Waste left behind from growing oyster mushrooms (Pleurotus ostreatus) was turned into bioethanol fuel using bacteria, enzymes, and leftover mushroom substrate made from sawdust and soybean hulls. The best process produced 7.07 g/L ethanol and boosted fuel output 15 times compared to the untreated version. Since mushroom farms can create 3–5 kg of leftover substrate for every 1 kg of mushrooms grown, this could help turn a huge waste problem into renewable energy.