Sustainable utilization of water treatment plant sludge as a pavement subgrade material in arid and semi-arid regions using biopolymers and lime

This study evaluates the stabilisation of water treatment plant sludge (WTPS) for pavement subgrade use in arid and semi-arid regions with biopolymers (xanthan gum, guar gum) and lime. The water treatment plant sludge (WTPS) specimens, treated with 0.5–2% biopolymer and 2–8% lime and cured for up to 28 days, were tested for unconfined compressive strength (UCS), California bearing ratio (CBR), Atterberg limits, and via scanning electron microscopy (SEM). Optimal stabilisation was achieved at 4% lime and 1.5% guar gum, increasing UCS from 109 kPa to 1,500 kPa and 3,649 kPa, respectively, in 28 days. The CBR values and plasticity indices improved with treatment and increasing binder content. The SEM images revealed denser microstructures and reduced pore connectivity upon treatment of WTPS with the biopolymers. The results highlight biopolymer–lime synergy in generating high-strength, low-plasticity, and environmentally resilient WTPS subgrade materials suitable for infrastructure in water-stressed environments.