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Effects of Mixing Duration on Biogas
Production and Methanogen Distribution
in Dairy Manure Anaerobic Digesters
Prepared by: Hui Wang, Rebecca A. Larson
March 31, 20151
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
1
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
3
2
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
3
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?
5
4
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
6
5
Fig.1 Design of the pilot-scale digester
Feed tank
Effluent
Pump
F
Sample ports
Heating band
Daily FeedingMixing
Temperature
6
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)
8
7
Cumulative Biogas Production
0
2000
4000
6000
8000
10000
12000
14000
16000
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Bio
gas
Pro
du
ctio
n p
er k
g T
S f
eed
(L)
Time (Days)
NO CON INT
9
80
Fig. 2 Cumulative Biogas Production with 3 Mixing Durations
Biogas Production
b
aa
b
a
ab
0
200
400
600
800
1000
1200
NO INT CON
Bio
gas
pro
du
ctio
n (
L)
Mixing Duration
L biogas per kg VS in digester L biogas per kg TS in digester
10
9
Fig. 3 Biogas production per kg of VS and TS in digester
Specific Biogas Yield
aa
b
0.0
0.5
1.0
1.5
2.0
2.5
NO INT CON
Sp
ecif
ic g
as
pro
du
ctio
n (
L M
eth
an
e
per
kg V
S d
estr
oy
ed)
Mixing Duration
L Methane per kg VS destroyed
11
10
Fig. 4 L methane per kg of VS destroyed with different mixing duration
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
1211
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
13
12
Total Solids Percentage in AD
14
13
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
Solids, Methanogens and Biogas Production
a a a
a a a
b a aa 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
Bio
ga
s p
rod
uct
ion
(L)
Mixing DurationVS % in the digester TS % in the digester
logSUMm (log, base=10) Archaea (log,base=10)
L biogas per kg TS in digesterTS
(%),
V
S(%
), S
um
M&
Arc
haea
(lo
g,
base
=10)
15
14
Fig. 8 Compare of solids, methanogens and biogas production
Methanogens in AD
16
15
Fig. 9 Archaea and sum of 5 tested Methanogens
Archaea
Sum M5
Sample time( day)
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.
17
17
18
Methanogens in AD
19
16
Fig. 10 Methanogens changes in the digester
Limitations
• This study does not take the whole range of mixing
duration: 0,1/4,2/4 . might also need ¾ and 4/4
20
17