PRACTICAL SYSTEM BOUNDARIES

Biorefinery/life cycle assessment reviews emphasize that cassava residues can be directed to multiple products (animal feed, biogas/bioenergy, volatile fatty acids/biochemicals, fertilizers), and that environmental outcomes are highly sensitive to system definition, substitution assumptions, and coproduct allocation [29]. A useful decision boundary is therefore: feed first when (i) residue quality can be stabilized and controlled, (ii) local dairy demand exists within a viable transport radius, and (iii) the alternative use is low-value disposal or low-efficiency energy recovery; whereas energy/industrial routes may dominate when residues are too dilute or contaminated for safe feeding, when dairy demand is distant, or when wastewater streams (often the largest residue by volume) are better suited to anaerobic digestion or volatile fatty acid platforms than to feed [151]. Importantly, emerging analyses warn against assuming universal climate benefits for waste-to-feed without robust, context-specific accounting, reinforcing the need to avoid overgeneralization and to report decision boundaries transparently [152]. Recent cassava starch and cassava-biorefinery studies make these system boundaries more concrete. In a 2026 factory-level study of a 120 t/day cassava starch plant in Vietnam, shifting excess biogas from a base case used mainly for drying to a biogas-to-electricity configuration reduced total fossil energy use from 3730.7 to 2901.7 MJ per ton and greenhouse gas emissions from 0.79 to 0.72 tCO₂-eq per ton. The same study reported annual production-cost savings of about USD 0.12 million, a payback period of 6.6 years, and a return on investment of 15%, illustrating that circularity claims are highly sensitive to energy-integration choices rather than to residue valorization alone [153].