Effect of retention time on gas production and fertilizer value of biodigester effluent

San Thy, T R Preston and J Ly*

University of Tropical Agriculture Foundation
Chamcar Daung, PO Box 2423, Phnom Penh 3, Cambodia
santhy@utafoundation.org
regpreston@utafoundation.org

* Present address: Swine Research Institute,
PO Box 1, Punta Brava, La Habana, Cuba
jlyca@yahoo.com

 

Abstract

Two experiments were carried out to study gas production and fertilizer value of the effluent in plug-flow, tubular, plastic biodigesters with hydraulic retention times of 10, 20 or 30 days (BRT10, BRT20 or BRT30). There were three biodigesters each of 510 litres liquid volume in each experiment which consisted of three consecutive periods (retention times) of 40 days arranged in a 3*3 Latin Square. In experiment 1, the quantity of fresh pig manure was 5.1 kg/day, mixed with 46, 20 or 12 litres of water to give retention times of 10, 20 or 30 days, respectively.  In experiment 2, the proportions of pig manure and water were maintained constant to give a total solids content of 60 g/litre in the influent, which was added at rates of 51, 25.5 and 17 kg daily for retention times of 10, 20 or 30 days, respectively. Gas production was measured daily by water displacement in inverted lightweight containers (tubular polyethylene supported by bamboo strips) suspended in 200 litre drums filled with water. Influent and effluent were analyzed at weekly intervals for DM, OM, pH and total nitrogen and ammonia-nitrogen. Gas production was measured daily but only the data for the last 10 days of each period were used in the statistical analysis.

With a fixed daily input of fresh manure, neither the rate of gas production (1.04, 1.20 and 1.12 volumes of biogas per unit liquid volume of the biodigester) nor the efficiency (493, 606 and 567 litres of biogas/kg of manure organic matter added to the biodigester), was influenced by retention time (10, 20 or 30 days, respectively). However, when the solids concentration of the influent was fixed at 60 g/kg, rates of gas production were reduced by increasing retention times (1.62, 1.19and 0.81 volumes biogas/unit liquid volume of biodigester for 10, 20 and 30 days retention); efficiency was better for 20 and 30 days retention (550 and 547 litres biogas/kg OM) than for 10 days (376 litres/kg OM). The proportion of ammonia-N in total-N increased from  a range of 0.077 to 0.12  in the fresh manure to a range of 0.46 to 0.65 in the effluent and did not appear to be affected by retention time or loading rate.

It is concluded that when fresh pig manure is the substrate in polyethylene plug-flow biodigesters the optimum retention time is between 10 and 20 days with a solids concentration in the influent of 60 g/litre.  The retention time apparently has no effect on the degree of conversion of organic N to ammonia-N. 

Key words: Biodigester, effluent, fertilizer value, gas production, pig manure, polyethylene, retention time
 

Introduction

The high population density is putting pressure on the use of natural resources causing damage to the world environment (Bui Xuan An and Preston 1998; Ding Jieyi and Han Yujin 1984). A particularly serious problem is the pollution caused by large commercial farms and factories (Preston and Leng 1989). The increased agricultural productivity has also had big effects on the soil fertility depletion. One of the ways to solve the problem is the promotion of integrated farming systems, with minimal external inputs and recycling of all wastes. The most important feature of this approach is the recycling of animal wastes in order to prevent deterioration of soil fertility through loss of nutrients and organic matter, erosion and salinity (Preston and Rodriguez 1996). For instance applying the manure to the soil can reduce environmental pollution and also improve the fertility of the soil through recycling of plant nutrients to the soil. 

Biodigesters play a crucial role in the conversion of organic matter to methane-rich biogas, with positive impacts on the environment and on human and animal health. and Soeurn Than (1994) have demonstrated that plastic tube biodigesters can be a low-cost source of energy and partly reduce the problem of severe energy shortage for households in rural areas of Vietnam and Cambodia.  However, it has been argued that few farmers used biogas in practice (Kristoferson and Bokalders 1991), although its use is increasing rapidly in some developing countries under extensive support from governments and agencies (Marchaim 1992; Karki 1996). Besides environmental preservation, biodigesters provide a very good source of fertilizer for crops on land and water, and for fishponds (Preston and Rodriguez 1996). Biogas used for cooking reduces the need for firewood and also the time and labor for finding other fuels. It also saves the costs of buying wood (NRC 1981).

