The materials used for the tests were one liter plastic containers, four in number; the 330 mL calibrated plastic containers, four in number, 160 mL plastic cup, 4.2 m × 0.8 cm diameter hose, plastic hollow bolts and nuts, rubber seals, washers and hose clips.
The method adopted in this study is similar to the method of Ahamed et al. (2016). Each of the one liter plastic containers was thoroughly washed and conditioned for growth of micro-organisms. They were perforated at the top, close to the lid and made airtight using washers and rubber seals. Hoses, clips and rubber seals were used to secure an airtight link between the one liter containers and 330 mL calibrated plastic container shown in Fig. 1. The 330 mL calibrated bottle for gas collection was made airtight and inverted. A 0.8 cm hose fitted into the inverted calibrated bottle was made to discharge water into a measuring container. At the start of each experiment, the calibrated plastic bottle was filled with water and inverted as in Fig. 1. As biogas was discharged from the digester, it entered the inverted jar filled with water which was made free from undesirable gases. The biogas was first measured by the calibration on the inverted jar and was then confirmed by the volume of displaced water collected in the measuring container. Four sample tests, T1, T2, T3 and T4 were carried out. The T1 contained cow dung (CD) only, the T2 contained jatropha fruit exocarp (JFE) only, the T3 contained a mixture of CD and JFE at the ratio of 3꞉1 and the T4 contained CD, JFE and 10 g of iron filings (Fe). The iron fillings used were in the solid state and of 10 micrometer particle sizes. The composition for each sample tested for anaerobic digestion is shown in Table 1. Slurry volume of 1000 mL was made for each sample from which 630 mL was used to make a mixture for the first test T1. The second test T2 used 640 mL, the third T3 and fourth T4 used 600 mL, respectively. The slurries were mixed with a stirrer and left for seven hours before being used for the experiment. The experiments were carried out between the months of April and June at an average temperature of 36℃.
Sample CD/g JFE/g CD, JFE/g Fe/g CD, JFE, Fe/g Slurry/ml Water in slurry/ml Inoculum/ml Total slurry/ml T1 (CD) 172.108 0 0 0 0 630 320 50 1000 T2 (JFE) 0 57.369 0 0 0 640 310 50 1000 T3 (CD, JFE) 0 0 229.477 0 0 600 350 50 1000 T4 (CD, JFE, Fe) 0 0 0 10 239.477 600 350 50 1000 Notes: CD, cow dung; JFE, jatropha fruit exocarp.
Table 1. Composition of treatments by mass and volume
An inoculum of 5% of the feedstock was used to assist the anaerobic digestion of the individual treatment and was prepared by composting fresh cattle dung for the period of six days, under ambient temperature (Bagudo et al., 2011; Wang et al., 2016). As a result of active micro-organisms contained in fresh cattle dung, 50 mL of inoculum were added to each of the four samples in order to compensate for the dead or weakened micro-organisms in the various sample treatments. The experiment carried out in duplicates and the averages were calculated.
The anaerobic digestion by chemical reaction stages are (Fig. 2):
Figure 2. Stages of anearobic digestion to produce methane (Ahamed et al., 2016)
n(C6H10O5) + nH2O → n(C6H12O6) (acid production) (1)
n(C6H12O6) → nCH3COOH (acid reduction) (2)
nCH3COOH → nCH4 + CO2 (methane formation) (3)