Livestock Research for Rural Development 17 (11) 2005 Guidelines to authors LRRD News

Citation of this paper

Effect of length: diameter ratio in polyethylene biodigesters on gas production and effluent composition

San Thy, P Buntha, T Vanvuth, T R Preston*, Duong Nguyen Khang**, Soukanh K***, Boualong Phouthone***, Choke Mikled**** and N Sopharoek

CelAgrid, UTA Cambodia,
thy.pfd@online.com.kh
*University of Tropical Agriculture Foundation, Colombia,
trpreston@mekarn.org
** Nong Lam University, Vietnam
*** National Agriculture Forestry Research Institute, Lao PDR
**** Chiang Mai University, Thailand

Abstract

The treatments arranged as a 4*2 factorial were: Length and diameter ratio and retention time  of plastic plug-flow biodigesters.  The design was a single changeover with experimental periods of 40 days on each retention time. The length: diameter ratios wee 8:0.6, 5:0.6, 3:0.6 and 2:0.6m with hydraulic retention times of 10 or 20 days.  For each retention time, the amount of manure (DM) was kept constant by adding water according to retention times. The manure solid concentration in the influent for 10 days retention time was 4 % and  for 20 days was 8 %.

The proportion of ammonia-N in total N increased markedly with different forms from the raw manure of 13.2±1.83, influent slurry in the range of 27.55± 3.05 and the effluent of 48.17±1.27 as pfor ent.

Biogas production as litres per 100 litres liquid volume biodigester, and ammonia concentrations in effluent, did not differ between different length:diameter ratios nor between retention times.

Key words: biodigester, length of biodigester, retention times, gas production, pig manure, effluent composition


Introduction

Biodigesters play an important role in the recycling of organic wastes, producing methane-rich gas for cooking, with positive impacts on the environment and on human and animal health (Preston and Rodríguez 1998). Soeurn Than (1994) and Bui Xuan An et al (1997a) showed that biodigesters made from tubular polyethylene were well accepted when introduced to households in rural areas of Cambodia and Vietnam because of the low price and simple installation. The impact of the technology in South Vietnam is demonstrated by the more than 30,000 units that have been paid for and installed by farmers between 1996 and 2002 (Duong Nguyen Khang et al 2000).  Biodigesters provide methane-rich gas for cooking while the effluent is a good source of fertilizer nutrients for crops growing on land and water (Kean Sophea and Preston 2001), and for fishponds (Pich Sophin and Preston 2001; San Thy and Preston 2003).

Many factors influence gas production and the fertilizer value of the effluent in tubular plug-flow biodigesters. Studies have been made on the effect of retention time, temperature, types of manure and concentration of solids in the influent (Boodoo et al 1979; Bui Xuan An and Preston 1999; San Thy et al 2003). However, the configuration of tubular polyethylene biodigesters, namely the ratio of diameter to length has not been studied. Most biodigesters of this type have been between 8 and 10m in length with a diameter of 1 m (Duong Nguyen Khang personal communication), equivalent to a ratio of 10.1:1 and 12.7:1 between length and cross-sectional diameter. Smaller biodigesters (2m long by 0.63m diameter; ratio of 6.4:1 of length to cross-sectional area)) were used by San Thy et al (2003) and appeared to be very efficient with production rates exceeding 100% of the biodigester liquid volume. It was therefore hypothesized that different  ratios of length to cross-sectional area in tubular polyethylene might influence the rate and efficiency of gas production.

Hypotheses


Materials and methods

Location

The experiment was conducted in the four countries participating in the MEKARN project (http://www.mekarn.org) : Cambodia (CelAgrid, Cambodia), Thailand (Chiang Mai University), Vietnam (Nong Lam University) and Lao (Livestock Research Centre). The activities in the different countries were initiated at different times: in Vietnam from September to November 2003, in Cambodia and Lao from February to April 2004; and Thailand from June to July 2004.

Experimental treatments and design

The treatments arranged as a 4*2 factorial were: Length: diameter ratio and hydraulic retention times. Each location  was considered as a replicate.

Length: diameter ratios

These were as follows (length: cross-sectional area is in brackets):

Retention time

This was 10 or 20 days.

The design was a single changeover with experimental periods of 40 days on each retention time (Table 1).

Table 1: Changeover arrangement of retention times within fixed lengths of the biodigesters.

