The use of enhanced anaerobic digestion process for energy recovery and phosphorus release from agro-industrial wastes

Abstract

Enhancement of anaerobic digestion process is an established strategy for ensuring the usability of diverse substrate-types, overcoming the recalcitrance of substrates to biological conversion and increasing biodegradability. It aids the supplementation of deficient but vital nutrients useful to methanogenic activities and constituting cofactors in anaerobic digestion enzymes as well as increasing both biogas production and biomethane content. Although these enhancements especially the use of iron-based additives aid substrate solubilization and improve biomethane yield, the iron ions react with other available ionized nutrients such as phosphate (P) to form non-degradable complexes and hinder recoverability of P or its release for plant usage. In response, this thesis investigates an integrated approach to biomethane recovery and P release using different enhancement options (accelerants and antagonists) as well as an assessment of the environmental impacts of the enhanced processes. Mixed methodological approach was employed in this study to select a novel substrate (okra biomass) and additive (Ppy/Fe3O4 NPs) for anaerobic digestion and assessment of the impacts of enhancement options on the environment. From all the enhancement options, most of them increased biomethane yield, but 20 mg/L of Ppy/Fe3O4 NPs gave the highest biomethane yield. The optimization of additives (accelerants and P antagonist) supplementation achieved the maximum biomethane yield of 502.743 mLCH4/gVS and P release of 168.674 mg/L at the optimum conditions of Ppy/Fe3O4 (20.0014 mg/L), HA (5.0018 mg/L), As (1.448 mg/L) and co-digestion (25.0001%). In considering the energy-environmental trade-offs of the enhancement options, a comparative assessment of the impact categories showed that co-digestion+Ppy/Fe3O4 NPs and Ppy/Fe3O4 only options improved biomethane productions and recorded lower impact category values.