Advanced Oxidation Process (AOP) in Pools: All You Need to Know

The Advanced Oxidation Process (AOP) is a chemical treatment that uses reactions with hydroxyl radicals to eliminate organic and occasionally inorganic substances from water. AOP enhances the decomposition of ozone into hydroxyl radicals using hydrogen peroxide, producing radicals faster than standard ozone decomposition. The process combines ozone and UV into one system to maximize disinfection, water clarity, and chloramine removal. AOP involves the generation of hydroxyl radicals to purify water using a combination of ozone, hydrogen peroxide, and/or ultraviolet light.

Is AOP a new technology?

No, AOP is not a new technology. Oxidation processes involving hydroxyl radicals have been in use since the 19th century. However, large-scale water treatment using these radicals did not occur until 1987. Today, there are approximately 500 commercial AOP installations worldwide used for drinking water.

How does AOP work?

The Advanced Oxidation Process (AOP) effectively eliminates organisms and contaminants either within the AOP chamber or close to the point of injection. As a result, many AOP devices must operate 24 hours a day. However, this does not guarantee safe water, especially if a biofilm develops on the pool walls or in the plumbing system.

How are Hydroxyl Radicals Introduced into Pool Water?

Hydroxyl radicals are introduced into pool water through an AOP device, similar to an electronic chlorine generator (ECG) or ozonator, installed in the plumbing line. The AOP device is integrated either directly into the main flow or set up as a side stream in commercial or public pools. When set up as a side stream, approximately 25% of the total flow is redirected through the AOP device. These devices often have a contact chamber to maximize hydroxyl radical contact with contaminants.

How is AOP Used in Water Treatment?

AOP is used in water treatment as a highly reactive and fast method for effectively removing taste, odor, and organic micropollutants that are difficult to remove with ozonation alone. In some AOP, H2O2 is fed as an aqueous solution, with H2O2 to ozone ratios between 0.2 and 3.0. The optimal ratio for removing most compounds ranges from 0.3 to 0.6. Both ozone and AOP processes form bromate in the presence of bromide, which is carcinogenic.

How Does AOP Compare to Ozone and UV?

AOP is the most potent oxidizer for pool or spa water, covering almost all oxidizable substances in the water and serving as an excellent disinfectant. Compared to UV, which effectively destroys viruses, bacteria, and pathogens but only affects water passing through the UV light chamber, AOP has a broader range of impact. However, the hydroxyl radicals in AOP have a very short lifespan, making it less likely to be used as a standalone disinfectant and oxidizer. On the other hand, ozone is excellent at eliminating viruses, bacteria, taste, odor, chloramines, combined chlorine, and most organic matter. But AOP reacts slowly with ammonia and does not cover all oxidizable substances in the water. It also has a short half-life in pool water and does not provide a lasting residual. Therefore, while AOP, ozone, and UV each have their strengths, they also have limitations and are often used in combination for optimal results.

Can Chlorine be Completely Eliminated Using an AOP Device?

No, chlorine is not completely eliminated using an AOP device. While UV, ozone, and hydroxyl radicals are powerful oxidizers and disinfectants, they do not build up in the water or protect against future contamination. They produce a fixed amount of hydroxyl radicals, which are not sufficient for increased demand.

What is the Efficiency of OHAOP in Oxidizing Water?

The efficiency of Advanced Oxidation Processes (AOPs), including those using hydroxyl radicals (·OH), in oxidizing water vary greatly depending on several factors. These factors include the specific contaminants present, the concentration of these contaminants, the pH of the water, the temperature, and the specific design and operation of the AOP system.

OHAOP, which produces hydroxyl radicals, is a more potent and efficient oxidizer than hypochlorite ion, hypochlorous acid, ultraviolet light, or ozone. Hydroxyl radicals are highly reactive and unstable, allowing them to react non-selectively with dissolved solids and decompose harmful substances into less harmful ones or even completely mineralize them. Hydroxyl radicals destroy various contaminants and microorganisms. However, their short half-life makes it impossible to build up a residual or measure it practically. The efficiency of the AOP process is affected by factors such as total alkalinity in the water, with the process being less efficient at higher alkalinity levels. The yield of hydroxyl radicals is significantly improved in the presence of other oxidants or irradiation.

