Advanced oxidation processes (AOPs) - AOPs combine ozone, UV, and catalysts to degrade persistent organic pollutants.

Advanced Oxidation Processes (AOPs) are a set of chemical water treatment procedures designed to remove organic and inorganic pollutants that are difficult to break down with conventional methods. These processes are particularly effective for treating wastewater, but are also increasingly used for drinking water purification.

The Core Principle
The fundamental principle of all AOPs is the generation of highly reactive species, primarily the hydroxyl radical (•OH). This radical is one of the most powerful oxidizing agents known, reacting with organic compounds at an extremely fast rate, often a million times faster than other oxidants like ozone or hydrogen peroxide alone. Unlike other disinfectants that leave behind residuals, the ultimate product of this process is typically water, carbon dioxide, and simple salts, a process known as mineralization.

The hydroxyl radical is a non-selective oxidant, meaning it can react with and degrade a wide range of organic compounds, including those that are biologically toxic or non-biodegradable. This makes AOPs an ideal solution for treating persistent organic pollutants (POPs) such as pesticides, pharmaceuticals, endocrine disruptors, and volatile organic compounds (VOCs).

Common Types of AOPs
AOPs are not a single technology but a family of processes, each using a different method or combination of methods to generate hydroxyl radicals. Some of the most common types include:

UV/Hydrogen Peroxide: In this process, high-intensity ultraviolet (UV) light is used to cleave the bond in hydrogen peroxide molecules, which produces two hydroxyl radicals. This method is effective and can be easily integrated into existing treatment systems.

Ozone: When ozone is exposed to UV radiation in water, it decomposes and forms hydroxyl radicals. This is a highly efficient process, often more effective than due to ozone's high UV absorption rate.

Fenton and Photo-Fenton Processes: The Fenton process uses a combination of hydrogen peroxide and iron salts to produce hydroxyl radicals. The photo-Fenton process enhances this reaction by adding UV light, significantly speeding up the degradation of pollutants.

Photocatalysis: This method uses a semiconductor material, typically titanium dioxide, which is activated by UV light. The activated catalyst generates electron-hole pairs that react with water and oxygen to produce hydroxyl radicals.

Electrochemical AOPs: These processes use electricity to generate hydroxyl radicals on the surface of an electrode, which can then oxidize pollutants directly.

Applications
AOPs are used in a variety of water and wastewater treatment scenarios, especially when conventional methods fall short. Their primary applications include:

Industrial Wastewater Treatment: AOPs are a preferred method for treating complex industrial wastewater from sectors like pharmaceuticals, textiles, and petrochemicals, where the waste streams contain toxic and non-biodegradable compounds.

Removal of Micropollutants: AOPs are highly effective at removing trace amounts of micropollutants that can pass through traditional treatment plants. This is crucial for ensuring the safety of drinking water and protecting aquatic ecosystems.

Sludge Treatment: AOPs can be used to treat and condition sludge, improving its biodegradability and reducing its toxicity, which makes it easier and safer to dispose of.

Water Reuse: By achieving a high level of purification, AOPs play a key role in making treated wastewater safe for reuse in applications like irrigation or for recharging groundwater.

While AOPs can have higher energy and chemical costs than some conventional methods, their effectiveness in degrading persistent and harmful contaminants has earned them the reputation as one of the most promising water treatment processes of the 21st century.