15 2T-15 2M-15 2B-15 3T-15 3M-15 3B-15
TS(%)
Fig. 5 Total Solids of 3 digesters at different location after 15 min settling
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13. Solids Sedimentation
0
1
2
3
4
5
6
7
8
TS(%)
Sampling time (min)
Top Middle Bottom
Fig. 6 Total Solids at top, middle and bottom of the digester in 2 hrs
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14. Total Solids Percentage in AD
14
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Fig. 7 Total solids percentage for AD with NO, INT and NO mixing in 4 weeks
0 7 14 21 0 7 14 21 0 7 14 21

15. Solids, Methanogens and Biogas Production
a a a
a a a
b a a
a a a
b
a a
0
200
400
600
800
1,000
1,200
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
NO INT CON
Biogasproduction(L)
Mixing Duration
VS % in the digester TS % in the digester
logSUMm (log, base=10) Archaea (log,base=10)
L biogas per kg TS in digester
TS(%),VS(%),SumM&Archaea(log,base=10)
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Fig. 8 Compare of solids, methanogens and biogas production

16. Methanogens in AD
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Fig. 9 Archaea and sum of 5 tested Methanogens
Archaea
Sum
M5
Sample time( day)

17. Summary
• No mixing has a higher biogas production when TS =
5.5%, compared with INT and CON mixing.
• No mixing will result in low specific gas production
• Recommendation： Operate the mixer to prevent
settling and other operational issues.
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18. 18

19. Methanogens in AD
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Fig. 10 Methanogens changes in the digester

20. Limitations
• This study does not take the whole range of mixing
duration: 0,1/4,2/4 . might also need ¾ and 4/4
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