## 1 Introduction  
The rapid expansion of the global population and industrialisation has significantly increased waste generation across domestic, agricultural, and industrial sectors [1]. These unwanted byproducts of human activity pose severe challenges to environmental sustainability. Domestic waste is a major contributor, with approximately 2 billion tonnes of municipal solid waste generated annually, a figure projected to rise to 2.59 billion tonnes by 2030 [2]. Agricultural waste, which includes residues from crop production (e.g., straw and husks), livestock activities (e.g., manure and bedding materials), and agroindustrial processes (e.g., fruit peels and bagasse), also contributes substantially to the global waste stream [3]. In less developed regions, a significant portion of agricultural waste is treated through open burning due to limited access to advanced waste management technologies, leading to environmental pollution and health risks [4]. Additionally, industrial waste, including byproducts from manufacturing, mining, and chemical processes, contributes another 20 million tonnes of hazardous and non-hazardous waste to ecosystems annually [5; 6]. Research has increasingly focused on innovative waste management strategies, particularly for industrial waste. For instance, [PERSON] et al. developed a whole-cell biocatalyst capable of effectively depolymerising polyethylene terephthalate (PET) plastic from industrial waste [7], while [PERSON] et al. investigated the upcycling of fly ash from incineration processes into green binders via flue-gas-enhanced wet carbonation [8]. Other studies have explored the use of lime sludge as a filler in polymer matrix composites, suggesting a sustainable technique that improves commercial viability and reduces manufacturing costs [9]. Amongst the various types of industrial wastes, alum sludge has drawn significant attention due to its environmental impact. Alum sludge is a byproduct of water treatment processes employing aluminium-based coagulants, such as aluminium sulphate (alum), to remove impurities. During treatment, alum reacts with water to form aluminium hydroxide, which aggregates suspended particles into larger flocs. These flocs settle as sludge, consisting primarily of aluminium hydroxide, organic matter, and other insoluble impurities, with up to 5% water by volume. The disposal of alum sludge is a persistent challenge for water treatment plants, with landfilling remaining the most common practice. However, increasing environmental regulations and public awareness have driven efforts to explore sustainable alternatives. Studies have investigated the reuse of alum sludge as an adsorbent [10], a component in building bricks [11], and a cementitious material [12]. Additional research has examined its potential applications in agriculture, for example, as a soil conditioner [13] or a substrate in constructed wetlands [14]. Despite these advancements, limited research has explored the use of alum sludge as a functional filler in polymer matrix composites, leaving a gap for further exploration in this area. Polymer matrix composites (PMCs) have become increasingly important due to their unique combination of lightweight, high-strength, and versatile properties. While polyolefin-based plastics are commonly used as matrix materials, there is growing demand for more environmentally friendly polymers, such as Poly(butylene adipate-co-terephthalate) (PBAT). PBAT is a biodegradable aliphatic-aromatic copolyester that offers high flexibility, excellent processability, and biodegradability, making it an ideal candidate for sustainable composite materials [15; 16]. PBAT has gained more attention in recent years for use in PMC manufacturing, thanks to its promising properties (e.g., high elongation at break) and environmental benefits (e.g., ability to break down naturally). To enhance the mechanical, thermal, and barrier properties, fibres or particles are incorporated with the polymer as reinforcements.