  Sewage sludge, a biosolid product of wastewater processing, is an
often-overlooked source of rich organic waste. Hydrothermal processing (HTP),
which uses heat and pressure to convert biomass into various solid, liquid, and
gaseous products, has shown promise in converting sewage sludge into new
materials with potential application in biofuels, asphalt binders, and
bioplastics. In this study we focus on hydrochar, the carbonaceous HTP solid
phase, and investigate its use as a bio-based filler in additive manufacturing
technologies. We explore the impact of HTP and subsequent thermal activation on
chemical and structural properties of sewage sludge and discuss the role of
atypical metallic and metalloid dopants in organic material processing. In
additive manufacturing composites, although the addition of hydrochar generally
decreases mechanical performance, we show that toughness and strain can be
recovered with hierarchical microstructures, much like biological materials
that achieve outstanding properties by architecting relatively weak building
blocks.
