The Prefecture of N’zérékoré is one of the 33 prefectures of Guinea. It is the largest city in Forestry Guinea, a region in the southeast of the Republic of Guinea. The city is also the capital Forest region. It is located between 7˚32 and 8˚22 north latitude and 9˚04 west longitude and extends over 47.3 km². The distance to neighboring prefectures is 39 km for N’Zérékoré-Lola, 62 km for N’Zérékoré-Yomou, 125 km for N’Zérékoré-Beyla, 135 km for N’Zérékoré-Macenta. Nzérékoré is at an elevation of 480 m and its relief is rugged. The plateau is dominated by hills that are sometimes gneissic (Gonia) and sometimes quartz (Gboyéba). The city has three important mountains: Götö (450 m), Hononye and Kwéléyé (350 m). Sheep breeding, goats and pigs is practiced throughout the commune. The pig herd is the largest in all areas of the N’Zérékoré. Cattle are imported from neighboring communes intended directly for butchery. The Map of the urban commune of N’Zérékoré is in Figure 1 .

To carry out this research, we used the following materials and equipment: plastic bottles, plastic flasks, cooler, gloves, graduated containers, electronic balance, analytical balance, valves, flexible pipes, clamps, liquid glue, Teflon, pH meter and temperature sensor. The physicochemical parameters of pig manure from N’Zérékoré are on average: humidity (53.83%); dry matter (44.26%); organic matter (81.39%); density (650.36%); Carbon (47.20%); Nitrogen (1.8%) and the ratio between Carbon and Nitrogen (26.22). For cow dung: humidity (82%); dry matter (22%); organic matter (52%); density (593.28%); Carbon (30.28%); Nitrogen (1.66%) and the ratio between Carbon and Nitrogen (18.27) [11] [14] .

The experiment was carried out at the Physics laboratory of the University of N’Zérékoré from 4 to 25/12/2020. Loading of experimental digesters with substrates began on 06/12/2020. The loading of the experimental digesters with the substrates began on 06/12/2020, the preparation of which is done as follows.

The substrate of pig manure and cow dung were each diluted in 2 liters of water in a ratio of (1/2) before being mixed in varying proportions and putting them in the different digesters as indicated in Table 1 .

For the design of the digester (D), we used a plastic bottle of 4.5 liters and 124 g empty mass, two others of the same volume, one of which is considered as a gasometer filled with water and the other empty to collect the water which is emptied from the gasometer under the pressure of the biogas produced. They are graduated in centiliter using graph paper in order to quantify the gas produced. The same device was made for the different types of substrates ( Figure 2 ).

The hydrogen potential (pH) of the solutions was measured using a Consort brand pH meter equipped with a combined Ag/AgCl glass electrode. Calibration is carried out using pH buffer solutions.

Monitoring of temperature variation in the different fermenters was carried out by a temperature sensor coupled to a millimeter.

The daily and cumulative biogas production of each type of substrate was measured on the gasometer graduation ( Figure 2 ).

For a constant volume of the gasometer, increased pressure was a result of increased volume of biogas generated. The pressure can therefore be calculated by the relation between the pressure and the volume of an ideal gas.

The daily biogas production and temperature evolution in the five digesters are illustrated in Figure 3 . It shows the biogas production did not start on the same day. After loading the digesters on 06/12/2020, we recorded the first production on the 4th day in digesters D1, D2 and D3 ( Figures 3(a)-(c) ) and the 2nd day in digesters D4 and D5 ( Figure 3(d) , Figure 3(e) ). The quantities are respectively: 0.15 liters for both (D1 and D2); 0.25 liters for (D3); 0.5 liter for (D4) and 1 liter for (D5). During the 25 days of digestion, the largest quantity of biogas was recorded on the 15th day in the digesters (D3 and D4) with a value of 2 liters ( Figure 3(c) , Figure 3(d) ). The same value was recorded on the 19th day in digester D2 ( Figure 3(b) ). The second largest value of biogas produced is 1.5 liters, recorded on the 8th day in digester D5 ( Figure 3(e) ). The smallest value 1 liter was recorded on the 11th day in digester D1 ( Figure 3(a) ). The daily average biogas production values are respectively: 0.32 liters for (D1); 0.62 liter for (D2); 0.86 liter for (D3); 0.79 for D4 and 0.68 liter for D5. It appears from these results the substrates in co-digestion with a high cow dung rate in (D3 and D4) remain the most productive. These results are in agreement with other research results [15] - [17] . The temperature in the digesters D1, D3 and D4 varied from 25˚C to 29˚C ( Figure 3(a) , Figure 3(c) , Figure 3(d) ). It varied from 25˚C to 30˚C in digesters D2 and D5 ( Figures 3(b)-(e) ). The average temperature value in the digesters are respectively 26.9˚C for D1, 27.54˚C for D2, 27.64˚C for D3, 27.9˚C for D4 and 28.73 for D5. These results show the average temperatures in the different digesters are relatively the same, with the highest value in the digester (D5) corresponding to 28.73˚C, which contains 100% BV. This is one of the reasons that justifies the co-digestion of cow dung with other substrates for optimal biogas production [18] - [21] .

