Xylem and Dutch water utility Dunea develop first sequential advanced oxidation process (AOP) system for drinking water containing bromide

Groundbreaking solution uses a unique process to advance sustainability by reducing micropollutants in water by up to 90%.

In 2005, the Dutch water utility Dunea, which supplies 1.3 million people with drinking water, began an ambitious research project with Xylem. The utility wanted to reduce micropollutants in its water, while keeping bromate levels well below recommended standards. Bromate, a potentially carcinogenic substance, can form when water containing bromide is treated with ozone. Xylem and Dunea found an innovative way to address this challenge.
Micropollutants in water supply sources are a growing problem for water utilities. These chemical traces come from industries, agriculture, households and hospitals. This category of pollutants can include pharmaceuticals, endocrine disrupting compounds, personal care products, industrial chemicals, pesticides and contrast agents used in medical imaging.

Getting a head start on a growing micropollutant problem

In 2005, Dunea experts noted an increase in micropollutants in the source of the utility’s water. Although the pollutant concentrations gave no reason for concern according to drinking water standards, Dunea decided to invest heavily in developing strategies and water treatment solutions to mitigate micropollutants. These long-sighted investments would help prevent the accumulation of micropollutants and improve the biodegradability of micropollutants in Dunea’s treatment train. In addition, the investments would help ensure compliance with any drinking water regulations that could arise in the future.
Micropollutants can effectively be removed from water with ozone oxidation. However, Dunea’s source water contains bromide, which eventually can be transformed to toxic bromate when ozone is used to treat water. Dunea’s goal was to find a solution for eliminating micropollutants while simultaneously forming 20 times less bromate than the World Health Organization (WHO) defines as safe for drinking water.

Finding the right combination of AOPs

The first phase of Xylem’s pilot study at Dunea, between 2009 and 2011, was to see what combinations of advanced oxidation processes would be best able to reduce four representative micropollutants.

There are three main, non-chlorine-based ways water utilities treat drinking water by oxidation:

1. Ultraviolet light (UV)-AOP.
2. Ozone or ozone-AOP
3. Combinations of the processes.

The goal of all of these processes is to distrupt the micropollutants at the atomic level, by using ozone or hydroxyl radicals. The hydroxyl radicals replace or add certain atoms to the micropollutants, causing them to lose their toxic properities. The reactions happen in fractions of seconds, after which the hydroxyl radicals are no longer present.

In the UV-AOP process, the contaminated water is mixed with low concentrations of hydrogen peroxide and irradiated with powerful ultraviolet light to form the hydroxyl radicals. In an ozone-AOP process, hydrogen peroxide is also used. Hydrogen peroxide is injected into the water, then ozone is dosed, which causes the hydroxyl radicals to form.

Ensuring quick reactions between ozone and hydrogen peroxide

Based on the results from the first pilot phase, Dunea and Xylem concentrated the second phase on adjusting the ozone and hydrogen peroxide doses from the first pilot phase, while keeping the bromate formation below Dunea’s standard.
When adding ozone to water containing bromide, the more ozone you add leads to more bromate being formed. To minimize bromate formation, Dunea and Xylem used an ozone loop reactor with six consecutive ozone injection points. First hydrogen peroxide is added to the water. Then the water passes the reactor where ozone is injected through the six dosing points. This leads to an increased hydrogen peroxide/dissolved ozone ratio in the water, which limits bromate formation and contributes to efficient micropollutant reduction.

The pilot study showed that very quick and efficient reactions of ozone and hydrogen peroxide are necessary to keep the bromate formation below the Dunea standard. During ozone injection, there must not be any spots where ozone gets time to react with bromide. Based on the loop reactor experiments, Dunea and Xylem developed the innovative and unique DOP reactor. It helps ensure immediate reactions between the hydrogen peroxide and ozone in order to minimize bromate formation and reduce micropollunts.

Two advanced oxidation processes for better water quality

After the water is treated using an ozone-AOP in the DOP reactor, it moves with residual hydrogen peroxide through an ultraviolet advanced oxidation process. The UV-AOP reduces micropollutants even more. Since hydrogen-peroxide-based UV-AOP doesn’t form bromate, it is an ideal addition to the upstream ozone-AOP. This sequential process ensures stronger micropollutant reduction and contributes an additional treatment barrier for harmful trace substances and pathogens.
Dunea’s advanced treatment train includes an ozone generator, the DOP reactor, UV lamps and UV reactors, all supplied by Xylem.

Installing an innovative AOP for micropollutant reduction

The close cooperation between Dunea and Xylem in the long-term pilot study led in 2018 to Xylem installing the world’s first sequential AOP for reducing micropollutants and limiting bromate formation.
In October 2018, the new solution was put to the test. The groundbreaking serial AOP performed extremely well, reducing atrazine (herbicide) by 73% and ibuprofen by 90%. While the World Health Organization’s threshold for bromate is 10 micrograms per liter, the new AOP process at Dunea keeps bromate below 0.5 micrograms per liter. In addition, Xylem’s low-energy solution uses only 0.15 kWh/m³ of energy for both the ozone-AOP and UV-AOP processes. This means that Dunea can meet it standards for water quality and reduce energy expenses by 30 to 50%.

Together with Xylem’s support, Dunea raised their existing treatment train to the next level with strong resilience against micropollutant contamination now and in the future, creating an innovative and sustainable solution for their community. More than 1.3 million people in the Netherlands now benefit from fresh and healthy drinking water with a quality much higher than current drinking water regulations.
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