This study aimed to harness the bio-stimulatory properties of two heavy metals, Zn (II) and Cd (II) at low concentrations in improving biogas yield from anaerobic digestion of cow dung (CD) and pig dung (PD). Proximate characteristics of the feedstocks were estimated by adopting standard methods. The batch fermentation experiment which was set-up in triplicates with equal weights of CD/PD and three concentrations: 0.02, 0.05 and 0.1 mM of the heavy metals in the reactors was conducted at ambient temperature range of 24 - 35 ± 2°C for 50 days. Results showed that the PD had remarkably higher volatile solids (55.50%) and lower carbon-to-nitrogen ratio (7.40%) whereas CD had higher carbon-to-nitrogen ratio (11.0%) but lower volatile solids (28.20%). Biogas yield increased with increase in concentration of the heavy metals except in 0.02 mM of Cd (II). The highest yield, 0.16 dm3/g VS was recorded at 0.1 mM of Zn (II) which amounted to 33.65 dm3 and 48.83% increase compared to the control. This was followed by 0.05 mM of Cd (II) with 0.15 dm3/g VS (32.85 dm3) and 45.29% increase. At higher concentration, 0.1 mM of Cd (II), a decline of 8.54% in biogas yield was recorded (0.14 dm3/g VS). Diauxic response in biogas production was observed in all the treatments. The adapted model, two-phase logistic function model adequately described the diauxic behavior with very high accuracy as indicated by the relative coefficient (R2) which was approximately 0.999 in all the tests. Diauxic biogas production responses from anaerobic digestion of lignocelluloses are ignored most often. This study has demonstrated the efficiency of two-phase logistic function model in the kinetic evaluation of diauxic biogas production from biomass with high accuracy.
Key words: Agro-wastes, heavy metals, biogas yield, diauxic response, logistic model.
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