Effect of effluent recycling on gas production in polyethylene biodigesters

Duong Nguyen Khang, Chhay Ty*, Soukarn Koenouchanh** and T R Preston***

Nong Lam University, Vietnam,
* CelAgrid, Cambodia,
** NAAFRI, Laos,
*** UTA-TOSOLY, Socorro, Colombia

Abstract

Four tubular polyethylene biodigesters of 64 cm diameter were used in a 4*2 factorial change-over design to study the effect on gas production of four rates of recycling the effluent and two retention times. The effuent recycling rates were equivalent to 0, 25, 50 and 75% of the liquid loading rate; retention times were 10 or 20 days. The biodigesters were charged with pig manure at a loading rate of 4 kg total solids per m³ of liquid volume per day. 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. Gas production was measured daily but only the data for the last 10 days of each period were used in the statistical analysis. Influent and effluent were analyzed at weekly intervals for DM, OM, pH, total nitrogen and ammonia-nitrogen. COD was measured in the effluent.

Gas production rate, as proportion of liquid volume and per unit of DM and OM in the slurry input, was higher with 20 than 10 days retention time, and showed a curvilinear trend with recycling rate of effluent, with production increasing from 0 to 25% level of recycling followed by a decline to lowest values at 75% recycling. DM content in the effluent and COD values were increased linearly as the rate of recycling of effluent was increased. There was a positive linear relationship between the DM content of the effluent and the COD of the effluent. pH values followed the same trend as gas production with the highest value for the 25% rate of recycling. Recycling rate had no effect on the N content of the effluent nor on the proportion of the N in the form of ammonia-N.

It was concluded that there was a slight (5%) increase in gas production with a 25% rate of recycling the effluent to the biodigester but production was reduced when the recyling rate was increased to 50 and 75%.

Key words:  Ammonia, COD, nitrogen, pig manure, retention time

Introduction

Low cost plastic biodigesters have had a major impact in Vietnam with more than 50,000 units installed during the past fifteen years (Khang et al 2002a). The technology is now becoming even more important in view of the escalating price of oil, and the associated increase in the price of liquid gas (now 1.7 USD per kg), used for cooking. Biogas is therefore attracting the attention of more and more farmers as a low-cost alternative to liquid gas. Another recent development is the initiative of some farmers to use the biogas as fuel in an internal combustion engine to produce electricity. Both these developments emphasize the need to increase the rate (gas output per unit volume of the biodigester) and efficiency (gas produced per unit of input substrate) of the plastic biodigesters.

Recent work in Cambodia (San Thy et al 2003) has shown that high yields of gas can be obtained when the loading rate is 6 kg DM/m³ of digester liquid volume, in plastic plug-flow biodigesters configured with a length: diameter ratio of 3:1.

The principle behind the low-cost plastic biodigester is one of “plug-flow”, that is the charge of manure: water moves slowly along the tube, pushed by the entry of new material. By comparison, high efficiency biodigesters are of circular construction, and use some form of mechanical mixing in order to accelerate the rate of fermentation (San Thy et al 2003).

This study aims to compare the effects on rate and efficiency of gas production of a method of mixing the contents of the plastic “plug-flow” biodigesters. We hypothesize that when effluent is returned to the input of the biodigester this could act as a form of mixing the contents and also could accelerate the rate at which the bacteria colonize the fresh manure.

Materials and methods

Location

The experiment was carried out in three locations of three participating countries of the Mekarn project. In Cambodia, the experiment was conducted at the ecological farm of the Center for Livestock and Agriculture Development (CelAgrid), located in Rolous village, Rolous commune, Kandal Stoeung district, Kandal province, about 26 km from Pnom Penh City, Cambodia. In Laos, the experiment was conducted at the Livestock Research Center, about 40 km from Vientiane, Laos. In Vietnam, the experiment was conducted at the experimental farm of Nong Lam University, about 20 km from Ho Chi Minh City, Vietnam.

Experimental treatments

Four biodigesters were made from tubular polyethylene film (internal diameter 0.64 m), enclosed in brick walls (Photo 1), with internal measurements of 60 cm width and 70 cm height to ensure the correct dimensions of the digesters, and to provide a liquid volume in the proportion of 75% of biodigester volume. The dimensions of the biodigesters were 2.52 m long and 64 cm diameter.

The experimental design was a 4*2 factorial single change-over with four rates of recycling the effluent and two retention times. The effluent recycling rates were equivalent to 0, 25, 50 and 75% of the liquid loading rate;

• P₀: No recycling of effluent
• P_0.25: Effluent recycled at rate of 25% of the liquid loading rate
• P_0.5: Effluent recycled at rate of 50% of the liquid loading rate
• P_0.75 Effluent recycled at rate of 75% of the liquid loading rate

Retention times were 10 or 20 days. The biodigesters were charged with pig manure at a loading rate of 4 kg total solids per m³ of liquid volume per day. 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 (San Thy et al 2003). Gas production was measured daily but only the data for the last 10 days of each period were used in the statistical analysis. Influent and effluent were analyzed at weekly intervals for DM, OM, pH, total nitrogen and ammonia-nitrogen. COD was measured in the effluent.

