The promulgation of the Biogas Act in South Korea has increased the number of organic waste treatment facilities and the amount of food waste digestion sludge (FWDS), a byproduct of the biogas process. FWDS recovery involves various challenges, which leads to the accumulation or improper disposal of sludge. Hence, FWDS needs to be treated in environmentally sound and safe ways. In this study, anaerobic digestion sludges were mixed with unused forest biomass to produce fuel. The results showed that pellets produced via mixing of FWDS with unused forest biomass had improved durability, bulk density, and fine particle performance compared to surface-carbonized wood pellets. Carbonized pellets manufactured with 30% FWDS had a moisture content of 11.746% and met all biosolid waste fuel (SRF) standards, except for moisture content. Carbonized pellets prepared with 15% FWDS met the L2 wood pellet standards for ash content (less than 3.0%) and bulk density (greater than 550 kg/m3), as well as all other standard values in both the industrial wood pellet quality standards and bio-SRF criteria. This study confirmed the potential and suitability of digestion sludge and unused forest biomass for fuel utilization by addressing their respective limitations.