Biogas Production Using Geomembrane Plastic Digesters as Alternative Rural Energy Source and Soil Fertility Management

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Ÿ PSA: plastic biodigester, single layered and constructed above ground.

Ÿ PDA: plastic biodigester double layered and constructed above ground.

Ÿ PSU plastic biodigester single layered and constructed under ground.

Ÿ PDA plastic biodigester double layered and constructed underground.

Ÿ FDU: Fixed dome biodigester constructed underground.

The manure was collected from Mersa Agricultural TVET College dairy farm and the nearby private cattle shed.

2.4. Geomembrane Plastic Construction Methodology

Materials used for the construction of geomembrane plastic biodigester were geomembrane plastic, PVC pipes, gate valves, GI caps, sockets, nipples, neoprene rubber hose and biogas stoves. Construction was done with the help of electrical geomembrane welding machine and CM-43 adhesive with other mixtures. The geomembrane plastic was cut at a dimension of 7 meter length and 4.50 meter width as per the design. Then, it was welded with the help of electrical plastic welding machine across the length and the circular part of the cylinder was fitted with the help of CM-43 adhesive and with other chemical mixtures. The biodigester has an inlet for entry of input materials, gas outlet for exit of produced gas and slurry outlet for disposal of fermented slurry. The digester and gas holder was made as one unit in a cylindrical shape.

Table 1. Technological Parameters of the Experimental geomembrane plastic biodigester

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2.5. Data Collection Procedures

Different data which were pertinent to the study objectives were collected following standard procedures. The following variables were measured and analyzed during the study. Amount of gas production, quantity of input and output slurry, temperature of the air, pH of the fresh cow dung and digested slurry, total–N and organic matter content of the substrate and the slurry. Daily rainfall, temperature of the air and slurry was also measured.

2.5.1. Input to the Digesters

The type of input material which was found feasible and available in the study area was cow dung as there was dairy farm at a distance of 25 meter from the experiential site. Manure inputs were measured using bucket of 25 liter. One bucket of cattle dung was mixed with 1 bucket of water ^[8]. Thus, as per the design three bucket of dung (75 Kg) were mixed with three bucket of pure water (75 litres) so as to produce 3 m³gas per day on May 5, 2007. After the slurry mixture has been fed in to the digester, 15 liters starter material prepared from cattle dung and water in 1:1 ratio and allowed to ferment for one month in a closed barrel were added at equal amount to all five biodigester to initiate and facilitate the fermentation process.

2.5.2. Measurement of Gas Production

The quantity of gas produced from the two models of biodigester were measured with the help of standard sized biogas burner which is, certified by ISO and manufactured by gas crafters in Bombe, India. The burner has a capacity of 0.45 m³per hour and it was adjusted with the help of its air shutter until blue flame comes to burn with its maximum capacity. Time to burn was taken by stop watch for consecutive hours of one day. So the daily gas production from the digester (s) is the sum total of the hours run by each burner and its gas consumption rate.

2.5.3. Temperature of the Air and Slurry

The temperature of the air was measured via mini-max thermometer in a metrological station found around the study area and the daily temperature of the slurry in the biodigester was measured using ordinary electronic thermometer.

2.5.4. Total-Solids (DM) Content

The dry matter content of the substrate and spent slurry was measured by drying a sample at 70°C in an oven and weighing the residue on a precision electronic balance.

2.5.5. The Organic Dry Matter (ODM)

Only the organic or volatile constituents of the feed material are important for the digestion process. For this reason, only the organic part of the dry matter content was considered and this was analyzed in the laboratory.

2.5.6. PH of the Fresh Cow Dung and Digested Slurry

The pH of the fresh cow dung and fermented manure was measured by pH-meter in the laboratory.

2.5.7. Quality of Output Slurry

The compositions of digested slurry produced by anaerobic fermentation in the two models of biogas plants were determined in the laboratory. Five digested slurry samples from all five biogas plant and two fresh manure samples from the cow dung fed to the two types of biogas plant were taken by random sampling technique. According to AOAC (1990) ^[10], total-N was determined by Foss-Tecator Kjeldahl procedures after taking 1 gram of manure sample and digested with sulpheric acid and salicylic acid and estimated by Kjeldahl method. The process of digestion took about three and half hours in the laboratory.