 

LR2

LR3

LR5

LR8

Period 1

10

20

10

20

Period 2

20

10

20

10

In each location, four experimental biodigesters were constructed according to the design developed by San Thy et al (2003). Tubular polyethylene film of 63 cm diameter was used to construct 4 biodigesters in each locaton(2, 3, 5 and 8m length).

Inoculation

At the beginning, the biodigesters were inoculated with effluent from a functioning biodigester. The ratios used were 60% of digester effluent and 40 % of water. A mixture of manure and water was added to give an initial solids concentration of 4% (Table 2). At the time of changeover, water was added at 50 % of biodigester liquid volume to facilitate the outflow of organic solid materials from the first period that might effect  biogas yield in the next period.

Table 2: Inoculation influent and manure water mixture

Biodigester length, m

L2

L3

L5

L8

Biodigester volume, m3

0.64

0.96

1.59

2.55

Liquid volume, liters

477

716

1194

1910

Influent input

 

 

 

 

  Effluent from other biodigester

286

430

716

1146

  Manure kg

38.2

57.3

95.5

153

  Water, liters

153

229

382

611

Construction of biodigesters

The 16 plug-flow biodigesters (in each location there were 4 biodigesters) were made from tubular polyethylene film (internal diameter 0.63m), mounted in shallow trenches lined with bricks (to ensure the dimensions were exactly the same size of plastic biodigester), to provide a liquid volume in the proportion of 80% of the total biodigester capacity (Photo 1). The biodigesters were installed in an area with the same microclimate condition by shading them with corrugated iron roof at 3 m above the ground. During the subsequent adaptation and data collection periods, the fresh pig manure and water were added in the proportion indicated for each loading rate treatment (Table 3).


Photo 1: The construction details of each unit of four biodigesters

Manure

Pig manure was used in each location. The loading rate was 4 kg DM per m of liquid volume. The manure was collected daily in the early morning from the pig pen and stored in a polyethylene sack. The pigs were fed a mixed feed formulated according to each location (Vietnam and Thailand fed by commercial feed and Lao and Cambodia fed by formulation feed, or high fiber feed such as water spinach, cassava silage,  distillery waste and brewery spentd grain mixed with commercial feed).

The biodigesters were charged daily at exactly the same time and with the amounts of fresh manure and water according to the treatments and the liquid volume of the biodigester (Table 3). The indicated amount of pig manure was fixed on a DM basis. The amount of water added was  determined by retention time (Table 3).

Table 3: Details of inputs of manure and water for the two hydraulic retention times in periods 1 and 2

 

L2

L3

L5

L8

Biodigester volume, m3

0.637

0.955

1.59

2.54

Liquid volume, liters

477

716

1194

1910

Period 1(1-40 days)

 

 

 

 

Retention time, days

10

20

10

20

Daily slurry input, liters

47.7

35.8

119.4

95.5

Fresh manure, kg

9.5

14.3

23.9

38.2

Water, liters

38.2

21.5

95.5

57.3

Solids concentration, %

4

8

4

8

Period 2 (41-80 days)

 

 

 

 

Retention time, days

20

10

20

10

Daily slurry input, liters

24

72

60

191

Manure, kg

9.5

14.3

23.9

38.2

Water, liters

14.3

57.3

35.8

152.8

Solids concentration, %

8

4

8

4

 

Data collection and analyses

The experimental data were recorded daily during the last 20 days of each experimental period. Samples of fresh pig manure and effluent were taken daily on days 21 to 40, immediately before (manure) and after (effluent) charging the biodigester.

The samples of fresh manure were bulked and mixed every 10 days, and effluent every 7 days, prior to taking representative samples for analysis of total N and ammonia-N using a Foss-Tecator Kjeldahl apparatus and for organic matter by ashing the samples in a furnace oven (AOAC 1990). DM content was determined by microwave radiation (Undersander et al 1993).

Gas production was measured daily using the system of water displacement developed by San Thy et al (2003) (Photo 2). The change in volume was recorded 2 to 3 times a day to determine daily gas production.


Photo 2: The inverted containers for collecting the biogas by water displacement

Statistical analyses

The data were subjected to analysis of variance (ANOVA) using the General Linear Model (GLM) of the MINITAB software Release 13.31 (2000). The model was:

Yij k = µ + Ti +Pj + Ak+ eijk

Where:


Results and Discussion

Manure at different locations

There were no differences in the DM, OM and total N concentration of the manure between locations (Table 4). Ammonia-N as a proportion of total N was highest in he pig manure in Lao and Vietnam and lowest in Cambodia and Thailand.

