Fresh cow dung samples used in this study were collected at the INP-HB cattle farm. Human urine was collected in the latrine and cassava effluent was collected from women in Yamoussoukro.

The pH was measured on 2 g of sample homogenized with 20 ml of distilled water using a pH meter (HAINA). The pH-meter is switched on and the probes are plunged directly into the different beakers containing the substrate. The value of these parameters is read directly on the digital display screen of the apparatus presented as follows [18] .

The dry matter was determined from 5 g of samples by differential weighing, after passing through the Heraeus T 5042 oven at 105˚C for 24 hours. The following formulas were used to calculate the dry matter (DM) and moisture (H) of cow dung [19] .

$\%\text{H}=\frac{M_0-M_1}{M_0}$(1)

$\%\text{DM}=100-\%\text{H}$(2)

With:

%H: percentage of moisture

M₀: initial mass of the sample before drying (fresh cow dung)

M₁: final mass of the sample after oven drying at 105˚C

%DM: dry matter

The organic matter (OM) content is obtained by weighing the difference between the mass of the dry waste (M₁) and the mass of the waste calcined at 600˚C (M₂) to a constant weight for 2 hours [19] .

$\%\text{MO}=\frac{M_1-M_2}{M_1}\times 100$(3)

With:

M₂: the final mass of waste calcined at 600˚C

M₁: final mass of the sample after drying at 105˚C

The organic carbon content was deduced from the organic matter content after calcining the sample in a muffle furnace at 600˚C for two hours. The following formula is used to calculate the organic carbon [20] .

$\%\text{C}=\frac{\%\text{MO}}{1.8}$(4)

Total Kjeldahl nitrogen takes into account two forms of nitrogen: total organic nitrogen and ammonia nitrogen. This determination consists of three steps: mineralization of organic nitrogen into ammoniacal nitrogen, distillation and titration [19] .

The 50 ml sample and 50 ml of distilled water (control) were introduced into matras mineralization tubes with the addition of 1 g of mineralization catalyst (selenium + potassium sulfate) and 10 ml of 95% - 97% sulfuric acid. The whole is heated to 350˚C for two hours in a fume hood. The reaction being exothermic, the tubes are cooled down to room temperature (25˚C).

After cooling, the acid is neutralized by adding 50 ml of soda 40% (NaOH). The mixture is placed in the distillation tank. The distiller’s coolant line is then dipped into a beaker containing 20 ml of boric acid with mixed indicator (methyl red + methyl blue). The solution turns from blue to violet. The distillation is done during 10 minutes. At the end of the distillation, the solution turns green.

The titration of the distillate is carried out with 0.1 N sulfuric acid. The return of the purple coloring marks the end of the titration, so the volume of 0.1 N sulfuric acid used is recorded.

The total Kjeldahl nitrogen (%N) is determined as:

$\%\text{N}=\frac{\left(V_1-V_0\right)\ast N\ast 14\ast 1000}{V_2}$(5)

With:

V₀: volume of H₂SO₄ used to titrate the control (distilled water);

V₁: volume of H₂SO₄ used to titrate the sample;

V₂: volume of test sample;

N: normality of H₂SO₄ (0.1 N);

14: Atomic mass of Nitrogen.

The digester used in this research work was manufactured using polyethylene (plastic) drums. The experimental device is composed of two parts. The first part consists of a polyethylene canister of capacity 25 L of diameter 25 cm and height 50 cm. This tank contains the reaction medium, i.e. the substrate obtained after mixing the dung and water. The second part consists of the car air chamber which will trap the biogas produced during the methanization. The air chamber acts as a gasometer. The gasometer is designed for two reasons:

Gas lines are used to connect the digester, air chamber and gas valve. The gas pipe is connected to the digester tank and the T-valve; and that the T-valve is connected to a rubber tube that connects a gas valve and the air chamber. The gas valve is used to burn the biogas. This prototype digester used in this work was manufactured in a workshop at the Mechanical and Energy Engineering Department of INP-HB.