The integration of livestock with trees, food crops and aquaculture is seen as the most appropriate way to use the natural resources in a system that is productive and sustainable in the long term (Preston 2000). Biodigester effluent can be applied to crops (rice field, cassava, and other perennial crops), vegetables (salad, tomatoes cabbage and water spinach) and in ponds (fish or water plants). In this connection, Rodriguez and Preston (1997) and Leng (1999) reported that yields of duckweed responded linearly to increasing levels of nitrogen in the effluent from pig manure. Similar responses were observed by Sophea and Preston (2001) from applying biodigester effluent to water spinach. Biodigester effluent is potentially superior to raw manure fertilizer because the anaerobic digestion process results in conversion of organic nitrogen in the manure to ionized ammonia (NH₄⁺), which can be used directly by plant roots (Forchhammer 1994).

According to previous research (Boodoo et al 1979; Bui Xuan An and Preston 1999), there are many factors that influence gas production and the fertilizer value of  the effluent, such as retention time, temperature, types of manure and concentration of solids in the influent. In the current study, it is proposed to evaluate the effect of hydraulic retention time of diluted pig manure in plug-flow polyethylene biodigesters on rate of gas production and the fertilizer value of the effluent.

 

Objectives

• This was to determine the gas production rate and the total nitrogen, ammonia nitrogen, pH, and chemical oxygen demand in the effluent from biodigesters with different retention times

 

Hypothesis

• It is hypothesized that increasing the retention time will increase the proportion of organic nitrogen converted to ammonia-N in the effluent

 

Materials and methods

Location

The experiments were conducted in the experimental farm of the University of Tropical Agriculture Foundation (UTA) located on the campus of the Royal University of Agriculture (RUA), Chamcar Daung, Dangkor district, Cambodia, about 10 km from Phnom Penh City.

 

Experimental plan

Two experiments were carried out:

• The first experiment “Effect of retention time in biodigesters charged daily with a constant daily amount of pig manure” began on May 10, 2002 and finished on October 9, 2002;

• The second “Effect of retention time in biodigesters charged with a constant manure concentration in the influent” began on June 14, 2002  and finished October 16, 2002.

 

Experimental treatments and design

In both experiments, the design of biodigester, the experimental design and the treatments were the same. The treatments were:

• BRT10 retention time 10 days

• BRT20 retention time 20 days

• BRT30 retention time 30 days

The experimental design was a 3*3 Latin square arrangement of the three retention times with 3 periods each of 40 days, applied to 3 biodigesters (Table 1). The first 30 days of each period were of adaptation to the new retention time followed by 10 days of measurements.

Manure

Pig manure was collected daily in the early morning from the pig pen of Mong Cai animals. The pigs were being fed a mixed feed formulated from wheat bran, broken rice, fishmeal, salt and premix as detailed in Table 2. 

Biodigesters

The six plug-flow biodigesters (in each experiment there were three biodigesters) were made from tubular polyethylene film (internal diameter 0.64m) and mounted in shallow trenches lined with bricks to ensure the dimensions were exactly the same (2 m length, 0.6 m depth and 0.6m width), to provide a liquid volume in the proportion of 80% of the total biodigester capacity. This was calculated to be 510 litres (Photo 1). The biodigesters were installed in an area with the same microclimate condition by shading them with plastic net 3 m above the ground. At the beginning of each experimental period, the biodigesters were inoculated with 140 litres of effluent from another biodigester, followed by a mixture of 5 kg fresh pig manure and 45 litres water applied each day for 10 days. During the subsequent adaptation and data collection periods, the fresh pig manure and water were added in the proportions indicated for each loading rate treatment.

