Used caustics contain compounds such as mercaptans, sulfides, and phenols. In addition, spent caustic effluents are highly alkaline and have a high chemical oxygen demand. There are different methods for purifying spent caustics. 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 the technologies used to treat spent caustic solutions. This review focuses on the prominent mechanisms and parameters that affect chemical oxygen demand (COD) reduction processes. The membrane process has a high efficiency (80-98%) and is important compared to other methods due to its advantages such as low energy consumption and easy design. However, it is not cost-effective due to the cost of cleaning the membranes and the generation of waste from the chemicals used to clean the membranes. The WAO process has high efficiency (70-96%) but is not recommended due to severe operating conditions. The production of Fenton’s reagent in an electrochemical cell using electricity as an energy source is one of the best options. The use of electricity as an energy source prevents pollution and increases the efficiency of the process up to (90-99%).
Access to clean and healthy water is one of the most basic human needs to live on earth. For this reason, wastewater is one of the most valuable water resources available today. Water quality is affected by pollution caused by human activities such as agriculture, mining and energy production . Fossil fuels are one of the most important branches of energy supply that have benefited mankind for many years. The increase in environmental pollution, global warming, and climate change are among the most important effects of using these fuels, which make people think of green and clean energies. [2-4]. Among the types of wastewater, industrial wastewater is more important due to its high consumption and variety of pollutants. Pollutants such as carbon dioxide, nitrogen oxides, sulfur dioxide, volatile organic compounds, heavy metals, are not easily decomposed and can pollute groundwater if improperly discharged into the environment . Therefore, it is necessary to remove these pollutants from the wastewater before the final discharge to the environment. Based on operating conditions, operating units and types of processed materials, different wastewaters are produced. Some wastewaters must be treated separately because their combination with other wastewaters makes treatment difficult [5،6].