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 Table of Contents  
Year : 2016  |  Volume : 2  |  Issue : 3  |  Page : 97-102

Improving Biogas Production Performance From Pomegranate Waste, Poultry Manure and Cow Dung Sludge Using Thermophilic Anaerobic Digestion: Effect of Total Solids Adjustment

Department of Environmental Science, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, Iran

Date of Web Publication2-Feb-2017

Correspondence Address:
Hadi Radnezhad
Department of Environmental Science, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/2423-7752.199293

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Context: Biogas is one of the most important sources of renewable energy and is considered as an environmental friendly energy source. One of the most important parameter influencing the production of biogas is total solids (TS). Aims: In this study, the effects of different total amount of solids, which consisted of 5, 10, 15, 20, 25, and 30% treatments, on the biogas production were examined. The solids were obtained at a thermophilic temperature (55°C) from a mixture of pomegranate rind, cow manure, and sludge in 15 days using one-liter glass bottles. Materials and Methods: The influences of TS, volatile solids (VS), pH, and carbon-to-nitrogen ratio on the biogas production volume from optimized TS treatment were also evaluated. In addition, pomegranate peel was pretreated for lignocellulosic destruction. Results: The results showed that the biogas production increased from 0.273 to 0.736 L/day with an increase in TS from 5 to 25%. The 25% treatment had the highest mean biogas production (i.e., 0.736 L/day). Significant difference was observed between the 25% treatment and all other treatments except the 20% treatment. The regression model showed that the VS was the only parameter that had a significant effect on biogas production. This parameter justified about 74.1% of the biogas production accuracy. Conclusion: Anaerobic digestion is an appropriate technology to achieve the organic fraction of solid wastes. Due to higher biogas production, dry anaerobic digestion is of more importance than wet and semi-dry anaerobic digestion.

Keywords: Anaerobic digestion, biogas, concentration of total solids, cow manure, pomegranate wastes, poultry fertilizer

How to cite this article:
Ardaji VG, Radnezhad H, Nourouzi M. Improving Biogas Production Performance From Pomegranate Waste, Poultry Manure and Cow Dung Sludge Using Thermophilic Anaerobic Digestion: Effect of Total Solids Adjustment. J Earth Environ Health Sci 2016;2:97-102

How to cite this URL:
Ardaji VG, Radnezhad H, Nourouzi M. Improving Biogas Production Performance From Pomegranate Waste, Poultry Manure and Cow Dung Sludge Using Thermophilic Anaerobic Digestion: Effect of Total Solids Adjustment. J Earth Environ Health Sci [serial online] 2016 [cited 2023 Dec 4];2:97-102. Available from: https://www.ijeehs.org/text.asp?2016/2/3/97/199293

  Introduction Top

Nowadays, the two of the important topics that have occupied the researcher’s and scientist’s minds are “environment and energy,” as it is estimated that energy demand in 2025 will increase by more than 50%.[1] However, the ease of access to fossil fuels has dropped during the past two centuries. Therefore, energy supply is one of the most important global problems in the future. In addition, combustion of fossil fuels such as gasoline, natural gas, and coal releases carbon dioxide (CO2), NOx, and SOx in the environment, which may lead to massive environmental problems and adverse effects on human health and the ecosystems causing acid rain and global warming. To solve these problems, the Europe Commission has set its target to achieve a 20% increase in energy production from renewable sources by 2020 compared with 8.5% in 2005. To achieve this goal, increase in the use of renewable energy sources and improvement in the production of renewable energy sources are needed.[2] On the other hand, the human society plays an undeniable role in waste material production. Each year, more than 400 million tons of agricultural forest and livestock wastes are produced.[3] The potential pollutants from livestock manures such as high decomposition of biological oxygen demand, pathogens, nutrients, methane (CH4), and ammonia emissions should be controlled before disposal.[4] Poultry waste management and inappropriate utilization of nitrogen and phosphorus on land as fertilizer could lead to soil contamination and eutrophication of surface water resources. Ammonia and greenhouse gas (CH4 and CO2) emissions from storage of the waste pollutants may also contaminate the air.[5] According to the United States of America Environmental Protection Agency in 2010, the lifetime of CH4 in the atmosphere is much shorter than CO2 and thus CH4 is more efficient than CO2 in trapping infrared radiation.[6] In general, the impact of CH4 on climate change is 20 times more than that of CO2 over a period of 100 years.

