Effects of Digestate from Cassava Effluent Methanisation on the Physicochemical Parameters of Cassava Roots

Anaerobic digestion of cassava-processing effluents produces digestate that could be used as an organic fertiliser, but its influence on the composition of harvested cassava roots requires further examination. This study assessed the effects of cassava-effluent digestate on the physicochemical properties of roots from the Yacé cultivar. A field experiment comprised five treatments: digestate at 75% (v/v), three mineral fertilisers (NPK 20-15-15, NPK 4-3-12 and NPK 7-21-7), and an unfertilised control, arranged in three replicated blocks. Roots were harvested 12 months after planting and analysed for proximate composition, cyanogenic content, antioxidant compounds and mineral elements. Treatment differences were assessed using analysis of variance and Tukey's test. Digestate-treated roots contained 97.83 ± 0.07% total carbohydrates, 4.03 ± 0.03% total sugars and 3.35 ± 0.05 g/100 g fibre on the reported dry-matter basis. Their hydrocyanic acid content was 152.55 ± 1.35 mg/kg fresh matter, compared with 236.25 ± 1.35 mg/kg in control roots. Digestate also yielded higher total polyphenol, flavonoid, potassium, calcium and sodium concentrations than the control. Nevertheless, arsenic, cadmium and lead were detected in digestate-treated roots, and the measured hydrocyanic acid concentration remained substantial. The findings indicate treatment-associated differences in root composition, while underscoring the need to establish the safety and composition of processed products before recommending food use.