Recent advances in the conversion of waste cooking oil into value-added products: A review

Waste cooking oil (WCO) is largely generated and disposed into the environment, especially in China and India, making it one of the largest pollutants in the environment. The disposal of high loads of WCO is causing many detrimental environmental problems, including blockages in the sewer, hindrance of sewage pretreatment at wastewater treatment plants, as well as water and soil pollution if WCO is dumped into municipal solid waste landfills. The most common solution to the aforementioned problems is to utilise WCO in biodiesel synthesis. Recently, owing to the similar properties of WCO to vegetable oils, the oleochemical potential of WCO has also been examined to explore its suitability to replace vegetable oils in the production of various oleochemicals. In this regard, the utilisation of WCO as a cheap feedstock to synthesise animal feed, green solvents, fermentative products, grease, biolubricant, etc. is promising and can be performed using existing technologies. Therefore, aside from the usage as energy sources, these applications have provided a secondary use to WCO which mitigate the adverse effects caused by WCO. The subsidies and financial support in major WCO producing countries are significant in pursuing the advancement of the current regulatory frameworks for WCO collection, transportation, and utilisation. Technological advancements via intensified approaches are not only upgrading WCOs recycling more efficiently but also allowing greater working flexibility. This review highlights the challenges in using WCO for producing value-added products and its future prospects.