Organic matter content was determined by the use of Toffle furnace by ash method. 10 gram of well mixed manure in dry nickel crucible or silica basin was weighted and was put in a low flame or hot plate till the organic matter begins to burn. The crucible in a muffle furnace was placed at about 550°C for 8 hours. The crucible with a grayish white ash formed was removed from the furnace cool in a desicator and weigh. Therefore, the residue represents the ash and the loss in weight represents the moisture and organic matter ^[11]. Fresh cow dung samples were taken on April, 2011 and fermented slurry samples were taken after gas was fully generated and measured i.e. on June, 2011 from five biodigester and two cattle sheds.

2.5.8. The Efficiency of Biodigester

It was evaluated by comparing its gas and slurry production with the amount of substrate fed in relation to the volume of the digester and compared by calculating their specific gas production. Specific gas production was determined by dividing the daily volume of gas measured by the amount of cow dung loaded into the plant.

3. Results and Discussion

3.1. Biogas Production

Gas was burnt and measured after gas has completely produced within the designed HRT of 40 days with the help of calibrated biogas burner and stop watch (Figure 2). As can be seen in Table 2, gas production as the proportion of biodigester liquid volume was higher for a single layered and above ground plastic biodigester than others and very less amount of gas was measured from the fixed-dome biodigester. This was because more sun light temperature (27.65°C-32.7°C) was absorbed in a black geomembrane plastic sheeting digester than reinforced concrete fixed-dome biodigester (Figure 3). Temperature is very important factor which positively or negatively affects the activity of microorganisms in the production of biogas.

Figure 2. Burning and Measuring of Biogas with a Biogas Burner after Gas Generation

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Figure 3. The Relationship between Temperature of the Slurry and amount of Gas Production for the Plastic and Fixed-Dome Biodigester

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Table 2. Total Values for Biogas Production (9 Am to 4 Pm) in Biodigester with Different Types, Layers and Location of Installation

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3.2. Temperature of the Air and Slurry

The average atmospheric temperature during fermentation time between May 5, 2007 and June 14, 2007 was 24.55°C. According to Grewal et al. ^[8], temperature is one of the factor affecting the growth rate of micro-organisms involved in the production of biogas and effective anaerobic fermentation is carried out in mesophilic temperatures averaging between 24°C and 45°C. The local average atmospheric temperature of Mersa was 21.12°C according to 12 years of data which was very suitable for biogas production.

According to Hu Qichun (1991) ^[12], a biogas plant could perform satisfactorily only where mean annual temperatures are around 20°C or above or when the average daily temperature is at least 18°C. With the range of 20-28°C mean temperature, gas production increases over proportionally. If the temperature of the biomass is below 15°C, gas production will be so slow that the biogas plant is no longer economically feasible. So, the temperature recorded in the study area was very suitable for normal fermentation and higher amount of gas was produced by mesophilic bacteria. The average temperature of the fermented material in the biodigester were in the range between 25.96°C and 32.7°C as described in (Table 3), which is greater than the critical 15°C. Thus, gas was completely produced within the designed HRT value of 40 days. As it is mentioned in Figure 3, R² is 0.945. Thus, 94.5% of the variability in the amount of gas produced by the biodigester was due to the variation in the slurry temperature.

The average temperature of the slurry measured in the geomembrane plastic biodigester exceed by 1.687°C to 6.737°C of the slurry in fixed-dome biodigester (Table 3). Thus, geomembrane bag was observed to have the best advantage of heating the digester contents easily and produce higher amount of gas (0.12-0.68 m³of gas per m³ of digester per day) than fixed-dome biodigester made from reinforced concrete (Table 3). Since its walls are thin and black in color, it can be heated quickly with an external heat source, such as the sun radiation of the same degree in the study area. Similar results were reported by Bui Xuan and Preston ^[13] that found average temperatures in bag digesters, compared with dome types, are 2°C-7°C higher in the bag (0.235-0.61 m³of gas per m³ of digester per day).

Table 3. Comparison of Average Slurry Temperature, °C and amount of Gas Produced, m³/Day of the Biodigester

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3.3. Characteristics of Bio-Digested Slurry (Effluent) and the Influent

Organic substances passed through biogas plants not only produce a source of energy, but also a large quantity of digested slurry, which provides excellent organic fertilizer. The net weight of slurry discharged daily from the geomembrane plastic and fixed-dome biodigester were 123 Kg and 112.5 Kg respectively. Thus, the annual slurry output was 44895 Kg (45 tons i.e. 18% of the total substrate was lost) and 41062.5 Kg (41 tons i.e. 25% of the total substrate was lost), which was produced from 27375 kg of fresh dung fed to the digester annually, which in turn is equivalent to 13468.5 kg of dried dung cakes. Likewise, the loss in the amount of slurry have been reported by UNESCO (1982) ^[14], and states that during digestion, about 20% of the total slurry is volatilized. Thus, geomembrane plastic biogas plant produced higher amount of slurry than fixed-dome biogas plant of the same capacity and using equal amount of input material. So, with the use of geomembrane plastic biodigester, the farmers could get 45 tones of fermented slurry which could be used to apply on the farmland as organic fertilizer. Thus, by conversion of cow-dung in to a more convenient and high-value fertilizer (biogas slurry), organic matter is readily available for agricultural purposes, thus protecting soils from depletion and erosion.

