Transforming primary sludge originating from pulp and paper industry into sustainable composites : technical feasibility and climate benefits

Abstract

This dissertation investigates the valorization of primary sludge from the pulp and paper industry as a filler material in high-density polyethylene (HDPE) composites. The research addresses the need for sustainable alternatives to sludge incineration, which is energy-intensive and misaligned with circular economy goals. The study explores whether primary sludge, a fibrous and mineral-rich by-product, can be effectively used to replace conventional wood flour in composite manufacturing.

The research combines experimental testing and a life cycle assessment (LCA) to evaluate the technical feasibility, mechanical performance and environmental implications of sludge-filled composites. Composite samples were prepared using HDPE, coupling agents, and either wood flour or dewatered primary sludge. Mechanical and physical properties were assessed through standardized testing, while environmental impacts were modelled using the Environmental Footprint 2.0 methodology.

The results show that primary sludge composites exhibit comparable or improved mechanical properties, including higher impact strength and moisture resistance, compared to wood flour composites. LCA results indicate that substituting virgin plastic with primary sludge can significantly reduce greenhouse gas emissions, provided that the substitution rates are sufficiently high. These findings support the use of primary sludge as a viable raw material for composite applications, contributing to resource efficiency and industrial decarbonization.

This work demonstrates how circular economy principles can be applied in the forest industry by transforming a low-value side stream into a functional composite material. The findings offer practical insights for advancing sustainable material innovation and reducing environmental burdens in pulp and paper production.