Low permeability sealing materials based on sewage, digestate and incineration industrial by-products in the final landfill cover system

This study explores repurposing municipal solid waste from the sewage, digestate, and incineration industries as landfill-sealing materials, aligning with circular economy principles. The main materials include digested sewage sludge (DSS), pretreated digested sewage sludge (PDSS), and digestate sludge (DS), with biomass bottom ash (BBA) and Al-anodizing waste (AAW) introduced as additives, complemented by 1.5 wt% water glass to enhance the sealing performance. The research evaluates the characteristics of the various treated and sourced sludges and additives, assessing their influence on water permeability, reaction products and the environmental consequences. Results demonstrate the achievement of low permeability in the sludge-based sealing materials, with optimal performance observed in the specimens prepared with DSS and AAW (k value = 3.78 ×10*-12 m/s). Thermal Pressure Hydrolysis pre-treatment in sewage plants reduces the organic content in PDSS, resulting in a
slight increase in permeability. DS-based specimens exhibit higher permeability due to their relatively lower organic content in DS. Gypsum is the primary reaction product attributed to leachable sulphate in BBA and AAW. Water glass addition in BBA-modified specimens promotes silica gel formation, while AAW effectively reduces matrix permeability as an externally added gel-like substance. Additionally, heavy metals (As, Pb and Cr) derived from the by-products are effectively immobilized in the sealing materials owning to the coagulation effect of organic matter in the sludge and sulphates in the products. Overall, this novel approach to landfill sealing materials exhibits promising applications in the Netherlands, offering cost savings and reduced environmental impact by recycling industrial by-products.