Ozone vs. UV-AOP: Choosing the right path for advanced water treatment

Both ozone and ultraviolet (UV) advanced oxidation processes (UV-AOP) have been around for years and are growing in popularity due to the increasingly complex demands in water treatment and their effectiveness in contaminant removal. While each technology is unique, they offer similar benefits in a range of applications, leaving treatment managers to decide which is best suited for their water quality and effluent requirements. To help guide this decision, this article explores different aspects of each type of system, including ideal applications, ease of operation, and more, as well as when they work best together. 

Ozone: Treatment train flexibility

Ozone treatment systems have been used for decades, with a boom in the early 2000s due to large cryptosporidium outbreaks and increased demand for safe, reliable drinking water. Since then, ozone technologies have advanced and are even more effective. As a powerful oxidant and disinfectant, ozone is used to treat a wide range of contaminants in many applications, such as municipal drinking water, industrial wastewater, food and beverage applications like bottling facilities, or “clean in place” processes. It is highly effective at removing taste and odor (T&O) compounds — e.g., MIB (2-Methylisoborneol) and geosmin — reducing organics and color and eliminating metals like iron and manganese. For surface water intakes, ozone is a powerful solution for treating harmful algal blooms. 

As a disinfectant, ozone can be applied at the end of a treatment train to minimize chlorine usage, particularly when discharging to sensitive ecosystems. However, this technology can be used almost anywhere in the treatment train. For example, it can be implemented at the front end of a drinking water plant as a pre-oxidation step or as an intermediate oxidation step to make organics more easily removable by downstream processes like carbon or other physical media filtration. The latter case is common when included in direct or indirect reuse systems. 

UV-AOP: End-of-line protection

The germicidal properties of UV light were discovered in the late 19th century, although it took much longer to develop practical systems for water disinfection. UV-AOP debuted in the late 1980s, creating new applications for the technology. Beyond targeting pathogens, the technology is often used to treat pesticides and herbicides, pharmaceuticals, industrial solvents such as 1,4-dioxane, T&O compounds, and more. 

Disinfection using UV is most effective when it comes at the end of a treatment train to replace or supplement chlorine. As such, while UV-AOP has the capacity to treat a range of chemicals, it is also most often used on the backend for peak effectiveness. Similarly, in industrial settings like bottling facilities, UV can be used to destroy residual ozone after the primary treatment step. This process ensures purified water moves to the next stage without a chemical residual. 

Comparing cost & performance

Due to its ability to break down a range of contaminants, ozone has an inherent advantage over UV alone. However, UV-AOP is comparable to ozone, depending on the substances being targeted and other treatment technologies being deployed with them. Still, ozone offers greater flexibility in terms of its placement in the treatment train, while UV (with or without AOP) is frequently the final step. 

From an operations and maintenance standpoint, ozone systems have a false reputation for being difficult to run. In reality, today’s systems are largely automated, and have minimal consumables requiring infrequent maintenance. By comparison, UV lamps and wiper rings need replacement, and the quartz could require cleaning. 

Although easier to operate, ozone systems have higher upfront capital costs than UV and UV-AOP. Moreover, retrofitting ozone systems into existing plants is challenging because of a large physical footprint and requires multiple subsystems. As such, ozone systems are often integrated as part of new construction or a fundamental plant upgrade. UV-AOP has a lower capital cost and requires much less space, making it easier to add to an existing operation.

Better together

Despite their differences, the two technologies can work synergistically, especially in water reuse as an alternative to reverse osmosis (RO) systems. In this setup, water is first dosed with ozone, typically followed by biologically active carbon (BAC) filters with UV-AOP acting as the final barrier. This combination is ideal for inland locations where RO brine disposal is a major challenge.

Ultimately, both ozone and UV are effective water treatment technologies with unique strengths that can work well together when needed. The choice between them depends on a careful evaluation of treatment goals, operational needs, budget, and physical plant limitations.