The disposal of water treatment sludge (WTS) remains a major environmental challenge, especially in developing countries. This study evaluates the incorporation of WTS into silty clayey soils, with and without nonwoven geotextile reinforcement, to assess the mechanical and hydraulic behavior of mixtures for sanitary landfill applications. Laboratory tests—including compaction, confined compression, hydraulic conductivity, and direct shear—were performed on mixtures with 0–10% WTS and pure sludge. Results showed that increasing WTS content raised void ratios, reduced maximum dry unit weights, and lowered hydraulic conductivity, suggesting potential as a low-permeability barrier. However, higher WTS contents also increased compressibility and reduced shear strength, which may compromise stability. The 7.5% mixture maintained shear strength comparable to natural soil, representing an optimal balance between performance and sustainability. Geotextile reinforcement further improved shear strength in all mixtures, particularly under higher confining stresses, expanding applicability in engineered barriers. Overall, the findings confirm the viability of reusing untreated WTS as a partial soil substitute in landfill construction, contributing to resource conservation and sustainable waste management.