Assessment of Some Mineral Nutrients in Digestate Slurry of Cow Dung, Poultry Wastes and Rice Husk

Anaerobic digestion of organic wastes provides a sustainable pathway for renewable energy production and nutrient recovery. Limited data exist on the combined influence of slurry concentration, single and co-digested substrates on nutrient composition. Cow dung, poultry waste and rice husk were prepared for analysis, with biogas production monitored over a hydraulic retention period of 48 days. Eight digesters D1-D4 and D5-D8 are prepared in solid to water ratios of 1:4 and 1:12 respectively. (D1, D2, D3, D5, D6, D7) contained pure substrates, while (D4&D8) contained co-digestate substrates and biogas yield was measured using the water displacement method. However, the digestate slurries have undergone a standard wet acid digestion method; and their mineral nutrients were evaluated through Kjeldahl; for nitrogen, AAS; for K, Mg, Ca and UV-Visible for P analysis. Spike recovery experiments were performed to assess the accuracy of the results. All analysis were performed in triplicate, and data were evaluated using ANOVA followed by Tukey’s HSD post-hoc test (α = 0.05). Biogas yields varied among digesters, ranging from 335 cm³ (D3) to 6145 cm³ (D2). Digesters D1 (4305 cm³) and D6 (3680 cm³) also exhibited high production, while D4, D5, D7, and D8 showed intermediate yields, demonstrating the influence of substrate composition and slurry concentration. Proximate analysis revealed moisture contents of 10–35%, ash contents of 23–85%, and organic matter contents of 5–45%, with poultry waste exhibiting the highest organic matter content and rice husk the highest ash content. Nutrient analysis shows N(0.60±0.32-3.20±0.09 mg/L), P(0.95±0.02-3.93±0.02 mg/L), K(0.83±0.02-2.70±0.02 mg/L), Mg(0.67±0.03-2.95±0.03 mg/L) and Ca(1.33±0.02-2.01±0.03 mg/L), generally with their %RSD of (<10%), P-values (<0.0001) and spike recoveries (>80%). In all cases, the ANOVA models explained a very high proportion of the total variance (R² > 99%), indicating a strong influence of substrate composition and digestion conditions on nutrient distribution in the digestate. The observed variations in biogas yield and nutrient concentrations among the digesters indicate that substrate composition and slurry concentration influenced digestion performance and digestate nutrient recovery. Further agronomic assessments are required to verify the suitability of the resulting digestates for agricultural application.