The manure was collected at a pig farm near the experimental site. A constant loading rate with 4 kg manure DM per 1m³ digester liquid volume was applied to biodigester P₀. For the 3 biodigesters P_0.25, P_0.5 and P_0.75, the manure loading rate was the same, but effluent was recycled to the input at the rate of 25, 50 or 75% of the liquid loading rate. The fresh manure was previously mixed with enough water (or water and effluent) so as to ensure a retention time of 10 or 20 days (Table 1 and Table 2). The experiment lasted for 2 months. The three locations were considered as replicates.

Measurements

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 by the method of KMnO4. The pH of the manure and effluent was measured daily using a glass electrode and digital meter. Gas production was measured daily using the system of water displacement developed by San Thy et al (2003).

Statistical analysis

Data were analyzed by ANOVA using General Linear Model and the Tukey Pair-wise comparison in the Minitab Statistical Software, version 13.31.

Results

Manure and slurry input

During the experimental period, there were only small changes in the chemical composition of manure and slurry (Table 3).

Biogas production

Gas production rate, as proportion of liquid volume and per unit of DM and OM in the slurry input, showed a curvilinear trend with recycling rate of effluent, with production increasing from 0 to 25% level of recycling followed by a decline to lowest values at 75% recycling (Table 4; Figures 1 and 2).

Gas production rate, as proportion of liquid volume and per unit of DM and OM in the slurry input, was increased when the retention time was increased from 10 to 20 days (Table 5).

Effluent characteristics

DM content in the effluent and COD values were increased linearly as the rate of recycling of effluent was increased (Table 6; Figures 3 and 4. As expected there was a positive linear relationship between the DM content of the effluent and the COD of the effluent (Figure 5). pH values followed the same trend as gas production with the highest value for the 25% rate of recycling. Recycling rate had no effect on the N content of the effluent nor on the proportion of the N in the form of ammonia-N.

Discussion

In general the rates of gas production were high (close to 100% of liquid digester volume) reflecting the loading rate of 4 kg manure solids per m³ digester liquid volume. Bui Xuan An and Preston (1999) showed that gas production rate in similar tubular polyethylene biodigesters was linearly related with loading rate over the range 0.66 to 2.76 kg manure DM per 1 m³. in the experiment reported by San Thy et al (2003), increasing the retention time from 10 to 20 days with a loading rate of 2.3 kg DM/m³ liquid volume decreased the rate of gas production (from 1.61 to 1.19 litres of gas/litre liquid volume) but increased the efficiency (from  289 to 431 litres/kg DM). Similar findings were reported by Duong Nguyen Khang et al (2002b).   In the present experiment with a loading rate of 4 kg DM/m³, comparable figures for 10 and 20 day retention times were 0.93 and 1.01 litres gas/litre liquid volume, and 248 and 259 litres/kg DM). The lower loading rate used by San Thy et al (2003) and Duong Nguyen Khang et al (2002b) may have been the reason for the differences in gas yield and efficiency.

The effect of recycling effluent to the biodigester input is mainly to increase the solids content entering the biodigester (effluent is obviously higher in solids that the displaced water). It is unlikely to have a mixing effect but there is the possibility that it might enhance microbial inoculation of the incoming manure solids.  However, there is no obvious explanation for the increase in gas production with 25% recycling followed by the decrease when the rate of recycling was increased to 50 and 75%.

Conclusions

·   Gas production rate, as proportion of liquid volume and per unit of DM and OM in the slurry input, showed a curvilinear trend with recycling rate of effluent, with production increasing from 0 to 25% level of recycling followed by a decline to lowest values at 75% recycling.

·   DM content in the effluent and COD values were increased linearly as the rate of recycling of effluent was increased.

·   There was a positive linear relationship between the DM content of the effluent and the COD of the effluent

·   Ammonia-N as percentage of total N in the effluent was not affected by concentration of effluent.

Acknowledgments

The authors are grateful to the Swedish International Development Authority (Sida) for funding this study.

References

AOAC 1990 Official methods of analysis of the Association of Official Analytical chemist (15^th Edn.), Washington, DC. 1: 69-90.

 

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/an92.htm

Bui Xuan An and  Preston T R  1999 Gas production from pig manure fed at different loading rates to
polyethylene tubular biodigesters.  Livestock Research for Rural Development 9(2): http://www.cipav.org.co/lrrd/lrrd11/1/an111.htm

Duong Nguyen Khang and Le Anh Tuan 2002a The result of the low plastic film biodigester technology transferring at University of Agriculture and Forestry, Ho Chi Minh. Proceeding of the 2002 International Workshop on the recent developments in recycling of livestock wastes through biodigesters and water plants. March 10-11, 2002. University of Agriculture and Forestry, Ho Chi Minh city, Vietnam. http://www.mekarn.org/procbiod/khang2a.htm

Duong Nguyen Khang, Le Anh Tuan, and TR Preston 2002b The effect of fiber level in feedstock, loading rate and retention time on the rate of biogas production in plug-flow and liquid displacement biodigesters. Proceedings biodigester Workshop. http://www.mekarn.org/procbiod/Khang.htm

San Thy, PrestonT R and Ly J 2003 Effect of retention time on gas production and fertilizer value of biodigester effluent. Livestock Research for Rural Development (15) 7, http://www.cipav.org.co/lrrd/lrrd15/7/sant157.htm

Undersander D, Mertens D R and Thiex N 1993 Forage analysis procedures. National Forage Testing Association. Omaha, pp 154

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