## 7 Conclusions  
The majorization of waste cooking oil (WCO) represents a promising pathway toward achieving circular economy goals, reducing environmental pollution, and mitigating dependency on fossil-based resources. Despite its abundance and potential environmental benefits, the valorization of WCO is hindered by several challenges, including feedstock variability, contamination, logistical constraints, and technical limitations in processing. Traditionally utilized in biodiesel production for combustion engines, WCO-based fuels, while renewable, still contribute to carbon dioxide and nitrogen oxide emissions during combustion, thus presenting limited environmental advantages. In contrast, the application of WCO in non-combustion sectors, particularly in transformer insulation, offers a safer, cleaner, and more sustainable alternative. This approach not only extends the lifecycle of WCO but also aligns with the global shift toward eco-friendly materials in energy infrastructure. Recent studies have shown that treated WCO possesses desirable characteristics for transformer applications, including good cooling performance and inherent fire safety. However, despite these advantages, WCO-based insulating liquids typically do not meet the strict K-class fire resistance standards, which require a fire point above 300 \({}^{\circ}\)C. Additionally, several physicochemical properties, such as acidity, oxidation stability, and cold flow behavior, exhibit significant variability due to differences in feedstock origin, frying conditions, and treatment methods. These inconsistencies raise concerns about the long-term stability and reliability of WCO in electrical insulation systems and warrant deeper scientific inquiry. One particularly critical but underexplored area is the compatibility between fatty acid methyl esters derived from WCO and other transformer components such as cellulose-based insulation paper. The solid-liquid interactions within the oil-paper insulation system are vital to the dielectric performance and aging behavior of transformers. However, the current literature offers limited insight into these interactions, particularly under accelerated aging or thermal stress conditions. Furthermore, the conversion process itself raises environmental concerns, particularly regarding energy consumption, chemical usage during pretreatment (e.g., transesterification, bleaching), and byproduct management. Therefore, integrating cleaner, low-energy technologies, such as supercritical or enzymatic processes, alongside life cycle and techno-economic analyses will be essential to fully assess the sustainability of WCO-derived insulating liquids. In addition, developing efficient and standardized WCO collection infrastructures is crucial for reliable feedstockavailability. Social and cultural barriers, informal markets, and inadequate regulatory oversight continue to hinder efficient WCO recovery in many regions. Promoting public awareness, incentivizing waste oil collection, and enforcing legislation against unsafe reuse are necessary policy steps to support a sustainable WCO supply chain. In summary, WCO offers a compelling opportunity for resource recovery and environmental preservation when applied in non-fuel domains such as transformer insulation. However, a successful transition requires a multidisciplinary research approach to address its physicochemical variability, material compatibility, environmental footprint, and regulatory integration. Future work should focus on advanced treatment protocols, optimizing performance additives, and evaluating aging behavior in real-life operating conditions. Furthermore, successful large-scale implementation depends not only on technological innovation but also on supportive regulatory frameworks, robust collection systems, and increased public awareness.