A 4000 L bio-digester was designed and fabricated using 5 mm mild steel plate with the main purpose of generating biogas and bio-fertiliser for Auchi Polytechnic Demonstration Farm. To achieve this, a varying ratio of 1.155 m3: 2.310 m3 mixtures of cow dung and water were anaerobically digested in the batch digester. The experiment lasted for 33 days during which the quantity of gas generated were evaluated. The volume of daily methane gas production ranged from 0 to 1.2 m3. The total volume of gas generated from the digestion was 21.68 m3 and comprised of 58% CH4, 25% CO2, 15% H2S and 2% other impurities. The results of physiochemical properties of the feedstock revealed a progressive increase of 6.0-6.2 in pH value from Day 4 after which it dropped sharply in value to 5.5 on the second week of fermentation. The minimum and maximum ambient temperatures of 24 and 30°C were observed while the slurry temperatures varied from 27 to 35°C. The average pressure built up of the digester was about 214,000 N/m2. The results further showed that biogas production is sensitive to pH and cannot produce optimally below the pH of 6.5. To achieve optimum biogas generation, a temperature range within mesophilic condition of 25 to 40°C and pH values of 7 should be maintained.
Key words: Biogas, mesophilic temperature, retention period, anaerobic digestion, digester.
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