Proceedings Biodigester Workshop March  2002

http://www.mekarn.org/procbiod/khang.htm

The effect of fibre level in feedstock, loading rate and retention time on the rate of biogas production in plug-flow and liquid displacement biodigesters 

Duong Nguyen Khang, Le Minh Tuan1 and T R Preston2

 Department of Animal Physiology and Chemistry, Faculty of Animal Husbandry and Veterinary Medicine, University of Agriculture and Forestry, Thu Duc distict, Ho Chi Minh City, VietNam
1 Biogas centre, University of Agriculture and Forestry, Thu Duc distict, Ho Chi Minh City, VietNam
2 University of Tropical Agriculture Foundation, Chamcar Daung, Phnom Penh, Cambodia 

Abstract

 

 

Introduction

All animal waste products contain organic and inorganic nutrients with potential to decompose in the environment with high Chemical Oxygen Demand (COD), methane and ammonia emissions and the release of excess nutrients and pathogens. Concerns have been expressed in recent years on the effects of pollutants from agricultural and industrial activities on sea, land and air (Lusk 1998) in relation to ecology and climatic changes. The governments of most Asia countries are signatories to the Kyoto protocol on mitigating factors that could affect global climate, and to cease all human emissions after the year 2020. The implementation of this protocol in the South Asia region places commercial pig producers in a highly precarious position, with the present system of waste disposal. However, agricultural and livestock waste products, if properly managed, could be converted to value-added products such as bio-fuel, bio-fertilizer, livestock feed and soil conditioners. Some farms use the aerobic digestibility process to convert the organic fraction of animal waste into biogas a renewable and sustainable form of energy. This gas is used for cooking, to heat water to create steam to power electric generators or simply burnt in gas engines to generate electricity (Bui Xuan an et al 1997; Piccinini et al 1998; Lusk 1998; Moog et al 1998). In-addition the aerobic digestibility process provides for the destruction of pathogens, odor and pollution prevention strategies (Doelle 1997).  

 In this study it is hypothesised that pigs fed high-fibre diets will produce feedstock for biodigesters that is fermented more slowly in the digester.  Thus a higher hydraulic retention time will be required for this type of feedstock.

Materials and methods

Location

The experiments were carried out at the experimental farm of the University of Agriculture and Forestry, Thu Duc district, Ho Chi Minh City, VietNam. The mean air temperature is 28.2 °C and the relative humidity 76.5 %.

Animals and housing

Eight Mong Cai pigs (5 - 6 months of age and 50 kg live weight on average) were used for the experiment. Animals were divided in two groups. Feedstocks of different fibre content were obtained by giving combinations of the following diets to the pigs (Table 1).

Table 1: Composition of the experimental diets (on DM basis)

 

Duckweed

ECL

Rice bran

Broken rice

SMB

Total

High fibre

 

 

 

 

 

 

DM basis

7

7

86

 

 

100

N*6.25

2.45

1.75

10.32

 

 

14.52

DM, g/d*

84

84

1032

 

 

1200

FM, g/d*

1527

240

1147

 

 

2914

Low fibre

 

 

 

 

 

 

DM basis

 

 

 

85

15

100

N*6.25

0

0

0

7.65

6.75

14.4

DM, g/d*

0

0

0

1020

180

1200

FM, g/d*

0

0

0

1133

200

1333

* for a pig of 30 kg liveweight

 

Clean water was available ad libitum during the whole experiment. The pig farm was located close to the laboratory and the samples of waste water were stored at 4°C until utilized.

Reactors

The laboratory scale digesters with liquid volume of 21 litres (working volume = 15 liters) were made from tubular PVC sanitary fittings to simulate the plug-flow and liquid displacement systems (Photo 1). The anaerobic digesters were fed once a day with different loading rates. Gas production was measured during 24 hours by a liquid displacement systems consisting of the container tube with the balance scale. Gas pressure was recorded at the time of  gas collection  by manometer water tube. The digesters were fed at different loading rates, different hydraulic retention times and two feedstocks from pigs fed high and low-fibre diets.

Experimental design

The experiment included 32 laboratory scale digesters and was designed as a 2*4*4 factorial with two levels (low and high) of fiber-containing feedstock, four levels (1, 2, 3, and 4 %) of  total solids in the feedstock, and four levels (10, 20, 30, and 40 days) of hydraulic retention times. The laboratory scale digesters were fed the fibre-containing feedstock over  periods of  six weeks.  Data were collected in the seventh week on each of  7 days.

Measurements
Chemical composition and characteristics of feedstocks

Feedstocks of different fibre content were taken for analyses of dry matter, neutral detergent fiber, acid detergent fiber, lignin, cellulose content, ash, COD, pH, temperature, fecal coliform and Escheria coli. The dry matter and ash of feedstocks were determined according to procedures of AOAC (1990). The neutral detergent fibre (NDF), acid detergent fibre (ADF), lignin and cellulose concentrations of feedstocks were determined according to the procedure of Van Soest et al. (1991). COD, Escheria coli and fecal coliform density of the feedstock samples of the output of the laboratory scale  digesters were determined according to standard methods (APHA 1985).

Biogas yield and pressure

Gas production was measured by the water displacement method during 7 days of the experiment. Gas pressure was measured by the differences in height of the water column in the manometer tube.

pH, temperature of input and output of batch reactor

pH and temperature of input and output of the reactor were immediately determined by pH meter. The samples of the input and output of the laboratory scale digesters were taken during 7 days of the seventh week.