A digital thermometer type TM-6 is connected to the digesters to observe the temperature variation in the digester. The synoptic diagram of these digesters is presented in Figure 1 .

The objective of this work is to produce biogas at an affordable cost for households. Also to realize a digester easy to move. Thus our choice of materials for the realization of the digester was focused on the plastic can, the inner tube of car and the gas pipes. These materials are available locally.

The organic waste used in this work is fresh cow dung collected from the cattle farm of INP-HB. Since pH plays an important role in considering the growth of microbial life during anaerobic digestion, a neutral pH is best suited for biogas production, since most methanogens grow at a pH range of 6.7 - 7.5. It is necessary to adjust the pH to maximize biogas production.

Kalloum et al. [10] studied the anaerobic digestion of household waste (kitchen waste) in two digesters, the first with pH adjustment around neutrality (pH = 7) with sodium bicarbonate and the second digester without pH adjustment; This study showed that the influence of pH on biogas production from household waste showed that adjusting the pH around 7 favors the growth of methanogenic bacteria that are responsible for methane formation. We also found that the amount of biogas produced in the adjusted pH case is twice as much as the amount produced in the unadjusted pH case [10] . Thus in this work, cassava effluent and human urine are used to neutralize the pH of the substrate. The urine of a healthy person. The characteristics of human urine and cassava effluent according to bibliographic sources are presented in Table 2 .

Cassava effluent is acidic due to its composition (high starch content, presence of cyanogenic compounds). About 90% of the nitrogen is concentrated in urine [23] . The main constituents of human urine (urea, sodium chloride, sodium sulphate) reveal that human urine has a basic character and can therefore replace chemicals such as lime, soda and sodium carbonate that are usually used to adjust the pH. According to these authors the pH of cassava effluent is 3.4 and human urine is 8.9 [22] .

For the start-up of our digester, the maximum amount of effluent entering the digester is maintained at 80% of the total digester volume to avoid overfeeding the digester, which often leads to inhibition of anaerobic digestion [24] . The fermentation materials entering the digester are calculated at 8% to maximize the biogas volume during anaerobic digestion [25] . The mass of waste and the volume of water to be used are calculated based on dry matter (DM). Thus the mass of waste and the volume of water to be added to the waste to feed the digester are calculated as [25] :

$m\left(\text{kg}\right)=\frac{80\ast \text{Vd}\ast 8}{100\ast \text{MS}}$(5)

$\text{Veau}\left(\text{L}\right)=m\ast \left(\frac{\text{MS}}{8}-1\right)$(6)

With:

Vd: The volume of the digester in liter

DM: Dry matter of the cow dung

Equations (5) and (6) are used to calculate the mass of cow dung and water to be used. The water to be added to the organic waste is heated to 40˚C, as heat is necessary for anaerobic digestion (AD) [26] . A mixture of waste and water is made until there are no lumps in the mixture and filtered [27] . The digester is stirred gently for three to five minutes each day after the introduction of the substrate [27] . The mixture of fresh cow dung and water at 40˚C is done as follows ( Figure 2 ).

The digesters are fed with 80% of the total volume of the reactors and the other 20% is reserved for the biogas that will be produced [27] . If the digester is overfed, acids will accumulate and methane production will be inhibited because microbacteria cannot survive in acidic conditions. Similarly, if the plant is underfed, gas production will also be low due to the alkaline solution, which is also not a favorable condition for anaerobic bacteria. Substrate sampling is performed before (inlet) and after (outlet) anaerobic fermentation in the different digesters. For the anaerobic digestion test, four experimental reactors were started one after the other.

Two digesters ( Figure 3 ) were placed in the black painted sheet metal box exposed to the sunlight while the other two were placed in a room. The increase of temperature in the digester maximizes the production of biogas. The temperature variation is a very important element to take into account because it can irreversibly affect the quantity of methanization microorganisms. To control this temperature variation, the digesters are installed in black painted metal boxes. The box is painted black to absorb the heat [28] . An empty space of 5 to 10 cm is left between the digester and all the walls of the box [29] . This empty space acts as insulation material. In order to guarantee the fermentation process, the temperature inside the digester must be uniform; temperature fluctuations should be avoided as much as possible. However, small temperature fluctuations can be tolerated by the microorganisms of 2˚C to 3˚C/h per day [29] . The use of the metal box also prevents light from entering the digester during anaerobic digestion. Indeed, sunlight favors the production of algae inside the digester which reduces the production of biogas.