Why is AOP gaining popularity?

AOP is gaining popularity as water shortages and droughts impose on water treatment facilities to explore alternative water sources and consider wastewater recycling. The Advanced Oxidation Process (AOP) is a recent advancement in oxidation that combines ozone and UV into one system, maximizing disinfection, water clarity, and chloramine removal. AOP is highly reactive and fast, effectively removing taste, odor, and organic micropollutants that are difficult to remove with ozonation alone. AOP is also used for the destruction of organic or inorganic contaminants in water. Despite its limitations, such as its short half-life and lack of a lasting or measurable residual in the water, AOP is becoming an increasingly popular choice in water treatment.

What are the Pros of AOP Devices?

The pros of AOP devices include their ability to kill organisms like Cryptosporidium parvum and Giardia lamblia, eliminate combined chlorine, chloramine, oxidation by-products, organics, microorganisms, and micro-contaminants. They are more powerful than chlorine and do not add lasting chemicals to the water. AOP devices reduce chlorine consumption by 50 to 90% depending on the device and conditions, and they provide better water quality.

What are the Cons of AOP Devices?

The cons of AOP devices include the fact that hydroxyl radicals last for 10 microseconds or less, they provide no residual in the water, and it difficult to verify the optimum level of operation of an AOP system. All oxidation and killing is done in the AOP device, so there is no control of biofilm or algae. They provide a fixed amount of ozone, UV, hydrogen peroxide, and hydroxyl radicals. High bather loads often make this quantity insufficient. AOP systems do not provide protection against future introduction of pathogens or person-to-person contact. Occasionally, these systems are marketed as being chlorine-free, even though they are not approved for that use. There is no test for hydroxyl radicals or AOP, and no AOP or hydroxyl radical dose guidelines or approvals. Their efficiency is affected by high humidity, temperature, high pH, high alkalinity, iron, copper, and manganese. The UV protective quartz sleeve needs frequent cleaning. The availability and cost of replacement or consumable parts represents an issue, and these systems are generally expensive.

How to Choose the Right AOP Device?

Choosing the right AOP device often presents a challenge. In the early 1990s, the ozone segment of our industry faced a similar issue with dosing and sizing. Manufacturers claimed different output amounts using various testing methods, resulting in ambiguity. Eventually, they agreed on a standard test method and dose amount, simplifying the process for customers to compare products.

What are the Challenges with AOP Dose and Output Amounts?

The determination of AOP dose level and device output pose significant challenges for AOP manufacturers. These challenges arise due to the lack of standard testing methods for output established criteria of hydroxyl radical dose levels. Without AOP manufacturers agreeing on these standards, the industry and potential clients will continue to face difficulties.

How to Determine if AOP is Working?

Determining if an AOP device is working is potentially difficult. The units have a light or sight glass to check if the UV bulbs are on. However, this does not guarantee the production of ozone, UV, hydrogen peroxide, or hydroxyl radicals. The only way to assess if an AOP device is working involves measuring the chloramines or combined chlorine through specialized test kits or digital meter designed for water testing, and observing whether their levels decrease or are eliminated upon the addition of AOP.

What are the Claims of AOP Manufacturers?

AOP manufacturers claim that their devices reduce chlorine use by 30 to 90% or even eliminate it completely. They also claim to eliminate chloramines, improve water quality, and destroy harmful organisms like cryptosporidium, giardia, or E. coli. However, manufacturers note that in the event of a fecal accident, there is no way to determine when the water will be safe again. They also acknowledge that while there are NSF certification standards for ozone or UV, there are currently no standards for AOP.

What is the Cost of Residential AOP Devices?

Residential pool AOP devices typically cost between 1,000$ and 3,000 $. Spa AOP units are less expensive.