Biogas production from Pongamia biomass wastes and a model to estimate biodegradability from their composition

Abstract

In this study, I investigated the chemical characteristics, biochemical methane potential, conversion kinetics and biodegradability of untreated and NaOH-treated Pongamia plant parts, and pod husk and press cake from the biodiesel industry to evaluate their suitability as an alternative feedstock for biogas production. The untreated Pongamia seeds exhibited the maximum CH₄ yield of 473 ml g ⁻¹ volatile solid (VS) added. Yellow, withered leaves gave a yield as low as 122 ml CH₄ g ⁻¹ VS added. There were significant variations in the CH₄ production rate constants, which ranged from 0.02 to 0.15 d ⁻¹, and biodegradability, which ranged from 0.25 to 0.98. NaOH treatment of leaf and pod husk, which were highly rich in fibers, increased the yields by 15–22% and CH₄ production rate constants by 20–75%. Utilization of Pongamia wastes in biogas digesters not only influences the economics of biodiesel production but also yields CH₄ fuel and protects the environment. The experimental data from this study were used to develop a multiple regression model, which could estimate biodegradability based on biochemical characteristics. The model predicted the biodegradability of previously published biomass wastes (r² = 0.88) from their biochemical composition. The theoretical CH₄ yields estimated as 350 ml g⁻¹ chemical oxygen demand destroyed are much higher than the experimental yields as 100% biodegradability is assumed for each substrate. Upon correcting the theoretical CH₄ yields with biodegradability data obtained from chemical analyses of substrates, their ultimate CH₄ yields could be predicted rapidly.