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Assessment of Anaerobic Digestate Amended with Wood Ash and Green Vegetable Matter and Impacts on Microbial Growth

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Anaerobic digestion (AD) is a waste management method worldwide, that results in biogas and digestate production. The digestate remains is a potential biofertilizer but may fall short of vital nutrients required for optimal plant growth, hence, requires amendment with other biomass residues. This study was aimed at investigating the microbial quality of digestate amended with wood ash and vegetable matter during treatment. Digestate from cattle rumen content (CR), food waste (FW), fruit waste (FRW) and their combinations were amended with wood ash, vegetable (green) matter and a blend of both. Amendment was carried out for 5 weeks, and samples were analyzed weekly. Enumeration and identification of bacteria and fungi were carried out on culture media. The rhizobacterial potential of the identified bacterial isolates was also investigated using standard microbiological procedures. The results showed high bacterial and fungal load in vegetable amended digestate from the first to the fourth week. Comparatively, cattle rumen content and food waste digestate amended with vegetables were highly significant, with mean values of 0.8 and 0.6, respectively. The identified bacterial isolates namely, Bacillus subtilis, Salmonella sp, Pseudomonas sp, Proteus sp, Enterobacter sp, Chromobacterium sp Bacillus spp. and Escherichia coli were found to be involved in the solubilization of phosphate, nitrogen fixation, ammonia production and induction of indole acetic acid. Specifically, feedstocks amended with vegetable (green) waste was found to have a significant influence on microbial growth (bacterial and fungal loads) between week 1 and 4. This suggests that digestate amendment is a crucial recipe for microbial growth which has considerable benefits in nutrient enhancement and increase in carbon levels.

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Digestate amendment and nutrient enhancement for plant growth

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Total Solid


Volatile Solid


Total Organic Carbon


Total Nitrogen


Cattle Rumen


Food Waste


Fruit Waste


Anaerobic Digestion


Polyvinyl Chloride


Sustainable Development Goal






University of Benin senior staff quarters


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This research was supported by the UKRI Collective Fund award UKRI GCRF RECIRCULATE: Driving eco-innovation in Africa: Capacity-building for a safe circular water economy, Grant Ref.: ES/P010857/1.


This research was supported by the UKRI Collective Fund award UKRI GCRF RECIRCULATE: Driving eco-innovation in Africa: Capacity-building for a safe circular water economy, Grant Ref.: ES/P010857/1.

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ESI: Experiments, Data analysis, MUA: Conceptualization, Methodology, Writing- Original draft preparation, Reviewing and Editing, CEO: Methodology, Writing, AA: Reviewing and Editing, AO: Experiments, Data analysis, VO: Reviewing and Editing, ISO: Reviewing and Editing, IT: Data discussion, NI: Reviewing and Editing, VIOE: Reviewing and Editing, NE: Reviewing and Editing, L-BAJ: Reviewing and Editing, CE: Supervision, Reviewing and Editing, LINE: Supervision, Reviewing and Editing, KTS: Supervision, Reviewing and Editing Supervision. All authors have read and approved the final manuscript.

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Correspondence to Mike U. Ajieh.

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Isagba, E.S., Ajieh, M.U., Oshoma, C.E. et al. Assessment of Anaerobic Digestate Amended with Wood Ash and Green Vegetable Matter and Impacts on Microbial Growth. Waste Biomass Valor 14, 3013–3025 (2023).

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