Organic waste management in institutional settings presents a persistent challenge due to the fluctuating macronutrient composition of daily waste streams. This study investigated the effect of macronutrient composition on Black Soldier Fly Larvae (BSFL, Hermetia illucens) bioconversion performance and larval development using four substrate types: tofu waste (protein-rich), leftover rice (carbohydrate-rich), green cabbage (fiber-rich), and a mixed substrate of equal proportions of all three. Conducted as part of an organic waste management initiative at Universitas Sumatera Utara, 7-day-old BSFL were cultivated over a 21-day feeding period, with larval length, weight, color, pre-pupal development, and chitin formation assessed at regular intervals. A preliminary antinutritional evaluation was further conducted using catfish (Clarias batrachus) fed a composite diet of 50% BSFL pupae and 50% commercial fish feed. Results demonstrated that substrate macronutrient composition significantly influenced all measured parameters (p < 0.05). Tofu waste exhibited the highest bioconversion rate, with complete substrate consumption within approximately 10 hours per feeding cycle, while green cabbage showed persistent residual substrate throughout cultivation. The mixed substrate achieved the highest total larval length at harvest (17.298 ± 3.341 mm), while tofu-fed larvae recorded the highest body weight. Pre-pupal formation was most advanced in tofu and mixed substrate treatments, with green cabbage larvae failing to initiate pre-pupal transformation within the cultivation window. Chitin yield was highest in mixed (18.136 ± 0.941%) and tofu (17.219 ± 0.530%) treatments at Day 28, while FTIR analysis confirmed that all substrates produced structurally identical α-chitin, which confirms substrate waste composition influences chitin quantity but not quality. A preliminary antinutritional trial indicated that partial substitution with chitin-bearing BSFL pupae (estimated 8–9% dietary chitin) significantly suppressed catfish growth over 14 days; as this short-term trial could not isolate the effect of chitin from other dietary differences, the finding is preliminary and requires confirmation through longer, dose-controlled studies.