Table 4: Pig manure characteristics from different countries

 

Cambodia

Lao

Vietnam

Thailand

SE/P

DM, %

33.1

35.2

29.1

26.8

2.82/0.356

OM,%

69.7

70.3

76.5

75.7

2.48/0.18

Total N, mg/kg

6221

7453

4350

7856

802.7/0.086

NH3-N, mg/kg

581

1114

654

803

77.65/0.006

NH3-N as proportion of total N, %

9.54

15.02

15.03

10.2

0.97/0.01

Influent

Ammonia-N as percentage of total N was higher for the more diluted influent (10 day retention time (Table 5).

Table 5: Effect of water added to pig manure according to retention times on slurry input

 

Retention times, days

 

 

10*

20**

SE

Prob

DM, %

4.78

7.38

0.286

0.001

OM, %

67.5

71.0

1.039

0.022

N, mg/litre

994

2156

161.7

0.001

NH3-N, mg/litre

297

402

22.3

0.003

NH3-N in Total N, %

32.2

20.8

1.89

0.001

* manure/water ratio: 1:4 for 10 days retention time and solid concentration with 4%
**manure/water ratio: 1:1.5 for 20 days retention time and solid concentration with 8%

OM content of the DM of the manure was lower in Lao and Cambodia than in Thailand and Vietnam (Table 6).  Correspondingly, NH3-N as proportion of total N was higher in Cambodia and Lao. 

Table 6: Influent characteristics from different countries

 

Cambodia

Lao

Thailand

Vietnam

SE/P

OM, % in DM

65.1

59.0

76.2

76.7

1.47/0.001

N, mg/liter

1188

1802

1923

1386

228.7/0.001

NH3-N, mg/liter

348

511

251

288

31.6/0.001

NH3-N as % of total N

31.1

30.7

23.4

20.7

2.67/0.022

 Ammonia-N as proportion of total N was higher in the influent than in manure , presumably reflecting microbial action between the time taken to sample raw manure, and the adding of water and stirring activities to make the influent slurry (Figure 1).

Biodigester effluent

There was no effect of the length of the biodigester on the DM, OM, total N and ammonia N (total ammonia as well as percent of total N) in the effluent (Table 7).

Table 7: Effect of biodigester length and retention times on effluent composition

 

Biodigester length, m

 

HRT, days

2

3

5

8

SE/P

 

DM, %

10

3.11

5.09

3.98

5.7

0.55/0.009

20

6.72

6.24

5.07

5.53

Mean

4.91

5.67

4.53

5.62

0.40/0.113

 

OM, %

10

62.8

57.5

62.2

62.0

2.90/0.938

20

71.6

67.6

68.8

69.9

Mean

67.2

62.6

65.5

66.0

2.08/0.423

 

N, mg/litre

10

1237

1214

1349

1248

240/0.088

20

1773

2266

1237

2063

Mean

1505

1740

1293

1656

172/0.256

 

Ammonia-N, mg/litre

10

615

536

746

582

191/0.136

20

1017

886

555

1270

Mean

816

711

650

926

137/0.479

 

NH3-N as proportion of total N, %

 

10

50.3

44.7

60.0

49.9

5.812/0.236

20

49.0

39.0

49.9

62.3

Mean

49.6

41.8

54.9

56.1

4.17/0.06

 

There was no effect of the manure dilution rate (retention time) on the proportion of total N in effluent in the form of ammonia (Table 8), nor of the biodigester length (Figure 2).

Table 8: Effect of retention times or water/manure on effluent composition

 

Manure/water ratio

 

 

1:4

1:1.5

SE

Prob

DM, %

5.76

7.36

0.30

0.001

OM, %

61.67

70.83

1.88

0.001

N, mg/litre

1534

2186

151.3

0.003

NH3-N, mg/litre

747

1095

128.8

0.061

NH3-N in Total N, %

49.4

46.9

3.82

0.638

San Thy et al (2003), using pig manure with different loading rates of 2.93, 1.46 and 0.92 kg DM/day/m3 liquid volume and hydraulic retention times of 10, 20, and 30 days, observed that the proportion of ammonia N to total N increased with longer retention time but when pig manure loading rate was fixed, it was not affected by retention time.