Examining the curves of daily biogas production and temperature evolution, for digester D1 we remarked from the start of biogas production (4th day) to 14th day an inverse relation between biogas production and temperature. In fact, each increase in biogas production over a day interval corresponds to a stabilization (plateau) of the temperature over the same interval ( Figure 3(a) ) while a stabilization (plateau) in biogas production over a day interval leads to an increase in temperature. From 15th day this trend is observed. For digester D3 a long period of temperature stabilization (plateau) is observed (from 6th to the 17th day) ( Figure 3(c) ) while biogas production increases and reaches its maximum value ( Figure 3(c) ). From Figure 3 , it can be concluded the substrate in Digester D3 is the best mixture of pig manure and cow dung for optimal biogas production.

The pH variation curves of the substrates is illustrated in Figure 4 . The pH variation curves of the substrates ( Figures 4(a)-(e) ) show that, during the digestion process, the pH varied from 5 to 8 in the five (5) digesters, with averages of 7.12 for the substrates of digesters (D1, D2 and D3) and 7.28 for the substrates digesters (D4 and D5). These average pH values are relatively similar and correspond to the neutral medium, which is favorable to the development of micro-organisms for an optimal production of biogas.

During the 25 digestion days, three phases of pH evolution were observed for each type of substrate: an acidic phase (pH around 6) until the 7th day for all the digesters ( Figures 4(a)-(e) ); a neutral phase (pH around 7) from the 8th to the 18th day for digesters D1, D2 and D3 ( Figures 4(a)-(c) ) and from 8th to 15th for digesters D4 and D5 ( Figure 4(d) , Figure 4(e) ); une phase basique (pH autour de 8) du 18eme au 25eme jour pour les digesteurs D1, D2 et D3 ( Figures 4(a)-(c) ) et 15eme au 25eme pour les digesteurs D4 et D5 ( Figure 4(d) , Figure 4(e) ). A basic phase (pH around 8) from the 18th to the 25th day for digesters D1, D2 and D3 ( Figures 4(a)-(c) ) and from 15th to 25th for digesters D4 and D5 ( Figure 4(d) , Figure 4(e) ). It should be remembered the variation in pH is one of the indices for appreciation of biogas production in an anaerobic medium. The pH value for optimal biomethanization is around neutral (6.8 - 7.5) [2] . This demonstrates the pH values recorded during this study remain favorable to biomethanization bacteria.

The cumulative biogas production profiles are illustrated by the curves in Figure 5 . The curves of cumulative biogas production of the five types of substrates are all characterized by low biogas production during the first week of digestion (latency phases), then an acceleration in production was observed from 8th to 19th day (exponential phase), then a slowdown of production during the last week of digestion (bearing phase) [22] [23] . The duration of these different phases depends on the nature of the substrate [24] [25] . Latency phase: is the first phase (substrate liquefaction period). It corresponds to the progress of hydrolysis, acidogenesis and acetogenesis. In the present study, it lasted: 7 days for substrates in D1 and D2, with a production of 0.15 liters each of them, and 6 days for other substrates, including 0.25 liters in D3 and D4 and 0.50 liters in D5. Exponential phase: is the second phase, which corresponds to methanogenesis. It lasted: 12 days (from 8th to 20th day) for the substrate in D1; 14 days (from 8th to 22nd day) for the substrate in D2; 16 days (from 6th to 22nd) for the substrate in D3 and 20 days (from 4th to 22nd day) for the substrates in digesters D4 and D5. Bearing phase: is the third phase, it corresponds to a very low or stopping of the biogas production under the effect of substrate depletion. It starts respectively from 21st for the substrate in D1 and from the 23rd day for D2, D3, D4 and D5.

The diagrams in Figure 6 show the cumulative biogas production during the 25 days of digestion for the substrates of the five digesters.