As part of a waste management system, use of anaerobic digestion reduces the emission of CH4 to the atmosphere. Another advantage of anaerobic digestion is biogas production with a high percentage of CH4 gas that can be used as fuel.[7] Therefore, biogas production using anaerobic digestion can be intended as one of the best technologies for reducing the size and mass of 50% of incoming wastes, reduction of pathogens, controlling the greenhouse gas emissions, and stabilizing crop biomasses before its use in agricultural activities.[4] The gas, as a clean and renewable energy, includes 60–70% CH4, 40–30% CO2, and slight amounts of N2, H2S, and water vapor.[8] Using anaerobic digestion for perishing waste, the total biogas production in Iran in 2011 was about 16,146 million m3,[9] which had a heating value of 5650 kcal/m3.[10]

In recent years, the anaerobic digestion process was used to obtain energy from lignocellulosic residues because of the high carbon content. Different types of lignocellulosic biomass exist based on the various percentages of its major constituents, which are cellulose, hemicellulose, lignin, organic materials, and inorganic minerals.[11] Pomegranate waste as a source of lignocellulose to generate high volume biogas has lesser digestibility and low protein content compared with other fruit wastes and, therefore, is not suitable as animal feed. Therefore, their storage as a superfluous material in factories is causing air pollution and public health problems. However, many studies have revealed that organic waste digest may contribute to improving the balance of nutrients, reducing the rate of acidification, reducing toxic compounds, lowering costs for preprocessing, and thus increasing the biogas production.[4],[12] Hence, cow manure as a cheap seeds and poultry fertilizer as nitrogen source can be effective in anaerobic digestion pomegranate peel wastes.

In addition to the type of raw material, many other parameters affect the amount of biogas produced. An important parameter in the process of biogas production is the total solids (TS).[13] On the basis of the amount of TS, anaerobic digestion process is classified as wet processes (6–10% TS), semi-dry (10–20% TS), and dry (≥20% TS).[14] The dry anaerobic digestion process is more impressive because of reduced water and energy consumptions, decrease in the size of the digester, and lesser latex produced.[15],[16] More than 60% of the anaerobic digesters built recently in Europe were based on dry anaerobic digestion systems.[17] Nevertheless, dry anaerobic digestion in the laboratory and industries has some difficulties because of a high concentration of solids.[18] For example, the solids make the destruction of the bed harder as well as the homogenization of the digester.[14] Thus, the optimum concentration of TS should be examined.

In this study, the biogas production from pomegranate peels, poultry fertilizer, and cow manure solids under various concentrations of solid materials from wet to dry processes in a thermophilic temperature in six digester batches was evaluated. The overall objectives of this study were the following: (a) to investigate the impact of the initial total amount of solids in the digester to produce biogas and select the optimal treatment, (b) to determine the effect of TS and volatile solids (VS) on biogas production in the optimized treatment, and (c) to evaluate the effect of pH and carbon-to-nitrogen (C/N) ratio on the biogas production under optimal treatment conditions.

  Materials and Methods Top

Material preparation

The organic wastes used in the anaerobic batch reactors were as follow:

  1. The pomegranate peels were collected from the pomegranate extraction manufactures that had stockpiled it as waste. Before preprocess activities on pomegranate peel wastes to provide appropriate feed for the digestion, smash and homogenize particle size of 2–6 mm according to Forster-Carneiro et al. [19] and boiling the pomegranate peel with 35% acetic acid and nitric acid 2% for half an hour for lignocellulosic tissue destruction in accordance with Xiao and Clarkson [20] was required.
  2. The poultry wastes were collected from the poultry farm at Azad University (Isfahan Branch). The wastes consisted of chicken feathers, shed seeds, and some soil particles. The waste was collected in sterile plastic bags and transported to the laboratory.
  3. Cow manure was obtained from the cattle farm at Azad University (Isfahan Branch). The cow manure was then mixed with water at a ratio of 1:1 in 20-liter plastic containers and kept at room temperature for 10 days to increase microbial activity. The materials prepared for the next stage were kept at 4°C.[21] The characteristics of the raw material before beginning the experiment are shown in [Table 1].
    Table 1: Initial characteristic of the raw material

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The reactors and operating conditions

  1. One-liter reactor (simple): The anaerobic digestion system was set with six reactors. In a laboratory scale, batch digestions with one-liter glass containers were used. The rubber hoses with a length of 1 m were placed into the gaps and were sealed using glue. As a gas duct into the cylindrical sleeves (1000 mL), a small plastic bucket filled with water was put down the cylinder using the clamp and the base set. The gas was measured by liquid displacement. A mixture of pomegranate peel waste, poultry fertilizer, and cow manure in the ratio of 1:1:1 (C/N = 22) with water to achieve the following six TS concentrations under anaerobic conditions of “wet” to “dry” were prepared: 5, 10, 15, 20, 25, and 30%. The experiment was executed at an initial neutral pH set using sodium bicarbonate for 15 days under a thermophilic temperature (55°C). The temperature monitoring was performed using the thermostatic bathroom [Figure 1].
    Figure 1: Digester set-up