These experimental data do not support the original hypothesis that shorter biodigesters and retention time would support a greater degree of conversion of organic-N in the influent to ammonia-N in the effluent. However, the substantial improvement in ammonia-N as proportion of total N in the transition from manure to influent (diluted manure) from 9.54-15.03% to 20.7-31.% to 41.8-56.1%) is in accordance with the  findings of Pedroza et al (2001) who reported increases from 20 in the influent to 60 in the effluent and San Thy et al (2003) who  reported  increases from 20 to 50-60 % of ammonia-N in total N in influent and effluent.


Figure 2: Effect of biodigester length on the proportion of NH3-N in total N in effluent

Biogas production
Adaptation period

After inoculation of the biodigester and an  adaptation period of about 8 days, the biodigesters were charged daily with manure and water according to treatment. The trend in rate of gas production during this phase (Figure 3) was similar to that reported by San Thy et al (2003).

 

Figure 3: Biogas production rate after adaptation after inoculation  (period 1)

 

Figure 4: Biogas production rate after adaptation (period 2)

 

Gas production (during collection period)

There were no differences between biodigester dimensions for gas production as litres per kg DM (and OM) of manure, or as proportion biodigester liquid volume (Table 9).

Table 9: The interaction of length and hydraulic retention times on gas production

 

Biodigester length, m

 

HRT, day

2

3

5

8

SE/P

 

Liters /day

 

10

233

404

670

962

40.2/0.43

20

272

437

633

1090

Mean

253

420

652

1026

31.6/0.001

 

Liters/kg DM

 

10

73.4

86.8

85.1

78.3

3.54/0.27

20

88.8

91.7

83.5

86.4

Mean

81.1

89.2

84.3

82.4

2.78/0.18

 

Liters/kg OM

 

10

102

124

118

112

4.97/0.21

20

127

127

119

120

Mean

114

125

118

115

3.91/0.16

 

Biogas production per liquid volume, m3/m3

 

10

0.49

0.56

0.56

0.5

0.03/0.27

20

0.57

0.61

0.53

0.57

Mean

0.53

0.59

0.55

0.54

0.02/0.17

 

% of  liquid volume

 

10

49.0

56.4

56.2

50.4

3.31/0.17

20

57.1

61.1

53.0

57.1

Mean

53.0

58.7

54.6

53.7

1.96/0.17

Gas production in different countries

The biogas production differed among the countries with highest values for Cambodia, followed by Lao, Vietnam and Thailand  (Table10). We have no obvious explanation for these differences.

Table 10: Biogas production from different countries (average of four dimensions of biodigester)

 

Cambodia

Lao

Vietnam

Thailand

SE/P

Liters/day

1122

754

365

110

30.3/0.001

Liters/kg DM

156

102

55.6

23.1

2.68/0.001

Liters/kg OM

224

146

72.7

32.4

3.76/0.001

% of liquid volume

104

72

32.5

11.6

1.89/0.001

All measures of gas production showed increases for the longer retention time of 20 days (Table 11).  This is in agreement with data in the literature (Figure 5) which showed a peak in production at around 20 day retention times and then a decline.

Table 11: Effect of retention time on gas production

 Retention time days

Litre/day

Litre/kg DM

Litre/kg OM

m3/m3

% as liquid vol

10

568

80.9

114

0.53

53.0

20

608

87.6

123

0.57

57.1

SE/P

23.6/0.25

2.08/0.03

2.92/0.04

0.02/0.06

1.47/0.06

 

Figure 5: Relationship between retention times and gas production (literature data:  Boodoo et al 1979,
San Thy et al 2004 b,  San Thy et al 2003, Safley et al 1987, Polprasert et al 1982a, Hayes et al 1979,
 Bui Xuan An  and Preston 1999,
Lotte et al 1996,  Khang et al 2002,  Bui  Xuan An et al  1995)

 

Conclusions

Acknowledgement 

The authors would like to thank the Swedish Agency for Research Cooperation with Developing Countries (SAREC) for funding this study through the regional MEKARN project, and all friends and research teams in riparian countries for their cooperation during the experiment.


References

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Received 10 June 2005; Accepted 20 October 2005; Published 1 November 2005

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