Reuse of water treatment plant sludge mixed with lateritic soil in geotechnical works

Abstract

This work investigates the possibility of reusing water treatment sludge (WTS) at as-collected water content mixed with lateritic soil in geotechnical works to avoid waste disposal and reduce the exploitation of natural soils. Three mixtures with different proportions of soil to WTS (5:1, 4:1, 3:1) were prepared. Leaching and solubilization tests were conducted for environmental characterization of the WTS. Particle size distribution, consistency limits and specific gravity of solids of the soil and the three mixtures were determined for geotechnical characterization. Compaction, permeability and triaxial compression tests were carried out to investigate the geotechnical behaviour of the materials. The sludge was classified as non-hazardous and non-inert by Brazilian standards. The addition of sludge to the soil did not significantly change its granulometry but led to a reduction in the specific gravity of solids and an increase in the plasticity index. In the compaction tests, the maximum dry unit weight decreased, and the optimum moisture content increased with WTS addition. The hydraulic conductivity of the mixtures was between 10 and 100 times that of the soil. In triaxial compressive tests, there was an increase in strain and a reduction in strength during shear, as well as an increase in the friction angle (between 1º and 4º) and reduction in cohesion (between 9 kPa and 16 kPa). Despite causing changes in soil behaviour, the addition of sludge does not preclude the use of the resulting mixtures in geotechnical works, since the parameters remain as established in the literature and technical. However, a case-by-case analysis must be carried out before using the mixtures.