Sustainable utilization of sewage sludge ash in stabilizing subgrade soil: an appraisal

Sewage sludge (SS), a liquid or semi-solid liquid, is a by-product obtained from municipal or industrial wastewater treatment operations. The amount of sewage sludge produced by any municipal or industrial wastewater treatment plant depends on several factors, such as population growth, urban planning, the expansion of sewage systems, and the industry's development. Due to presence of pathogens, inorganic pollutants, and organic pollutants, the huge amounts of SS generation pose severe environmental issues. The inadequate disposal of SS such as open dumping, landfill, and composting causes alteration the ecosystems including air, water, and soil pollution, thus it represent a real threatening to human health. Moreover, the burned or incinerated SS, in the form of sewage sludge ash (SSA) were utilized in the past studied as a supplementary cementitious material to produce brick, roof tiles, lightweight aggregates, ceramic materials, and subgrade soil stabilization. In addition to this, the utilization of SSA in pavement subgrade soil stabilization leads to substantial cost savings and solves the disposal problems of SSA. This paper review the basic properties of SSA, such as its physical, chemical, pozzolanic, and microstructural characteristics, followed by their effective utilization in subgrade soil stabilization. The paper summarized the plasticity, compaction, strength (unconfined compressive strength and California bearing ratio), and durability (wetting–drying cycle) properties of subgrade soil treated with the SSA-based stabilizer. The findings indicate that adding SSA up to certain amount to the subgrade soil led to reduction in plastic index and volumetric strain. Moreover, the addition of SSA improves the compaction characteristics, unconfined compressive strength, and California bearing ratio of treated soil. The paper also highlights the geoenvironmental suitability of SSA in stabilizing soil subgrade, and thus it promotes both sustainable development and zero-waste achievement. From the detailed review of literature, it can be concluded that it is advantageous to incorporated SSA with other additives such as cement, fiber and nano-material to improve the compressive strength and California bearing ratio of weak subgrade soils.