The biogas produced is recorded in an air chamber connected to the digester. Thus the calculation of the volume of biogas recorded is as follows ( Figure 4 ):

The volume of biogas at the end of digestion can be calculated as [30] :

$V=2{\pi }^2\ast R\ast r^2$(7)

The four series of experiments presented above were conducted as follows:

All the digesters were operated in a discontinuous manner and were manually agitated on a daily basis before any sampling and measurements.

During the realization of each experiment, the filling of the digesters is done as follows:

The biogas production is calculated at the end of the experiment after 22 days. It is assumed that after 22 days the biogas produced reaches its maximum volume. The gas from the digester was stored in a tube for a flame detection test and biogas analysis.

To show that the gas that had filled the storage chamber (car tire) was flammable, a gas burner or valve is used.

The quality of the gas obtained is the best key to appreciate each recorded production. The flammability test of the gas would allow to have a more precise idea on the nature of the gas, i.e. to recognize if the gas obtained is indeed biogas [3] . The analysis of the biogas allows knowing its composition (methane, carbon dioxide, etc.). The analyzer presented in Figure 5 was used to analyze the biogas.

In order to evaluate the capacity of the waste to produce quality biogas, these parameters (dry matter (DM), organic matter (OM) and C/N ratio) must be determined. Equations (2), (3) and (4) were used to determine these parameters. Table 3 shows the values obtained from cow dung.

The dry matter of cow dung used in this work is estimated at 15.5%. This value is in the range of 12% to 17%, the range in which the dry matter of cow dung oscillates [25] . Moletta [31] reports that the dry matter (DM) content of the substrate to be anaerobically digested generally determines the choice of anaerobic digestion processes and the type of anaerobic digester. The dry matter content of the study substrate (15.5%) confronts us with a “wet” anaerobic digestion which is adapted to the treatment of wastes having between 5% and 20% of dry matter (DM). The OM of 95.82% (DM), is slightly higher than the usual characterization of this waste. The range of OM of cow dung varies between 80% and 90% [1] [6] . High organic matter (OM) contents indicate preferred substrates for microorganisms in anaerobic digestion. A compound with high OM is rich in carbon because the relationship (4) shows that the percentage of carbon (%C) is related to the organic matter (OM). A substrate rich in carbon (C) produces a high proportion of methane, up to 90% [11] .

The C/N ratio of cow dung is in the optimal zone for biogas production. Indeed a ratio (C/N) carbon-nitrogen of 20 to 30 represents an optimum for biogas production, while the excess of nitrogen or carbon sources can lead to inhibition.

From the values obtained from the cow dung (DM, OM and C/N), it appears that the cow dung is relatively biodegradable, easy to use for a biogas plant, already contains the right bacteria able to produce quality biogas.

The physico-chemical characterizations of the urine and cassava effluent used are given in Table 4 :

The physico-chemical characteristics of the cassava effluent show an acid effluent with a pH below 7 and a low quantity of nitrogen which is 0.41 g/L. These cassava effluents are biorecalcitrant because they are acidic (pH < 4) and present a nitrogen deficiency (0.6 - 0.8 g/L) according to [22] . The PH value of human urine used in this work is greater than 7 and rich in nitrogen (2.64 g/L). These values are not fixed; they can vary from one individual to another. Acidic human urine is sometimes found. However, the composition varies according to several factors such as the time of urination, type of food consumed and the physical conditions of the person concerned. The high content of nitrogen is explained by the high content of urea in human urine as mentioned in the thesis work of Youssef Abarghaz [23] . Because human urine is rich in nitrogen, it therefore has a basic character.