Chemical Oxygen Demand (COD), Escheria coli and fecal coliform density

COD, Escheria coli and fecal coliform density of the feedstock samples of the output of the laboratory scale batch digesters were determined according to standard methods (APHA 1985).

Statistical analysis

Data were analyzed by ANOVA using the General Linear Model option of the Minitab Statistical Software version 12.21.

 

Results and Discussion

Chemical composition of feedstocks

During the experimental period (50 days), there were only small changes in the chemical composition of feedstocks of different fibre content. 

Table 2. Chemical composition of feedstocks of different fibre content (% DM basis, except for dry matter which is on fresh basis )

 

High fibre

Low fibre

Dry matter

28.8±0.23

61.7±0.29

NDF

34.4±0.10

18.5±0.11

ADF

22.8±0.20

17.9±0.17

Lignin

12.5±0.14

9.0±0.06

Ash

20.7±0.33

41.5±0.15

Cellulose

11.9±0.04

9.36±0.13

 

Characteristics of the wastewater

Characteristics of the waste used in the experiment are shown in Table 3.  

Table 3: Characteristics of the wastewater

 

High fibre

Low fibre

COD, mg/litre

709±42

782±45

Fecal coliform (x 10-7 MPN/g)

5.07±0.39

4.54±0.33

E coli, x 10-7 MPN/g

3.05±0.24

2.64±0.17

Temperature, 0C

28.5±0.19

29.4±0.22

pH

7.07±0.01

7.09±0.01

Effects of different levels of fibre on feedstock on parameters of reactors

Table 3. Performance of reactors on feeding different levels of fibre

 

High fibre

Low fibre

SE

P

Gas yield (litres/kg OM)

430

250

11.4

0.001

Pressure (mm)

6.09

3.61

0.09

0.001

COD (mg/litre)

48.0

675

3.96

0.001

Fecal coliform (x10-4 MPN/g

5.15

2.85

0.56

0.01

E. coli (x10-4 MPN/g)

1.65

0.11

0.26

0.001

Temperature (0C)

28.1

29.0

0.17

0.001

pH

6.98

7.01

0.01

0.05

 

Effects of manure concentration on parameters of reactors          

Table 4. Performance of reactors with different loading rates (1 to 4% of dry matter in input substrate)

Dry matter in input (%)

1

2

3

4

SE

P

Gas yield (litres/kg OM)

309

315

360

375

16.1

0.01

Pressure (mm)

3.58

4.25

5.39

6.19

0.13

0.001

COD (mg/litre)

310

345

376

416

5.6

0.001

Fecal coliform (x10-4 MPN/g

1.56

2.99

5.00

6.46

0.79

0.001

E. coli (x10-4 MPN/g)

0.79

0.84

0.88

1.01

0.37

0.98

Temperature (0C)

28.4

28.5

28.5

28.6

0.24

0.95

pH

7.01

7.00

7.00

6.98

0.01

0.36
 
Effects of retention time on parameters of reactors

Table 5. Performance of reactors with different retention times (10 to 40 days)

Retention time (days)

10

20

30

40

SE

P

Gas yield (litres/kg DM)

199

332

01

426

16.3

0.001

Pressure (mm)

5.22

5.55

4.64

4.00

0.13

0.001

COD (mg/litre)

114

205

437

690

5.6

0.001

Fecal coliform (x10-4 MPN/g

4.46

4.20

4.02

3.33

0.79

0.77

E. coli (x10-4 MPN/g)

2.09

0.64

0.57

0.22

0.37

0.01

Temperature (0C)

28.1

28.5

28.7

28.7

0.24

0.32

pH

6.96

6.99

7.01

7.02

0.01

0.001

 

Conclusions

 

Acknowledgments

The authors are grateful to the Swedish International Development Authority (Sida/SAREC) for equipment and funding for this study. We are also grateful to the University of Agriculture and Forestry, Ho Chi Minh City for allowing two of the authors to participate in this study and to use their facilities to accomplish the work. The technical assistance of Mr. Trieu is also gratefully acknowledged. 

References

AOAC 1980 Official methods of analysis 13th. Edition.  Association of Official Analytical Chemists. Washington, DC

APHA 1985 Standard methods for the Examination of Water and Wastewater, 16th edition. American Public Health Association, American Water works Association and Water Pollution Control Federation, Washington, DC

Doelle H W 1997 Biofuel Production. In: Workshop/Seminar Proceedings on The Role of Biotechnology in Health, Food, and Energy Supply for a Sustainable Development of the Pacific Island Nations.

Bui Xuan An, Preston T R and Dolberg F 1997 The introduction of low-cost polyethylene tube biodigesters on small scale farms in Vietnam. Livestock Research for Rural Development  (9) 2: http://www.cipav.org.co/lrrd/lrrd9/2/an922.htm

Lusk P 1998 Methane Recovery from Animal Manures: The Current Opportunities Casebook. NREL, Colorado USA.

Moog F A, Avilla H F, Agpaoa E V, Valenzuela F G and Concepcion F C 1998  Promotion and utilization of polyethylene bio-digester in small farming systems in the Philippines. Internet Conference on Integrated Bio-Systems. (Editors: Eng-Leong Foo and Tarcisio Della Senta).

Piccinini S, Fabbri C and Verzellesi  F 1998 Integrated Bio-systems for Bio-gas Recovery from Pig Slurry: Two Examples of Simplified Plants in Italy. Internet Conference on Integrated Bio-systems. UNESCO, MIRCEN and Institute for Advanced Studies

Steel R G D and Torrie J H  1980 Principles and procedures of statistics. Biometrical approach. 2nd edition. McGraw-Hill Book Company New York, NY.

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