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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, 2015 1

Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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Page 1: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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

Page 2: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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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Page 3: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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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Page 4: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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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Page 5: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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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Page 6: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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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Page 7: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

Fig.1 Design of the pilot-scale digester

Feed tank

Effluent

Pump

F

Sample ports

Heating band

Daily FeedingMixing

Temperature

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Page 8: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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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Page 9: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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

Page 10: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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

Page 11: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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

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Fig. 4 L methane per kg of VS destroyed with different mixing duration

Page 12: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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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Page 13: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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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Page 14: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

Total Solids Percentage in AD

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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

Page 15: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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

Page 16: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

Methanogens in AD

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Fig. 9 Archaea and sum of 5 tested Methanogens

Archaea

Sum M5

Sample time( day)

Page 17: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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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Page 18: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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Page 19: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

Methanogens in AD

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Fig. 10 Methanogens changes in the digester

Page 20: Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

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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