Therefore, the farmers should be advised to use geomembrane plastic biogas plant so as to utilize the dung for dual purposes such as the produced gas as fuel for cooking and the remaining large quantity of slurry as organic fertilizer which helps to increase crop production by preventing it from burning in the form of dry dung cake and ashing down the good manure creating unhygienic conditions in the kitchen.

3.4. Characteristics of Total-N in the Slurry and Influent

The average total nitrogen of the substrate and digested manure of the geomembrane plastic and fixed-dome biodigester were 0.37%, 1.13% and 1.15% respectively as described in Table 4. Similarly, Grewal et al. ^[8], reported that the total-N content of fresh dung as 0.242% which is 34.6% less than the result of this study. Thus, fermented slurry contained larger nitrogen content than fresh cow dung in both models of the biodigester because in an air tight biogas digester more organic acid such as acetic acid, prop ionic acid, butyric acid, ethanol and acetone was produced doting anaerobic fermentation of soluble simple organic substances which helps to absorb and fix ammonia and minimize the loss of nitrogen thus conserving the fertility of the manure. So, the higher quantity of nitrogen was converted in to the useful nitrate and ammonia which is the most important ingredient for plant growth.

There was also a greater conversion of organic substrate to nitrogen during 40 days of the experiment because microbial anaerobic degradation was facilitated with higher temperature.

According to Hu Qichun ^[12] (1991), during anaerobic fermentation, part of the total nitrogen is mineralized to ammonium and nitrate. Thus, it can be more rapidly taken up by many plants and in a number of applications, slurry from biogas plants is even superior to fresh dung especially when the slurry is spread directly on fields with a permanently high nitrogen demand (e.g. fodder grasses) or when using slurry compost to improve the structure of the soil.

Processing evidence suggests, however, that slurry is much more effective than dung when applied as fertilizer, French (1979) ^[15], discusses that slurry is 13 % more effective than dung, and Van Buren (1974) ^[16], reported that the ammonia content of organic fertilizer fermented for 30 days in a pit in china increased by 19.3%.

As can be seen in Table 4, the single layered geomembrane plastic biodigester constructed above ground produced higher amount of total nitrogen than others. This was due to higher amount of slurry temperature (Table 3) absorbed in the single layered above ground biodigester activated anaerobic micro-organisms to convert more simple organic substances of the substrate in to simple organic acid so as to fix more ammonia. Therefore, total-N was affected by material and position of biodigester construction. Mersa ATVET dairy farm fermented cow manure contains 1.0164%-1.2026% N and 50-75% Organic matter. Thus, the annual output of slurry equivalent to 13468.5 kg of dry dung cake converted to 152.2 kg of N and 8127 kg of organic matter on the average. In the slurry which has higher total nitrogen content there is higher proportion of ammonium which constitutes the more valuable form of nitrogen for plant nutrition ^[17].

Table 4. Effect of Material & Position of Biodigester Construction on the Composition of the Effluent

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3.5. Characteristics of Organic Matter in the Slurry and Substrate

The application of digested slurry to crop serves as a dual purpose; a soil conditioner as well as a source of plant nutrients. According to Table 4, the effluent in PSA showed larger organic matter content than the effluent in FBU. This could be because the amount of organic matter in the influent of plastic biodigester (IPB) was higher than that in the influent of fixed-dome biodigester (IFB). Higher value of organic matter was recorded from fresh cow dung than the slurry (Table 4). Even if higher value was recorded for fresh manure, the use of excreta of dairy cattle in biogas plants was found to be better than the farmyard manure in several ways. A part of nitrogen which is ammonia, found in the slurry becomes available to the plants. The ability of the wet digested slurry to aggregate soil particles immediately after application is also very unique. The digested manure was available in 40 days as compared to 4-6 months taken in the usual method of composting in a manure pit. According to Grewal et al. ^[8], almost all plant nutrients are retained in the digested slurry in such finely divided state that, it mixes up with the soil quickly and thoroughly, and the soil bacterial activity increases substantially. Thus the application of digested slurry gives better yields for all crops as compared to farmyard manure (FYM) made from the same quantity of cattle dung. The digested slurry in this study was thin and used directly to the crops through the irrigation channels and by direct splashing on the farmland.

