Abstract Microwave-assisted wet torrefaction (WT) was investigated for the co-torrefaction of optoelectronic sludge (OS) and fruit waste. The factors of torrefaction temperature (120–180 °C), reaction time (10–40 min), biowaste types of Mangifera indica seeds (MIse) and Passiflora edulis shells (PEsh), and blending ratio of OS and biowaste were investigated. Blending OS with two biowaste types separately at different blending ratios increases the biofuel quality and mitigates the problem of waste disposal. Blending OS with MIse at a ratio of 25/75 under the same WT condition further increased the HHV of 19.0 MJ/kg while retaining a high energy yield of 92.1%. Thermogravimetric analysis and derivative thermogravimetry results confirmed that the properties (particularly dehydration and devolatilization) pertaining to the thermal stability of co-torrefied biochar were superior to the raw OS. Furthermore, the 10%–20% co-torrefied biochar can replace sub-bituminous coal and reduce the global warming potential. Co-firing the obtained biochar with bituminous coal could aid in achieving high combustibility (fuel ratios of 1.60–1.82), high energy efficiency (energy return on investment of 14.7), and environmental sustainability (approximately 6.6%–13.2% less greenhouse gas emissions than bituminous coal). In summary, microwave-assisted WT is a potential technology for producing renewable energy.