Relationship between Different Anaerobic Digestion Parameters in a Pig-dung Aided Water Hyacinth Digestion Process

Abstract

This study is aimed at assessing the relationship between different anaerobic digestion parameters (biogas quality, retention time, pH, temperature, biogas pressure, volume of biogas produced and cumulative volume of gas produced) in a Pig-dung (PD) aided Water Hyacinth (WH) digestion process in order to maximize biogas yield in terms of quantity and quality. 25 - litre capacity plastic prototype digesters were used in the study and eleven (11) mix ratios of PD and WH were prepared namely: 10 WH: 0 PD, 9 WH: 2 PD, 8 WH: 2 PD, 7 WH: 3 PD, 6 WH: 4 PD, 5 WH: 5 PD, 4 WH: 6 PD, 3 WH: 7 PD, 2 WH: 8 PD, 1 WH: 9 PD and 0 WH: 10 PD. The digestion process was evaluated for a retention period of 40 days. A bivariate Pearson correlation analysis was carried out to examine the relationship between the quality of gas produced and other variables. The results revealed that daily gas production yields greatly improved in the co-digestion runs with mix 3 WH: 7 PD recording the highest maximum daily yield of 9.5 L with a cumulative gas volume of 140 L. For this mix, the methane content of the gas produced increased from 5.8% on day 4 to 69.2% on day 20. The least quantity and quality of gas was produced by mix 10 WH: 0 PD with a maximum daily yield 2.34 L and a cumulative gas yield of 32.18 L. The digestion of all the mixes occurred within a mesophilic temperature range of 28.2 to 31.4 0C and an increase in temperature within the digestion resulted in an increase in the quality of gas produced. The gas pressure ranged from 1 bar to 3.324 bars with an increase in gas pressure leading to a corresponding increase in volume of gas produced. The pH of the substrates ranged from 6.1 to 8.4 with the values low at the start of the digestion process and gradually increasing to its maximum at the end of the digestion process. The results revealed a very strong, positive and significant association between the quality of the biogas produced and other digestion parameters.