By Greg Reyneke, MWS
Today’s water improvement landscape is quite different from years past; there are contaminants being discovered in water now that we are still grappling to fully understand. Legislators and water treatment professionals are working more closely together now than ever before to evaluate and recommend the necessary solutions to provide clean, safe, affordable water to as many people as possible.
Advanced oxidative process (AOP) technology has been in use to address water quality challenges since the 1970s. AOP involves the generation of hydroxyl (•OH) ions to unleash their strong oxidative power to address problematic water quality issues in both fresh and wastewater applications. Chemical oxidation processes involve oxidation-reduction (redox) reactions which are essentially an exchange of electrons between various chemical species. This electron exchange affects the valence (oxidation state) of the chemical species involved. Carbon bonds are broken as a result of this electron exchange and the organic compounds are either completely destroyed or simply converted to smaller, less hazardous compounds.
While AOP technology is familiar to industrial wastewater operators, this is a new concept to many freshwater/potable water professionals and should be studied to enhance one’s ability to aid clients. AOP technology has been effectively used internationally to address many contaminants (see Table 1).
Hydroxyl ions might sound familiar to you, since the hydroxide anion (OH–) is used to regenerate weak-base anion (WBA) ion exchange resins. The hydroxyl radical used in advanced oxidation processes (•OH) is the neutral form and is entirely different, being highly reactive and very unstable.
- •OH destroys compounds that cannot be oxidized by conventional oxidants such as O2, O3 and Cl–.
- •OH reacts with dissolved waterborne contaminants in a series of oxidation reactions until they are completely mineralized.
- They are non-selective in their mode of attack and are able to operate at normal temperature and pressure.
- They oxidize almost all reduced materials present in wastewater without restriction to specific classes or groups of compounds.
- AOP differs from other treatment processes because contaminants are degraded rather than concentrated or transferred into a different phase.
- No secondary materials are generated, so there is generally no need to dispose of or regenerate media.
AOP is especially interesting in developing markets, since the electricity required for some methods can be generated onsite via renewable energy methods (see Table 2). There is little ongoing need for consumables that are difficult to transport and store. Some innovators are even proposing harnessing magnified equatorial sunlight as a source of UV radiation for photocatalytic AOP processes.
There are many AOP methods currently in use, as well as a growing portfolio of theoretical applications that have yet to be deployed outside the laboratory. The two most common ways of generating hydroxyls through AOP are with and without added ozone. Processes that add ozone are generally regarded as being cleaner and less complicated than non-ozone processes, but the ozone equipment itself requires a significant capital and maintenance investment that shouldn’t be overlooked when evaluating the cost-effectiveness of AOP options.
Most common ozone-based methods
These methods rely on ozone (O3) as an oxygen donor (see Table 3). Depending on the specific process, ozone is injected into the water stream and then reacted with the water and subsequent part of the process to develop •OH radicals. Remember that ozone will react with anything oxidizable in the water before participating in the AOP reaction, so one should inject the ozone sufficiently upstream and in a properly calculated dosage and concentration to satisfy the base ozone demand before beginning the AOP reaction.
Ozone and ultraviolet. Ozone reacts with ultraviolet light at specific wavelengths and intensity to develop hydrogen peroxide and then further to develop hydroxyl radicals. This method is relatively simple and quite effective as long as the system integrator/designer calculates the potential interference factor like turbidity, suspended solids and hardness minerals that could interfere with UV transmission.
Ozone and titanium dioxide/ozone react with the titanium dioxide surface to create an electron hole pair. The ozone develops into hydrogen peroxide and then further to hydroxyl radicals while in contact with the TiO2 catalyst. Naturally, the most effective (and complex) ozone-based method is to leverage the benefits of both ozone and UV light catalyzed by a titanium dioxide-doped surface.
Most common non-ozone methods
These methods rely on reagents like hydrogen peroxide along with a catalyst to develop •OH radicals. This method can be beneficial when working with high-turbidity waters that would significantly interfere with ultraviolet light’s ability to pass through the water.
Fenton’s reagent. The most common non-ozone method is to simply add Fenton’s reagent to the water being treated. Fenton’s reagent is a unique combination of hydrogen peroxide and a ferrous compound that act together to develop hydroxyl radicals and conventional hydroxyl anions. Fenton chemistry has great potential when properly applied and when the operator allows for the inherent exothermic reaction and pH swing.
Hydrogen peroxide and UV. By injecting hydrogen peroxide into the untreated water stream, overcoming the initial ozone demand then irradiating with UV light at an appropriate wavelength and intensity, •OH radials are developed. This method is very effective if the proper balancing chemistry is performed, with the only major drawback being sourcing and storing hydrogen peroxide onsite.
UV and titanium dioxide. A powerful non-ozone AOP technology is the synergy of UV and TiO2. Titanium dioxide reacts with UV light to create an electron hole pair that catalyzes the development of ozone, hydrogen peroxide and then, hydroxyl radicals. This catalysis is highly effective and requires little energy. Naturally it is negatively affected by the turbidity of the water, as well as adhesions that occlude the titanium surface. As manufacturers develop more cost-effective ways to create a high-density TiO2 matrix, this technology will drive the installed cost of AOP down significantly. A downside to this method is the necessity for a reasonable amount of free dissolved oxygen to achieve maximum effectiveness.
PFAS, PFOS and related compounds?
Currently, specialized carbon and anion resin media are being effectively deployed for capturing many perfluorinated compounds found in water. While this is a good start, we’re merely kicking the can down the road. These contaminants are environmentally persistent and we will inevitably have to deal with safe disposal of media that is contaminated with these compounds.
The ideal way is to break the offenders down into safer and more manageable compounds. While AOP has enough energy potential to break the strong fluorine chemical bonds that comprise PFOS and other perfluorinated compounds, it currently is not an effective solution at the municipal level. In multiple-pass batched operations, AOP can be effective and should be considered as such. As integration and diffusion technologies improve, this will continue to become a more viable technology for this growing environmental threat.
Looking at the impressive arsenal of AOP tools available and the potential to address so many contaminants, one could be tempted to apply AOP technology as a cure-all technology. Sadly, AOP has complicating factors as well as operational restrictions that vary depending on the operating environment, water quality challenge and specific technology deployed. When evaluating methods of drinking-water treatment, sanitation and wastewater management in residential, commercial and industrial markets, AOP should be seriously considered as an alternative or adjunct to more conventional technologies.
About the author
Greg Reyneke, Managing Director at Red Fox Advisors, has two decades of experience in the management and growth of water treatment dealerships. His expertise spans the full gamut of residential, commercial and industrial applications, including wastewater treatment. In addition, Reyneke also consults on water conservation and reuse methods, including rainwater harvesting, aquatic ecosystems, greywater reuse and water-efficient design. He is a member of the WC&P Technical Review Committee and currently serves on the PWQA Board of Directors, chairing the Technical and Education Committee. You can follow him on his blog at www.gregknowswater.com