From the value of the dry matter (DM), we calculated the mass of fresh cow dung to be used as well as the quantity of water which it is necessary to add to the cow dung from the formulas (5) and (6). For a tank filled to 75% of its total volume, we used 9 kg of cow dung and 8.5 liters of water, heated to 40˚C to feed each digester (see Table 5 ).

In each experiment, all the digesters were fed with 9 kg of cow dung + 8.5 liters of water at 40˚C. The feed substrate of the digesters in experiments 2 and 4 was adjusted with 200 mL of human urine and 1 liter of cassava effluent to have the pH of the substrate around neutrality (PH = 7). Before feeding the digester, the pH and the oxidation-reduction potential (ORP) were measured using a pH meter. After a retention period of 22 days, these same parameters were measured as well as the volume of biogas recorded. Table 6 gives the different values obtained in each experiment:

The digital pH meter described above was used to measure pHinitial, pHfinal, ORPinitial and ORPfinal. The digital thermometer connected to the digester makes it possible to measure the temperature in the digester. The biogas being stored in an air chamber. Once the inner tube is filled, the volume of biogas can be calculated using Equation (7).

This digester operated with a temperature varying between 25˚C and 28˚C without adjustment of the substrate. The initial pH of the substrate is 7.8 at the start of anaerobic digestion and 5.8 at the end of anaerobic digestion.

It is therefore obvious that the pH has dropped in the acid zone, therefore the methanogenic bacteria producing flammable biogas cannot survive in the event of acidification of the substrate. This therefore made it possible to obtain a non-flammable biogas at the end of the anaerobic digestion. This is visible through the redox potential which is also positive.

Higher or even positive values of the oxidation-reduction potential are the sign of a fermentation defect. This further confirms that biogas-producing methanogenic bacteria were not active in the digester during methanogenesis.

The difficulties encountered with the previous digester linked to the acidification of the pH led us to create a second digester always at a temperature varying between 25˚C and 28˚C but this time with the adjustment of the pH with 1 liter of effluent of cassava and 200 mL of urine. The initial pH after adjustment was 7. On the other hand, at the end of anaerobic digestion, the pH dropped to 6.4. The favorable pH range for anaerobic microbes being 6.8 - 7.5.

Adjusting the pH with human urine and cassava slurry helped improve the pH around the optimal pH zone which is 6.8 - 7.5. The pH approaches around neutrality and the oxidation-reduction potential remained positive at the end of the anaerobic digestion. The biogas obtained is not flammable. As in the previous case, the methanogenic bacteria were killed in this digester.

The temperature varying between 25˚C and 28˚C did not allow the growth of methanogenic bacteria in this digester as long as the pH was adjusted. It is therefore clear that the temperature has a great influence on the growth of microorganisms [29] . The methanogenic phase takes place between 32˚C to 42˚C with 35˚C being the optimal value.

From these two experiments we found that at the temperature varying between 25˚C and 28˚C and unadjusted pH; at the temperature varying between 25˚C and 28˚C and adjusted pH it is impossible to obtain a flammable biogas. Therefore we found it judicious to observe the effect of the increase in temperature alone on the production of biogas. The digesters will no longer be inside the laboratory but the digesters will be exposed to the sun’s rays. So we made digester 3 placed in a box painted black and exposed to sunlight without adjusting the pH. When the sun’s rays strike the black metal box, it absorbs heat, which will help to increase the temperature in the digester. The empty space of 10 cm left between the digester and the walls of the box acts as an insulator which will prevent a large variation (sudden variation) of the temperature in the digester [28] .

The average temperature in digester 3 measured using the thermometer is 31.5˚C, the initial pH is 7.8 and at the end of anaerobic digestion the pH is 7.4. The pH remained slightly above neutral; But the oxidation-reduction potential at the end of methanization was negative (−3.9 mV) which resulted in the production of a flammable gas of 15.8 liters. This volume was calculated using relation (7). The temperature increase of 31.5˚C favored the growth of methanogenic bacteria. The choice of a temperature of 31.5˚C makes it possible to improve bacterial kinetics while favoring the metabolism of active microflora in the organic load confirmed.