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  2. Nine-liter reactor (with sampling): This discontinuous reactor was equipped with a total capacity of 9 L with a heating system, stirrer, pressure meter, pH adjustment, and injection and suction systems [Figure 2]. The reactor was equipped to allow sampling to determine the influence of parameters such as TS, VS, the ratio of C/N, and pH on the production of biogas by optimal treatment over a period of 15 days. This experiment was conducted under a thermophilic condition (55°C) and 25% TS.
    Figure 2: General view of anaerobic reactor load

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Analytical method

The parameterssuch as TS, VS, C/N ratio, andpH of the optimal treatmentwere measured once in every three days. The TS and VS were measured using standard methods for water and wastewater.[22] To measure TS, the samples were dried in an oven at 105–110°C, and to test the VS and organic carbon, the samples were burned in an oven at 5 ± 550°C. The pH was determined using a pH-meter (Sartorius PB-10, Germany), and total nitrogen was measured by Kjeldahl method. Biogas production was measured on a daily basis using water displacement.[23] The performance of biogas was based on biogas production volume that was measured by the total VS.

To analyze the results, the one-way analysis of variance method was employed and, to find the optimum conditions for the biogas production (optimization) in regard to different concentrations of TS and their interactions with each other, the least significant difference test was used at P < 0.01. The statistical analysis and plotting of graphs were done using the SAS (Statistical Analysis System) Software and Excel, respectively.

  Results and Discussion Top

Effect of TS amount on the performance of anaerobic digestion

The cumulative averages of biogas production of reactors at six different levels of TS (treatments) during the experiment are presented in [Figure 3]. Each curve represents the average of three replications. Biogas production was observed immediately after setting up the digesters for all values of TS that reflects the basic rate of anaerobic degradation. Results of Maamri and Amrani[21] also confirmed gas production and bed destruction in the early hours of the digester setting up.
Figure 3: Cumulative biogas production according to the TS content

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After setting up the digesters, cumulative biogas production in the treatments increased by enhancing the amount from 5 to 25% TS, indicating the importance of a high solids anaerobic digestion (25% TS). Although the 30% TS has less biogas production [Figure 4], this shows that TS levels above 30% reduce the destruction of the substrate and material mass transfer and thus the production of biogas. It also makes digester mixing and homogenization more difficult. In this regard, Karim et al.[24] showed that on increasing the concentration of TS in the reactor, stirring was an important factor to improve the anaerobic digestion and thus biogas production. Under similar conditions, the transfer was likely to be done by diffusion processes, which were highly dependent on the porous medium and thus on the amount of water.[25] Resistance to mass transfer may have an effect on anaerobic digestion in the dry systems.[26] Therefore, there would be some problems related to high concentrations of TS (30%) in anaerobic digestion operations. Fernandez et al.[27] and Forster-Carneiro et al.[19] reported better performance in anaerobic reactors 20% TS compared to 30% TS.
Figure 4: Influence of TS content on anaerobic digestion performance

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Biogas production and performance

According to [Table 2], the lowest and highest biogas productions at 0.273 and 0.736 L/day were related to 5 and 25% TS, respectively. The biogas production amount increased by enhancing the TS value up to 25%. This trend could be attributed to the reason that by increasing the TS, a simultaneous increment in the substrate (organic matter) and microflora occurred, which provided the requirements for biological degradation. A study of Maamri and Amrani[21] also corroborated the notion that a high concentration of TS, nutrient rich substrate, and adequate amounts of carbon, hydrogen (H2), nitrogen, phosphorus, potassium, calcium, magnesium, and a number of elements are necessary for the growth of anaerobic bacteria. This could optimize the interaction between acidification and CH4 formation, which was the most important step in the anaerobic digestion process. Thus, by increasing the concentration of TS, biogas production and performance would increase. Igoni et al.[28] also reported that the amount of produced biogas was related to TS concentration. In another study, Forster-Carneiro et al.[19] showed that the CH4 production yield decreased about 60% by an increase in TS from 20 to 30%. Similar results were observed in several other studies. It was also reported that high TS levels (30–35%) might prevent gas transmission and resulted in the accumulation of CO2 and H2, and prevented the accumulation of methanogens.[17]
Table 2: Rate of biogas production and performance

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TS at 30% concentration had biogas production of 0.441 L/day and performance of 0.221 L/g VS, which had the highest rate of biogas production and performance when compared to TS at 5% concentration. This represents a material transfer limitation and reduces the hydrolysis in this amount of TS. Yang et al.[17] showed that the amount of water used was affective in facilitating the mass transfer. Anaerobic digestion with high solid amounts might lead to slow mass transfer between bacteria and raw materials and thus might result in less biogas production and low performance.