Moreover, it was also put in a fish pond of Mersa agricultural T.V.E.T. College and fishes were nourished. Thus, it was proved that, the waste that comes out of the digestion process as slurry was very useful both as feed and organic fertilizer. Studies by Sokoine Agricultural University in Tanzania have shown that slurry from biogas improves productivity of land and maintains soil quality that can support crop production over a long period of time ^[18].

3.6. Characteristics of pH of Fermented Slurry

It is generally recommended that the pH inside the biodigester should be above seven for normal fermentation and maximum gas production ^[8]. In the present study, the pH of the fermented slurry and fresh cow-dung was 6.8-7.6 and (Table 4) with a hydraulic retention time of 40 days. Thus, the condition inside the digester was very comfortable for anaerobic micro-organisms to accomplish normal fermentation, higher gas production and fertile manure.

3.7. Efficiency of the Biodigester

The average specific gas production of the single layered above ground geomembrane plastic biodigester was greater than others (Table 2). That was due to higher amount of sun light was absorbed by the digester (32.7°C) that used to facilitate the activity of micro-organisms and increase the amount of gas production per weight of the substrate (Figure 2). Thus, the construction of single layered aboveground biodigester is better than other biodigester as efficiency of conversion of substrate organic matter to biogas was higher.

3.8. Economic Evaluations

In order to compute the value of biogas in terms of the value of traditional fuels saved by utilizing biogas, it was necessary to estimate the amount of dried dung cakes or fire wood needed to produce an equivalent amount of energy. Thus, according to UNISCO ^[14], Van Buren ^[16], used in this study assume that 1 m³ biogas substitutes for 3.47 kg of fire wood, 12.3 kg of dry dung fuel and 0.62 liter of kerosene oil.

Economic evaluation was also done for all biodigester using cost-benefit analysis which is the most commonly employed method used by many extension officers. It was able to evaluate the relative advantage of a geomembrane plastic biogas plant investment as compared to fixed dome biodigester on the basis of the anticipated minimum interest rate and economic life for the alternative designs. A discount rate of 18 % has been applied throughout the analysis according to Amhara regional state credit and saving institution (2007).

3.8.1. Market Price of Inputs

Dung: According to Senait Seyoum ^[19] (2007), the cost of dung was estimated in terms of.

Table 5. Summary of Market Value of Inputs and Outputs Used in the Analysis

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Ÿ Dung’s value as fertilizer determined by the cost of an equivalent amount of commercial fertilizer, or.

Ÿ The market value of dung cakes, if dung is sold.

Ÿ 1 ton of DAP is 16 tone of dry manure.

Price of 1 tone of DAP according to 2006/2007 year was 1450 ET Birr. This was determined from a receipt voucher issued to Mersa agricultural T.V.E.T College purchasing office. The price of dried cow dung according to Woldya market in 2006/2007 ranged from EB 5.00 to EB 8.00 per 50 kg sack, averaging EB 0.50 per kg of dried dung.

Water: This was valued according to the price charged by the water authority of Habru District, Mersa town. I.E. EB 2.00 per 5 m³.

Labor: The Labor used to collect water and spread slurry was valued at EB 15 day^-1.

3.8.2. Market Price of Outputs

Biogas: The biogas produced by the digestor was valued at the market value of fire wood or dried cow dung cakes, which it replaces. The observed price of fire wood in Mersa was between EB 15 and EB 20 per bundle weighing 15-20 kg. It was estimated that firewood averages EB 1.00 kg^-1.

Slurry: Output digested slurry was valued at the official price of DAP (diammonium phosphate). N and P contents of DAP roughly approximated the proportions of these nutrients in dried cow dung. According to Senait Seyoum ^[19], 1 ton of DAP is roughly equivalent to 16 ton of dry manure. The 2006/2007 price of DAP was EB 1450 tone^-1, thus each ton of dry cow dung which has less nutrient content than the fermented slurry is worth EB 90.62. Therefore, the cost of 1 ton of fermented slurry was assumed to be EB 90.62 to the minimum.

Three important technical assumptions were made with respect to gas production and use, and the efficient use of inputs and their conversion in the analysis.

Table 6. Summary of total costs and total benefits of geomembrane plastic and fixed-dome biogas plants

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