The advanced oxidation wastewater treatment process was initially proposed for drinking water treatment, but now its application includes industrial wastewater treatment as well. The advanced oxidation process (AOP) is a complex method in which hydroxyl radicals (OH-) or sulfate radicals (SO4-) are generated to remove refractory organic pollutants. These radicals can also treat organic contaminants certain organic pollutants and increase wastewater biodegradability as a prior treatment.
In this article, we will go through various aspects of AOPs, how they work, their different types, and much more. Let’s begin with the basics.
The Advanced Oxidation Process (AOP) are the oxidation process in which hydroxyls are generated to effect water treatment. It is much different from other oxidation methods like chlorine and ozone, which have a dual nature of decontamination and disinfection. AOPs are utilised to destroy both organic and inorganic pollutants present in wastewater. When AOPs are used for treating wastewater, the radicals act as a powerful oxidising agent, destroy wastewater pollutants, and transform them into less or non-toxic products. Thus, AOPs are a highly reliable way of wastewater treatment.
There are various types of radicals in the Advanced Oxidation Process. Let’s understand each in detail:
Hydroxyl radical is the most reactive oxidising agent in the wastewater treatment. It has oxidation potential between 2.8 V and 1.95 V. OH· is very nonselective in its behaviour and rapidly reacts with numerous species with rate constants on the order of 108–1010 M−1 s−1. There are four methods by which it attacks organic pollutants present in the wastewater:
A series of carbon-centred radicals are formed during the reactions of hydroxyl radicals with organic compounds. Due to their short lifespan, hydroxyl radicals are only created in situ during application using a variety of techniques, such as a combination of catalysts (like Fe2+), irradiation (like UV light or ultrasound), and oxidising agents (like H2O2 and O3).
Ozone(O3) is another strong oxidative agent after hydroxyl radical. It has an oxidative potential of 2.07 V. In some conditions, OH- is generated from O3 to initiate the indiscriminate oxidation as it is an indirect mechanism. The OH· production can be greatly increased in the presence of additional oxidants or radiation. For instance, in the so-called peroxide (O3/H2O2) system, hydroperoxide (HO2 −) generated from H2O2 decomposition enhances O3 decomposition and OH· generation.
In UV-based AOPs, UV light is required, and an oxidant like hydrogen peroxide or chlorine is required. The process rapidly occurs in the UV chamber with oxidising radicals attacking and decomposing contaminants so they don’t remain harmful. Here is how it works:
Among all metals which are capable of active hydrogen peroxide and produce hydroxyl radicals in water, iron is the most frequently used. It is called the Fenton process. In this process, H2O2 reacts with Fe2+ to generate strong reactive species. Although alternative molecules such as ferryl ions have been suggested, the reactive species generated are conventionally identified as hydroxyl radicals.
S2O8 is a strong oxidant with an oxidative potential of 2.01 V. After getting activated by heat, UV irradiation, or transition metals; it can form more powerful sulfate radicals to initiate sulfate radical-based advanced oxidation processes.
Just like hydroxyl radicals, sulfate radicals are highly reactive species with a short lifespan. However, both radicals have different reaction patterns. In hydroxyl radicals, C=C bonds are formed or abstract H from C-H bonds during their reactions with organic compounds. On the other side, sulfate radicals tend to remove electrons from organic molecules that are subsequently transformed into radical cations.
There are multiple chemical procedures in the Advanced Oxidation Processes for the removal of organic or inorganic pollutants present in wastewater. They are useful because they can break down stubborn substances that are normally challenging to get rid of using traditional treatment techniques. They are effective because they can break down recalcitrant compounds that are typically difficult to remove using regular treatment methods.
Here are the major applications:
The first major application of AOP is to reduce the overall organic content present in the wastewater. This results in lower levels of chemical oxygen demand (COD), signifying a lower amount of organic pollutants.
AOPs are highly reliable in the specific destruction of pollutants like persistent organic pollutants (POPs). These processes can effectively break down complex chemicals into simpler, harmless compounds.
The colour and odour of wastewater can be unpleasant. AOPs address this problem by oxidising the molecules responsible for these characteristics.
The adaptability of AOPs enables a wide range of applications. They can be used as a stand-alone treatment or in conjunction with traditional wastewater treatment plants to improve their efficiency. As environmental regulations tighten and industrial pollutants become more complicated, AOPs provide a reliable solution to modern wastewater management difficulties.
AOPs for wastewater have a sustainable and innovative future. They are highly anticipated to evolve by integrating more catalysts for better reactions and high efficiency while lowering energy consumption. We can also expect that there will be a potential rise in the combination of AOPs with biological processes, targeting the destruction of specific pollutants in a more eco-friendly manner.
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