Maleic anhydride (MA) is a chemical intermediate used in the production of various polymers, resins, and coatings. However, the manufacturing process of MA generates a large amount of wastewater that contains high concentrations of organic pollutants, such as MA, maleic acid, and acetic acid. These pollutants pose serious environmental and health risks if discharged without proper treatment. Therefore, it is essential to develop an efficient and economical wastewater treatment process for MA industry.
In this article, we review the current state of the art of MA wastewater treatment process, focusing on the main methods, mechanisms, advantages, and disadvantages. We also provide some suggestions for future research directions and improvements.
Physical-Chemical Treatment
Physical-chemical treatment is a conventional method that involves the use of coagulation, flocculation, sedimentation, filtration, and adsorption to remove the suspended solids, turbidity, color, and some organic pollutants from MA wastewater. The most commonly used coagulants are aluminum sulfate, ferric chloride, and polyaluminum chloride. The most commonly used adsorbents are activated carbon, zeolite, and clay.
The advantages of physical-chemical treatment are that it is simple, fast, and low-cost. However, the disadvantages are that it produces a large amount of sludge that requires further disposal, and it cannot remove the dissolved organic pollutants completely. Moreover, some coagulants and adsorbents may introduce secondary pollution or toxicity to the treated water.
Biological Treatment
Biological treatment is an advanced method that utilizes microorganisms to degrade the organic pollutants in MA wastewater. The main types of biological treatment are aerobic, anaerobic, and combined aerobic-anaerobic processes. Aerobic processes use oxygen as the electron acceptor and produce carbon dioxide and water as the end products. Anaerobic processes use other substances, such as nitrate or sulfate, as the electron acceptor and produce methane and hydrogen sulfide as the end products. Combined processes use both aerobic and anaerobic microorganisms in different stages to achieve better performance.
The advantages of biological treatment are that it can remove the organic pollutants more effectively and reduce the sludge production. However, the disadvantages are that it requires a longer retention time, a higher operational cost, and a more strict control of environmental factors, such as pH, temperature, dissolved oxygen, and nutrient balance. Moreover, some microorganisms may produce toxic or odorous compounds during the degradation process.
Advanced Oxidation Processes
Advanced oxidation processes (AOPs) are emerging methods that employ strong oxidizing agents, such as ozone, hydrogen peroxide, ultraviolet light, or ultrasound, to generate highly reactive radicals that can oxidize the organic pollutants in MA wastewater. The main types of AOPs are ozonation, Fenton reaction, photocatalysis, sonolysis, and electrochemical oxidation.
The advantages of AOPs are that they can degrade the organic pollutants more completely and rapidly than biological treatment and produce less sludge and toxicity than physical-chemical treatment. However, the disadvantages are that they require a higher energy consumption, a higher capital investment, and a more careful optimization of process parameters. Moreover, some AOPs may generate harmful by-products or intermediates that need further treatment.
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