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

The quantity and characteristics of the materials that were used in the construction of the biodigesters and the installation process are set out in Appendix 1. 

Manure input

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 (Tables 3 and 4). 

Experiment 1

The amount of fresh pig manure was fixed at 5.1 kg (1.18 kg DM) per day with addition of different amounts of water (Table 3) in order to give the different retention times. This resulted in concentrations of dry matter in the input material of 2.3, 4.6 and 6.9 % (weight / volume basis). 

 

Experiment 2

The concentration of manure in the influent was maintained constant at 6.0% solids (DM). This required the addition daily of 10.7, 5.4 and 3.6 kg fresh manure, and 40.5, 20.2 and 13.5 litres of water (Table 4).

 

Data collection and analyses

The experimental data were recorded daily during the last 10 days of each experimental period. Samples of fresh pig manure and the corresponding effluent were taken daily on days 31 to 40, immediately before (manure) and after (effluent) charging the biodigester. They were stored in a refrigerator at -12^oC. Gas production was measured daily throughout the experiment but only the data for the last 10 days were used in the statistical analysis.

The samples of fresh manure were bulked and mixed every 5 days, and of effluent every 3 days, prior to taking representative samples for analysis of nitrogen and ammonia 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). Chemical oxygen demand (COD) was measured in representative samples of effluent taken two times during each collection period, and analysed immediately (Andrew et al 1996). The pH of the manure and effluent was measured daily using a glass electrode and digital meter. VFA was determined by steam distillation in three samples of effluent taken at 3 day intervals during each measurement  period. 

Gas production was measured daily by collecting the gas in inverted light-weight containers (a plastic-covered bamboo frame) of 200 litres capacity, floating inside oil drum containers filled with water, and permanently connected to the gas outlet of each biodigester (Photo 2). The change in volume was recorded four 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:

Y_{ij k} = µ + T_i +P_j + A_k+ e_ijk

Where:

Y_{ij k}: Dependent variable

µ: Overall mean

T_i: Treatment effect

P_j : Period effect

A_k : Biodigester effect

e_ijk : Ramdom error

 

Results

Experiment 1: Effect of retention time on gas production and fertilizer values with input of constant quantity of manure

Manure and slurry input

There were differences in the DM content of the manure and the pH, and in the total N concentration during the successive periods of experiment 1 (Table 5). Ammonia-N as a proportion of total N was in the range of 0.023 to 0.029 and did not vary between periods. Ammonia-N as proportion of total N was a little higher in the input slurry (0.04 to 0.08 mg/litre) presumably reflecting microbial action between the time taken to sample the raw manure and add water to make the influent slurry.

The differences in DM and N content, among retention times,  reflect the effect of the addition of water. There were no differences due to retention time in pH, OM, VFA, nitrogen, NH₃-N and the proportion of NH₃-N in total N.

 

Biogas production 

At the beginning, the biodigesters were inoculated with 140 litres of effluent from another biodigester, followed by 5 kg of fresh pig manure and 36 litres of water per day for 10 days. Gas production began on the 7^th day from the start of the inoculation period (Figure 1 and gradually increased during the adaptation period before reaching a plateau on or about day 25 of the adaptation phase in period 1 (Figure 2). Subsequently gas production rates were similar on all three retention times (Figures 2, 3 and 4).

 

Figure 1: Initiation of gas production during the ten  day period of inoculation

 

Figure 2: Biogas generation during period 1.

 

Figure 3: Biogas production during the second period

 

 

Figure 4: Biogas production during the third period

 

Mean values for rates and efficiency (litres gas per input of DM and OM or per liquid biodigester volume) of gas production during the 10 days of data collection (Table 6; Figures 5, 6, 7) were not influenced by retention time, period or biodigester. The rate and efficiency of gas production tended to be lower for 10 day retention time with no differences between 20 and 30 days (Figure 7). There were no differences in any of the measurements between biodigesters.  The air temperature varied from 26 to 31 ºC during the experiment and did not differ among periods.