Admittedly, digester 3 gives flammable biogas, but the pH remained slightly above neutral; it is therefore important to see the effect of the pH adjustment. The results showed that the volume of biogas with digester 4 is twice the volume of biogas obtained with digester 3.

The significant quantity of biogas recorded in the case of digester 4 (in the black box and exposed to the sun’s rays) is at two levels:

The addition of human urine, which has a basic character, and the acid cassava effluent in digester 4 raised the pH. The high production and flammability of biogas in digester 4 neutralized with human urine at the start of the experiment would be due to the almost neutral pH of the reaction medium and the increase in temperature (26˚C to 37˚C). This neutral pH would have made it possible to initiate methanogenesis in the reactors. These same results were obtained by Kalloum et al. (2007) [11] . According to these authors, methane production is high in the temperature range of 35˚C to 40˚C. The digester in the box exposed to the sun’s rays with pH adjustment produces a high quantity of biogas (430,750 liters).

The values of the oxidation-reduction potentials remained negative at the end of the methanization leading to a flammable biogas. A low oxidation-reduction potential (ORP) is necessary to maintain the activity of methanogenic bacteria. Normally, the redox potential of non-methanogenic bacteria is in the range −100 to 100 mV, while that of methanogenic bacteria is in the range of −400 to −150 mV. The results obtained in this work are in agreement with Kalloum et al. (2007) [10] where a neutral pH and a mesophilic temperature favor the production of biogas.

The anaerobic digestion parameters are significantly optimized when the digester is installed in the metal box and the substrate adjusted with human urine and effluent. Thus the biogas produced in this case is therefore analyzed in order to know the proportion of each constituent.

Attempts to burn the gas produced were satisfactory at the end of the methanization, which means, and according to the literature, that the composition of our produced biogas is such that it is possible to recover it [3] . Figure 6 gives the blue flame obtained in experiment 4 in a lighted (a) and dark (b) room.

The methane contained in the biogas produced in experiment 4 is 67.1% and 21.1% carbon dioxide. The production of biogas with dung contains 67.9% methane and 27.9% carbon dioxide according to the work of Ukpai et al. (2012) [9] . This present work improves the quality of biogas given the decrease in the proportion of carbon dioxide (21.1%) ( Table 7 ).

The fresh cow dung we used in this work has a C/N ratio of 28.78. When the sample was made by adjusting the pH with urine and cassava slurry, it had a lower C/N ratio. This drop in C/N is explained by the fact that urine is rich in nitrogen, so the addition of urine increases the amount of nitrogen in the substrate, so a reduction in the C/N ratio will be observed in particular. Human urine is generally used to reduce the C/N of organic materials with a high C/N ratio [23] . The sample adjusted with cassava effluent and human urine after analysis gives the results below. These different parameters are obtained before anaerobic digestion and after anaerobic digestion ( Table 8 ).

During anaerobic digestion, the rate of organic matter in cow dung dropped from 83.40% to 59% at the end of methanation. There was a reduction in organic matter by more than half. During anaerobic digestion the rate of organic matter in many wastes or biodegradable by-products is halved. The pH at the end of anaerobic digestion is 7.2; this shows the stability of the digester and the process of anaerobic digestion. This reduction in organic matter shows a large consumption of carbon during anaerobic digestion [11] . The reduction of nitrogen during anaerobic digestion is low, it goes without saying that the digestate obtained after anaerobic digestion is rich in nitrogen as we can see in Table 6 . The high proportion of nitrogen is due to the presence of urine in the final digestate. Urine contains the three most important plant nutrients that farmers buy and use as artificial fertilizers. Namely, nitrogen (80% of the organic nitrogen contained in our excrement is in the urine) [23] , phosphorus and potassium are all the essential elements that plants need for their growth during their cycle. of development, which consists in passing from the state of seed to the production of another generation of seeds. Urine is therefore an excellent source of nitrogen for plants [23] ; this digestate is therefore an excellent fertilizer, and can therefore be used by growers as fertilizer.