COD removal from industrial spent caustic wastewater: A review

Spent caustics contain compounds such as mercaptans, sulfides, and phenols. In addition, spent caustic solutions are highly alkaline and have high chemical oxygen demand. There are different ways to treat caustic waste. This paper for the first time, presents a comparison of a wide range of caustic waste treatment methods in terms of their economic costs and efficiency. Wet air oxidation, advanced oxidation processes, electrochemical processes, as well as biological and membrane systems are among technologies used to treat spent caustic solutions. This review focuses on the mechanisms and prominent parameters that affect the chemical oxygen demand (COD) reduction processes. The membrane process enjoys high efficiency (80-98 %) and is important because of advantages such as low energy consumption and easy design, compared to other methods. Nevertheless, it is not affordable because of the cost of cleaning the membranes and producing waste caused by the chemicals used to cleanse the membranes. The WAO process has high efficiency (70-96 %) but is not recommend due to severe operating conditions. Production of the Fenton reagent in an electrochemical cell using electricity as an energy source is one of the best options. The use of electricity as a source of energy prevents pollution and increases the process efficiency (90-99 %).