Fossil fuel is currently the world’s main source of energy and is prevalent in forms such as crude oil, hard coal, and natural gas. Such fuels are not renewable energy sources as they were formed over hundreds of millions of years but are consumed at a much faster rate than the rate at which new fossil fuels are being formed. One of the largest disadvantages associated with the use of fossil fuels is that harmful greenhouse gasses such as carbon dioxide are released when it is burnt during energy production processes. This is especially true during the production of electricity via coal-fired power plants [1].
In recent years, the energy sector has received increased attention due to the concern of an oil shortage. Additionally, concerns such as the greenhouse effect and the general depletion of our energy reserves have played a significant role in the debate. This has caused the development of a range of new energy technologies such as wind energy, solar energy, and biomass energy [2,3].
The anaerobic digestion of biomass to produce biogas is considered to be a model for choosing the best alternative sources of energy for rural areas using the reasoning that it is cheap and can be locally produced and used. Also, the biogas produced can be used for a number of purposes such as heating, lighting, fuel for cooking, and local or on-the-grid electric power generation [4].
Animal manures can be used as sources of biomass-based conversion processes, especially in bio‐energy and bio‐fertilizer production. Today, developed countries tend to decrease the number of farmers but increase the number of animals. This trend is also transforming livestock production in developing countries [5]. Livestock contributes to nearly 40% of the total agricultural production in developed countries and 20% in developing countries, supporting the livelihoods of at least 1.3 billion people worldwide, since 34% of the dietary protein supply comes from livestock [6].
The specific amount of cattle manure per animal relies on many aspects such as feeding regime, stage of the process, type of production system, etc., and the method of housing used [7]. Livestock activities have an environmental impact when manure is not effectively managed [8,9]. On the other hand, animal manure is considered an attractive natural resource for renewable energy production, and can also replace industrial fertilizers and improve soil fertility [10,11].
Anaerobic digestion can take place at psychrophilic temperatures below 20 C but most reactors operate at either mesophilic temperatures or thermophilic temperatures, with optima at 35 C and 55 C, respectively. The methane yields that are obtained at a temperature range of 15–20°C is about 26–42% of the yields achieved at 35°C [12]. The percentage of methane in biogas produced under thermophilic conditions (55°C) is on average 2% higher when compared with biogas produced under mesophilic conditions(35°C). It has been shown that temperature has almost no effect on the ultimate methane yield of beef cattle manure for temperatures between 30 and 60°C [13].
In a study evaluating the influence of diet and of the period on the anaerobic digestion of cows, Orrico et al [14] observed that only the diet had an effect under the digestion process. The authors observed that the proportion with the highest amount of concentrate (40% roughage and 60% concentrated) led to greater efficiency in the gas production compared to the 60% roughage and 40% concentrated diet with a biogas production potential of 420 mL/g.
Barros et al. [15] evaluated the biogas production in an Indian digester with a capacity of 7 m³, using cow manure as substrate. In the two-month period, the researchers observed a cumulative production of 5.025 L. In addition, Weber [16] while studying the biogas production from cow manure by using a vertical continuous digester, with a capacity of 20 m³, observed production of 396.850 L of biogas in four months.
Several studies to develop strategies to increase biogas production and methane yield have shown that co-digestion of organic wastes, such as animal manure combined with industrial, agricultural, and municipal wastes, is a viable option [17]. However, the low rate of biodegradation of fibrous wastes, such as manure, proves that an anaerobic digestion process for biogas production based solely on these substrates may be difficult, whereas the addition of substrates with lower fiber contents facilitates a more rapid initiation and increase in the biogas yield.
Previous studies have investigated the use of cow manure that was co-digested with different wastes to increase biogas production and methane yields. Cunsheng et al.[18] reported that co-digestion of cattle manure with food waste in batch mode at an optimal ratio of 2:1 (manure : food waste) increased methane production by 41%, from 2624 ml to 3725 ml, compared to mono-digestion.
Benali et al. [19] Effect of solid concentrations on anaerobic digestion of cow manure. This research aim to determine the optimum water dilution which will produce the high biogas yield. Three batch set-ups; CM1, CM2, and CM3 of uniform amounts of cow manure were prepared with different water dilution conditions. The results of accumulated biogas yield at the end of an experiment were 5.38L, 3.96L, and 3.4L for CM2, CM1, and CM3.
Benali et al. [20] comparative study of biogas production from cow, chicken, and sheep manure, this research aim to determine the best sample from animal waste for the production biogas, where the study has proven that the maximum value of biogas production for chicken manure is followed by sheep and cow respectively.
In Libya, even if the production of biogas started in the last years, still there is too much need to optimize the biogas resources. This study provides strong evidence that the concept of improving biogas yield through the paint the reactor black color and determining the optimum pH of cow, sheep, and poultry livestock manure.