Characterization of freshwater sludge generated in Singapore: exploring opportunities for a circular economy

Abstract

Freshwater sludge (FS), a by-product of drinking water treatment, is produced in large quantities globally but remains underutilized, predominantly ending up in landfills. As landfill space becomes increasingly scarce and environmental regulations tighten, there is an urgent need for sustainable management strategies for FS. By adopting circular economy principles that emphasize reuse and recycling, potential solutions can be found. This study analyzed the chemical and mineral compositions as well as leaching behavior of ten FS batches collected from various Singapore waterworks. It explored the potential applications of FS, addressing challenges and proposing solutions for its repurposing. The results revealed that FS primarily consists of aluminum oxide (34.94%–57.2%), sulfur trioxide (5.56%–8.98%), and silicon dioxide (1.24%–6.85%). FS also contains minerals such as aluminum sulfate, bayerite, kaolinite, and quartz. The primary variation among different batches of FS was the organic content (28.89%–52.3%), which mainly consisted of carbon, hydrogen, and oxygen. The leaching test indicated that the heavy metal concentrations in FS were within safe limits, suggesting its safety for various applications, especially in high pH environments. The substantial organic content of FS makes it a candidate for conversion into biochar and hydrochar for soil amendment. The rich aluminum content in FS opens avenues for its application in construction material fabrication, soil enhancement, and the remediation of heavy metal-laden wastes, soils, and wastewater.