Investigation of the relationship between biogas production and some characteristics

The trend of TS and VS, pH, and the ratio of C/N of the optimal treatment for every three days is provided in [Table 3]. The simultaneous trends of changes in these parameters and the produced biogas are also presented in [Figure 5],[Figure 6],[Figure 7],[Figure 8].
Table 3: Trends of changes optimal treatment parameters every three days

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Figure 5: The changes in volume of biogas produced by TS

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Figure 6: The changes in volume of biogas produced by VS

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Figure 7: The changes in volume of biogas produced by pH

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Figure 8: The changes in volume of biogas produced by C/N ratio

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The stepwise regression model showed that among the studied parameters, that is, TS, VS, pH, and C/N ratio, only the VS parameter had a significant effect on biogas production. This parameter confirmed about 74.1% of the accuracy of produced biogas. The obtained regression model is the following:

Total solids and volatile solids

[Figure 5],[Figure 6] show a declining trend in TS and VS changes and, from the beginning until the ninth day of the anaerobic digestion process, a considerable amount of biogas seemed to be produced. The main reason for high biogas production during these few days was the digestible solids by microorganisms in the microbial mixture. VS changes over the time curve also reveled this issue. Most of the VS changes occurred the first day until the ninth day. Vindis et al.[29] reported that the maximum production of biogas digestion occurred in the first week. The removal of VS and biogas production showed a significant decline from the ninth day of the experiment until the last few days of the process. This observation might be because of constant microbial growth, which was consistent with the results of Maamri and Amrani.[21]


[Figure 7] shows the pH changes during the process of anaerobic digestion. As it can be seen, the primary pH value of 6.9 was close to neutral pH. Ogiehor and Ovueni[30] reported higher yields of biogas starting from pH 7. The pH tended to decrease in the first three days from 6.9 to 6.4, which could estimate that the hydrolysis process had occurred in the reactor, especially, in the first three days. This result was consistent with the study of Maamri and Amrani.[21] On the sixth day, the pH rose again to neutral with a value of 6.9. The process continued until the ninth day of the experiment and then pH reduced. The acidic pH at the end of digestion could be attributed to the rapid accumulation of volatile fatty acids (VFAs), which usually stopped and reduced the production of biogas.[30] In other words, according to the pH value of biogas production from the first day to sixth in the hydrolysis and acid step, and on the sixth day till the ninth, the biogas production process was related to the methane step. Sitorus et al.[7] expressed that by reducing the pH, digestion in the acidification process while the pH is stable and neutral, phase it is the beginning of methane digester phase. This result indicates that digestion is done more in acidic pH values (i.e., 5.5–6.5) compared to the optimal CH4 pH (6.8–7.2) to anaerobic digestion process. The VFA accumulation probably led to a decrease in pH, which indicated an increase in acid producer activities as compared to the methane producers and a decrease in methane producer bacteria. As Sitorus et al.[7] showed, acid accumulation was created by high concentrations of VFA in the digester. When the acid producer bacteria act, the produced organic acids and digester pH decrease.

C/N ratio

In this study, according to the mixture ratio of 1:1:1 of the materials, the C/N ratio was equaled to 22, which was the optimal value of C/N ratio. Therefore, the digester was expected to be at an optimal condition for biogas production. According to the results, the C/N ratio showed no significant change with time, and C/N ratio remained in an optimized range during the 15 days of the process [Figure 8]. Thus, it is expected that a sudden decrease or increase in the pH and biogas production would not happen. Otherwise, with an increase in the C/N ratio, a rapid consumption of nitrogen by microorganisms might happen and thus gas production was lower, which led to C/N ratio decrease, high pH (5.8), and a high concentration of ammonia in anaerobic digestion systems. This construction status is toxic for methane producer bacteria. Jiang et al.[31] reported that the suitable C/N ratio for anaerobic digestion was in the range of 20–30. In this ratio, the digester could reduce the pH reduction and thus the failure of anaerobic digestion system.

  Conclusion Top

Anaerobic digestion is an appropriate technology to achieve the organic fraction of solid wastes. Dry anaerobic digestion (≥20% TS) has more importance as compared to wet and semi-dry anaerobic digestions (6–20% TS) because of higher biogas production. By increasing the TS of pomegranate peels, cow manure, and sludge of waste mixture from 5 to 25%, the biogas production rate significantly increased. The best performance of biogas production was the reactor with 25% TS with 0.276 L/g VS. However, it should be noted that the dry anaerobic digestion had some limitations including slow hydrolysis rate and mass transfer of materials.

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Conflicts of interest

There are no conflicts of interest.

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]

  [Table 1], [Table 2], [Table 3]

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