 

 

Figure 5: The effect of retention time, period and biodigester on daily gas production (Experiment 1)

 

Figure 6: Biogas production per liquid biodigester volume with manure quantity constant

 

Figure 7: Biogas production per kg manure OM with constant amount of manure daily

 

 Effluent characteristics

The DM,  total N and ammonia-N  concentrations in the effluent increased with retention time reflecting the DM concentrations in the influent. pH and ammonia-N as proportion of total N increased with retention time as did the COD values (Table 7 and Figure 8).  

 

Figure 8: Effect of retention time on proportion of ammonia-N to total N in effluent
from biodigester  with constant daily manure input

 

Experiment 2: Effect of retention time on gas production and fertilizer values with constant manure concentration

Manure and slurry input

There were no differences in the characteristics of the manure, nor in the input slurry, among the three periods in Experiment 2 (Table 8). Ammonia-N as a proportion of total N increased when the manure was diluted with water prior to charging the biodigester and reached values of between 0.5 and 0.6 in the effluent (Figure 9). 

Table 8: Manure and slurry input characteristics
Manure characteristics	Period
	1	2	3	SEM	Prob
pH	6.33	6.21	6.86	0.091	0.048
DM, %	30.6	27.2	28.4	2.03	0.656
OM, %	78.0	73.7	84.3	2.97	0.263
VFA, m-equiv/100g	88.9	73.6	62.4	5.104	0.124
Slurry input	Treatments
	BRT10	BRT20	BRT30	SEM	Prob
pH	6.93	6.73	6.89	0.131	0.696
DM, %	2.70	2.77	2.82	0.182	0.936
OM, % in DM	81.5	81.0	80.9	3.940	0.996
VFA, m-equiv/100g	5.00	5.37	2.14	2.352	0.715

Biogas production

The initiation of gas production was observed after some 7 days during the period of inoculation (Figure 10).

 

Figure 10: Gas production during the inoculation period in Experiment 2

Production rate then increased steadily until reaching a plateau after 20 to 25 days in the adaptation period, with production rates almost always higher for the 10 day retention treatment than the 20 and 30 day retention times (Figures 11, 12 and 13).

Figure 11 : Gas production in the adaptation phase of the first period with constant manure concentration

 

Figure 12: Gas production in the adaptation phase of the second period with constant manure concentration

 

Figure 13: Gas production in the adaptation phase of the third period with constant manure concentration

Mean values for gas production rate during the 10 day collection period were highest for the 10 day retention treatment and lowest for 30 days (Table 9; Figures 14, 15).  By contrast, efficiency of gas production was lowest for 10 day retention time with no differences between 20 and 30 days (Figure 16). Gas production tended to be higher during the second and third period and efficiency 40 to 50% greater during these periods compared with period 1. The ambient temperature was 2 to 3 ºC higher in periods 2 and 3 compared with period 1.  There were no differences in any of the measurements between biodigesters.

 

Figure 14: Effect of retention time, period and biodigester,  on rate of gas production, with constant
concentration of solids in influent (Experiment 2)

 

 

Figure 15: Biogas production per liquid biodigester volume with constant DM concentration in the influent

 

Figure 16: Biogas production per kg manure OM with constant DM concentration in influent

Effluent characteristics

There was no effect of retention time on the characteristics of the effluent (Table 10). Ammonia-N as proportion of total N tended (P=0.17) to increase with retention time. COD values were low and not affected by retention time.

 

Discussion

Gas production

The two experiments used contrasting methods of varying the hydraulic retention rate: in Experiment 1the daily input of manure was held constant while in Experiment 2 the concentration of manure in the influent was held constant. Thus loading rate, expressed as kg manure DM/m³ of digester liquid volume, was constant in Experiment 1 (2.29) for all retention times while in Experiment 2, the loading rates were 6.1, 3.0 and 1.96 for retention times of 10, 20 and 30 days, respectively. It was apparent from the results that retention time was poorly related with gas production and that the determining factor was the loading rate as illustrated by the regression of gas production on loading rate (R² = 0.90) using data from both experiments (Figure 17).  This conclusion is supported by the data in the literature (Table 11; Figure 18), which in spite of the wide range in locations showed a relatively strong relationship (R² = 0.68) between rate of gas production (m³ gas/m³ digester liquid volume) and the loading rate.

