Safety control of waste cooking oil: transforming hazard into multifarious products with available pre-treatment processes

The increase in worldwide vegetable oil consumption has produced a large increase in hazardous waste cooking oil (WCO) production. The improper disposal of WCO has been a significant problem from both an environmental and economic perspective. Therefore, it is advantageous to transform WCOs into valuable products efficiently and effectively in order to contribute towards the establishment of a circular economy. In this review, the current state of WCO is discussed in relation to WCO production and valorisation. The valorisation rate of each country can be categorised into three groups related to the consumption of vegetable oil, production and valorisation of WCO, as well as the production, conversion and valorisation factors. Before any valorisation and processing of WCO can be carried out, pretreatments are crucially needed in refining and upgrading WCOs, specifically to reduce their free fatty acid (FFA) contents. This could help to produce refined WCOs with appropriate feedstock properties suitable for value-added applications in oleochemical industries. Hence, several pretreatment methods (e.g., solid impurities removal, FFA reduction, moisture removal) have been summarized and evaluated in depth. The transformation of WCO into valuable products including alkyd resin, green solvent, soap, plastics and plasticizer are also reviewed. Recent technological advances have made WCO feasible as sustainable feedstocks for oleochemical production, but not limited to biofuel production which in turn maximizes the value of this hazardous waste and turns WCO into a sustainable source.