Comparison of different strategies to treat challenging refinery spent caustic effluents

Worldwide, spent caustic effluents from Merox units are a challenge for refineries given their very hazardous nature in terms of toxicity as well as their extreme pH (approximately 12–13). Although this is a global concern, the spent caustic effluent generated by Galp Refinery located in Sines, the major refinery in Portugal, was used as case study. To tackle this problem, two strategies were addressed in this study to treat spent caustic effluents: (i) neutralization, followed by Fenton oxidation post-treatment (approach 1) and (ii) neutralization, followed by liquid-liquid extraction (approach 2). Approach 1 allowed removals of 70% for total organic carbon (TOC), 80% for chemical oxygen demand (COD) and 95% for polar oil and grease (O&G). The Vibrio Fischeri toxicity test showed a considerable decrease in the acute toxicity of the spent caustic effluents after this treatment (approximately 70%). Approach 2 allowed for a mineralization of 97%, COD reduction of 96% and polar O&G removal of 99%. Both sets of technologies permitted a direct discharge of treated spent caustic into the effluent pre-treatment system of the refinery. Since the proposed treatment approaches enable a considerable improvement in the effluent quality and noteworthy annual savings were determined compared with current effluent management strategies (1.39 and 1.58 M € for treatment approach 1 and 2, respectively), this study provides useful information for the development of industrial scale plants in petroleum refineries for the treatment of spent caustic effluents.