Advanced Oxidation Based Treatment of Petroleum Industry Effluent

The utilization of water is the core of our lives at every moment, through direct and indirect use. Indeed, industries are a significant player in water consumption, for various functions such as processing, diluting, cooling and washing. These processes often leave contaminated water streams. ~15% of the water globally used ends up as industrial wastewater of varied composition and extent of contamination. Treating industrial wastewater is crucial to meet discharge standards, but conventional methods struggle with complex organic compounds that resist treatment. These compounds, referred to as recalcitrant organics, pose hazards like toxicity and bioaccumulation if left untreated. Conventional treatments like physical, thermal, or chemical methods are energy-intensive, require frequent maintenance, and may not fully meet regulatory standards [1-4]. Thus, more advanced and sustainable wastewater treatment technologies are required. Advanced oxidation processes (AOPs) are processes for treating wastewater by creating radicals in-situ that break down recalcitrant pollutants via a multi-step degradation pathway. AOPs are considered a 'green' approach due to their ability to convert harmful compounds into safer forms or mineralize them. These processes have garnered significant research attention for their potential in industrial wastewater treatment [5]. AOPs can be classified into three types: additive-based, catalyst-based, and electrochemical-based processes. Additive-based AOPs use oxidants such as hydrogen peroxide, ozone, or chlorine-containing compounds. Catalyst-based AOPs utilize Fenton's reagent or photocatalysts to realize the oxidation process. Electrochemical AOPs involve using electrical means to oxidize pollutants in wastewater. These categories showcase the diversity and effectiveness of AOPs in addressing various pollutants. Their adoption can greatly contribute to sustainable water management and environmental preservation, offering a promising solution for treating industrial wastewater and reducing its impact on ecosystems. The application of these AOPs have been demonstrated on various effluent types. Additive-based AOPs consume at least the stoichiometric quantity of oxidants and more often than not, more than stoichiometric quantities of oxidants for treatment. Whereas, energy-based (electrochemical, microwave-based etc.,) necessitate input of energy to initiate chemical reactions for treatment. On the other hand, catalyst-based processes have the potential to lower the loading of oxidants and possibly consume less energy. Thus, offering the potential for intensified oxidation. Some instances of catalyst-based AOPs achieving treatment targets are: a Fenton's oxidation was applied to olive mill wastewater (COD0 = 9740-ppm), reporting a 90% removal in 120 minutes. A comparable removal was reported for a polymeric wastewater (hydrolyzed polyacrylamide wastewater) of COD0 ~ 10,000 -ppm [6]. While the Fenton's process is efficient in abating the organic load, as demonstrated in these couple of instances, the sludge generated in the process need further proper treatment. Among other AOP based catalyst processes, the photocatalysis route is a promising one. In other words, theoretically no (or minimal external) chemicals are needed to initiate oxidation. The photocatalysis process comprises the use of catalyst materials such as ZnO, TiO2, Bi2WO6, Fe2O3, Nb2O5, ZnS and so on [7] with an appropriate illumination source, to receive a light of energy which equals or exceeds its bandgap energy. By this photo-absorption process, the catalysts produce oxidants in the liquid phase which can be utilized for the oxidation of organic pollutants in wastewater. A typical schematic of the photocatalytic process for ZnO and SiC materials are shown in Figure 1. The PC technique offers various advantages such as low operating and installation costs and does not suffer from disadvantages such as the formation of sludge, which is common in other catalyst-based processes (such as Fenton's oxidation). Based on the implementation of the PC process (either suspended or immobilized), some disadvantages of the process are the agglomeration, mass transfer limitations and requirement of a secondary separation step to recover the catalyst.