<p>Sewage sludge, a biosolid product of wastewater processing, is an often-overlooked source of rich organic waste. Hydrothermal processing has shown promise in converting sewage sludge into valorized materials with potential application in biofuels, asphalt binders, and bioplastics. Here we characterize the physicochemical properties of hydrochar, the carbonaceous solid phase product of hydrothermal processing, and investigate its use as bio-based filler in additive manufacturing. We find that the presence of metallic and metalloid dopants in sewage sludge, which are not typically found in biomass wastes, yields unusual results in organic material processing such as decreased graphitic ordering after thermal activation. We further find that addition of hydrochar generally decreases mechanical performance of additive manufacturing composites, however, some properties such as toughness can be recovered with nature-inspired architecting into gyroid microstructures. These findings demonstrate that more investigation is required to optimally valorize sewage sludge and similarly disordered waste streams.</p>