Incorporation of water treatment plant sludge into expanded lightweight aggregates: waste valorization to promote the circular economy

ABSTRACT Water Treatment Plant Sludge (WTPS) is a growing environmental challenge that requires sustainable management solutions. This study proposes valorizing WTPS by producing expanded clay lightweight aggregates (LECA) to promote the circular economy. Aluminum- and iron-based WTPS, derived from the potabilization process for public water supply, were evaluated for their feasibility as partial replacements for clay. The methodology included preliminary tests and a Doehlert experimental design, considering the following process variables: sludge content (10–90%), sawdust content (3–7%), and curing time (3–15 days). The chemical and geoenvironmental characterization for the materials to produce LECAs was provided. The technical feasibility analysis for the produced LECAs with WTPS was analysed using compressive strength (N), water absorption (%), thermogravimetric, mineralogical, and microstructural characterization and the results were compared with those of commercial LECAs. The environmental risk by produced LECAs was assessed based on the concentration of metals in the leachates from the representative LECAs. Formulations containing up to 30% WTPS achieved compressive strengths higher than those of commercial LECAs, and it’s possible to produce LECAs with 70% of the WTPS, when subjected to adequate curing times. Only aluminium LECAs leached Al in relevant concentration. Statistical analysis (ANOVA) revealed a strong correlation between process variables and mechanical performance, while water absorption was mainly influenced by sludge type. The findings indicate that WTPS-based LECAs can be tailored to specific construction applications and represent a viable circular economy strategy, simultaneously diverting waste from disposal and reducing the demand for virgin clay in lightweight aggregate production.