The cumulative production of biogas from pig manure and cow dung substrates in the proportions indicated in Table 1 are: D1 (7.95 liters), D2 (15.60 liters), D3 (21.50 liters), D4 (19.65 liters) and D5 (17.05 liters) ( Figure 6 ). The cumulative production total is 81.75 liters. It appears from these results that the substrate of digester D3 (50% Pig manure and 50% Cow dung) has the highest cumulative value of biogas products (21.5 liters) following by D4 (25% Pig manure and 75% Cow dung), 19.65 liters showing thus the co-digestion substrates remain the most favorable in anaerobic digestion for optimal biogas production [26] [27] .

After following the evolution of biomethanization parameters of substrates (temperature, pH), we presented in this subsection the cumulative biogas production and the evolution of the pressure generated by their production. This is illustrated in Figure 7 for biogas from the five digesters. It is observed that the pressure curves from the five digesters are the same trends. During the digestion process, the pressure of cumulative biogas varied in the five (5) digesters with different average values. 3.15*10⁵ Pascal for the substrate of digester D1 ( Figure 7(a) ); 1.61*10⁵ Pascal for digester D2 ( Figure 7(b) ); 1.16*10⁵ pascal for digester D3 ( Figure 7(c) ); 1.28*10⁵ pascal for digester D4 ( Figure 7(d) ) and 1.48*10⁵ pascal for digester D5 ( Figure 7(e) ).

We observed three phases for the pressure curves ( Figures 7(a)-(e) ). Phase 1: A rapid increase of pressure followed by rapid decrease (5th day to 7th day) for digester D1 and D3 ( Figures 7(a)-(c) ), (5th day to 9th day) for digester D2 ( Figure 7(b) ), (3th day to 8th day) for digester D4 ( Figure 7(d) ) and (3th day to 6th day) for digester D5 ( Figure 7(e) ); Phase 2: a small increase of pressure (7th day to 8th day) for digester D1 and D3 ( Figures 7(a)-(c) ), (9th day to 10th day) for digester D2 ( Figure 7(b) ), (8th day to 9th day) for digester D4 ( Figure 7(d) ). This phase do not exist for digester D5; Phase 3: A decrease of pressure (10th day to 25th day) for digester D2, (8th day to 25th day) for digester D3, (9th day to 25th day) for digester D4, (6th day to 25th day) for digester D5, and (8th day to 14th day) for digester D1.

In order to obtain an overview of their daily evolution, we represented in Figure 7 , the evolution the biomethanization parameters (biogas production, temperature, pH) and pressure of the cumulative biogas production for the substrates of the different digesters on the same graph. The results confirm the same interpretations of the Figures 1-7 .

For daily biogas production ( Figure 8(a) ), it can be seen that during the 25 days of digestion, the largest quantity of biogas was recorded on the 15th day in the digesters (D3 and D4) with a value of 2 liters. The same value was recorded on the 19th day in digester D2. The second largest value of biogas produced is 1.5 liters, recorded on the 8th day in digester D5 and the smallest value 1liter was recorded on the 11th day in digester D1. For daily temperature evolution ( Figure 8(b) ), the temperature in the digesters D1, D3 and D4 varied from 25˚C to 29˚C. It varied from 25˚C to 30˚C in digesters D2 and D5. For daily pH evolution ( Figure 8(c) ), the pH variation curves of the substrates show that, during the digestion process, the pH varied from 5 to 8 in the five (5) digesters. For daily pressure evolution, it is observed the pressure curves in the five digesters are the same trends and during the digestion process, the pressure varied in the five (5) digesters with different average values.

Biogas is a mixture combustible gas if the methane content is greater than or equal to 50%. The combustion of biogas is characterized by the release of a yellow or blue flame depending on the methane content. A persistent blue flame confirms the presence of methane in significant proportion (50%) or more [28] . The results obtained during this experimental study show that the biogas produced from the different types of substrates is combustible ( Figure 9 ). The combustibility test revealed that the biogas produced by the substrates during the first two weeks was non-flammable. It was from the 15th day that the combustibility of the biogas produced began, and it is from the 20th day that a significant quantity of methane with a stable flame was observed. It appears from this test, the quantities of biogas produced by the substrates (50%PM + 50%CD, 25%PM + 75%CD and 0%PM + 100%CD) respectively in the digesters (D3, D4 and D5) were very combustibles (Figures 9(a)-(c)). These test results confirm the importance of co-digestion of pig manure with cow dung in different proportions for combustible biogas production [29] .