 

 

Figure 17: Relationship between loading rate and gas production (experiments 1 and 2)

 

 

Figure 18: Relationship between loading rate and gas production (data from Table 11)

 

Effluent fertilizer value

In both experiments the increase in retention time resulted in slight increases in the proportion of  ammonia-N in total N (from 0.40 to 0.53 in Experiment 1 and from 0.50 to 0.60 in Experiment 2, for 10 and 30 day retention times, respectively). In both experiments the proportion of ammonia-N in total N was low in the raw manure (range 0.02 to 0.04) and increased markedly in the effluent (range of 0.40 to 0.60). These data support the original hypothesis that longer retention times would support a greater degree of conversion of organic-N in the influent to ammonia-N in the effluent. The effect of the retention time on this parameter is discussed in subsequent experiments to verify this (Santhy et al 2003).  The substantial improvement in ammonia-N as proportion of total N in effluent as opposed to the influent (from 0.02 to 0.50) is in accordance with the findings of Pedroza et al (2001) who reported increases from 0.2 in the influent to 0.60 in the effluent.

 

Chemical oxygen demand

The COD values in effluent registered in both  experiments were at the low end of values reported in the literature (Table 12). Only in experiment 1 was there a relationship with retention time, with higher values for the longer retention times. However, as retention time was confounded with concentration of DM in the effluent, the higher COD values for the longest retention time could have been due to the higher solids concentration creating less favourable circumstances for breakdown of organic matter.  The negative effect of the longer retention is the opposite of the findings reported by Polprasert et al (1985) where COD was lower with longer retention times (24.8, 17.7 and 14.9 mg/litre  for retention times of 30, 50 and 70 days. Khang et al (2002) reported that COD increased from 48 to 675 mg/ litre as the manure concentration was increased from 1 to 4% DM in the influent). The trend for effects of retention time were similar to our findings, although of a higher order of magnitude,  as COD increased from 114 to 690 mg/litre with  retention times increasing from 10 to 40 days.   

 

Conclusions

With a fixed daily input of fresh manure, neither the rate of gas production (0.97, 1.2 and 1.12 volumes of biogas per unit liquid volume of the biodigester) nor the efficiency (546, 619 and 598 litres of biogas/kg of manure organic matter added to the biodigester), were influenced by retention time under study (10, 20 or 30 days, respectively). However, when the solids content of the influent was fixed at 60 g/kg, rates of gas production were reduced by increasing retention times (1.62, 1.19 and 0.81 volumes biogas/unit liquid volume of biodigester for 10, 20 and 30 days retention), while efficiency was better for 20 and 30 days retention (550 and 547 litres biogas/kg OM) than for 10 days (376 litres/ kg OM).

With a fixed concentration of manure DM in the influent, the proportion of ammonia-N in total-N increased from a range of 0.077 to 0.12 in the fresh manure to a range of 0.50 to 0.60 in the effluent and did not appear to be affected by retention time.

Analysis of the data from the two experiments and from reports in the literature showed that the major determinant of rate of gas production was the loading rate expressed as kg of manure solids per m³ of liquid digester volume.

It is concluded that when fresh pig manure is used as substrate in polyethylene plug-flow biodigesters the optimum retention time is between 10 and 20 days with a solids concentration in the influent of 60 g/litre.

Acknowledgements

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 UTA staff (Lylian Rodriguez, Pol Samkol, Hean Pheap and Srey Sam An) for their help during the experiments. The senior author expresses deep gratitude to his parents  and wife for their encouragement and very strong support during this study. This paper formed part of the MSc thesis (MEKARN-SLU, Uppsala, May 2003), of the senior author.

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