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.

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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

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