Advanced Oxidation Processes

Many emerging contaminants, including some pharmaceutical and personal care products, endocrine disrupting compounds, and industrial contaminants, as well as naturally occurring taste and odour compounds, cannot be removed efficiently from water by conventional treatment. The need to remove or degrade these contaminants has led to the application of alternative treatment processes. One such group of processes is Advanced Oxidation Processes (AOPs). A number of AOP technologies have been identified, though many of these are either in their infancy, or have been used only in remediation applications.

Various criteria are used in selecting the appropriate AOP technology, including:

• experience to date with the technology at full scale

• capability and experience to date with treating potable water vs. other types such as wastewater, etc.

• process integration

• previously documented AOP treatment performance treating the contaminant of interest

• regulatory approval of AOP technology

• ancillary considerations such as chemicals, power and residuals management.

This article provides an overview of advanced oxidation mechanisms and highlights some emerging technologies used for trace contaminant removal and taste and odour control.

Generating the hydroxyl radical

The key to all AOPs is generating a highly reactive species, the hydroxyl radical (OH•).

The various processes differ as to which chemical or catalyst is used to generate the hydroxyl radical and the means to initiate the reaction. Some processes use ultraviolet (UV) light energy as part of the process, such as UV/H₂O₂, and some use catalysts such as titanium dioxide (TiO₂). Other processes use ozone (O₃), usually in combination with hydrogen peroxide (H₂O₂). The type or the extent of AOP treatment will determine the degree to which a contaminant will be mineralized.

The rate of oxidation or degradation of a target pollutant is dependent on three main factors: the concentration of radicals, oxygen concentration, and the concentration of the target pollutant. Maintaining a sufficient radical concentration in the water matrix is dependant on pH, temperature, the presence of ions in solution, the type of pollutant targeted, and finally the presence and concentration of radical scavengers. ¹

Compounds that are sensitive to photochemical transformation may be degraded with UV light alone in a process called direct photolysis. Many other compounds, however, are poor absorbers of UV radiation and require the oxidizing power of hydroxyl radicals before they will degrade. During the UV/H₂O₂ process, the key mechanism the cleavage of peroxide molecules into hydroxyl radicals2, which then react with contaminants. This process, termed indirect photolysis, has been proven effective in the removal of several different organic pollutants, including some emerging trace contaminants (e. g. some pharmaceuticals and personal care products, endocrine disruptors, and industrial contaminants), as well as taste and odour compounds.

The UV/H₂O₂ process typically requires a substantially higher UV dose than would be used for disinfection alone, and it is thus more energy intensive. Several factors affect the efficiency of this process, some of which include the UV lamp type, UV fluence, mixing efficiency, and water quality characteristics. It is also important to optimize the hydrogen peroxide dosage, as it must be present in sufficient quantities to produce high levels of hydroxyl radicals, yet it can also be a scavenger of hydroxyl radicals.

Still other technologies are emerging in the drinking water industry that also incorporate the use of a catalyst with UV light, termed photocatalysis. The catalyst absorbs light and promotes thermodynamic reactions without undergoing change to itself. ³ Titanium dioxide (TiO₂) is the most popular catalyst used, typically in the form of a powder slurry. A centrifugation or microfiltration step is needed for subsequent TiO₂ particle separation and recycling.

Advanced oxidation processes using ozone and hydrogen peroxide, termed peroxone, are also used in the water treatment industry to remove pollutants and taste and odour compounds. This process requires more space and an additional chemical system compared to the UV-based AOPs. Hydrogen peroxide can be added before, after, or concurrently with ozone. The preferred sequence of addition depends on the treatment goals, i. e. if ozone is being used both as a direct oxidant and disinfectant, as well as serving as a source of hydroxyl radicals, or if the goal is to deter the formation of undesirable by-products. Affecting the efficiency of this process is competition for the ozone: not all ozone reacts with the H₂O₂ present.

Water quality considerations

Several water quality parameters will impact the efficiency of an AOP technology. Radical scavengers are chemicals that compete for hydroxyl radicals, thereby impeding the hydroxyl radicals’ ability to react with the contaminant of interest and adversely impacting the efficiency of the process. Bicarbonate, alkalinity, and chloride are common radical scavengers and the extent of their scavenging potential is dependent on general water quality parameters such as pH, temperature and organic levels.

The selection of AOP technologies should also consider the potential of byproduct formation. All AOP technologies have drawbacks and are not appropriate for all types of source water conditions. For example, ozone-based technologies may not be suitable for waters containing bromide, [since they could result in] the undesirable byproduct, bromate.

Quenching residuals

AOPs that use hydrogen peroxide as part of their treatment leave a certain amount of the peroxide in the treated water because not all of the peroxide is consumed during the process. In typical full-scale installations, 70-80% of the hydrogen peroxide dose can be expected to remain in the treated water as residual. Depending on the peroxide dose required, this [residual} can have a significant impact on cost, the overall treatment train, and water quality. To quench residual hydrogen peroxide in the treated water, chlorine is typically used. Granular activatis ed carbon (GAC) can also be used to quench peroxide residual, at a greater capital and maintenance cost.

Plant retrofits

An important step prior to proceeding with the conceptual design of a retrofit at a water treatment facility is to identify the treatment objectives, select the preferred AOP and identify the preferred process location within the overall treatment [train]. To enable this to occur, various other considerations regarding the installation and design must be established for the various preferred AOPs through the bench-and pilot-scale testing initiatives. These considerations include:

• impact on upstream and downstream treatment process performance

• overall plant hydraulics and existing equipment operating capabilities

• equipment requirements and sensitivity to contaminant increases

• physical location of AOP facility considering existing site restrictions

• impact on operations and maintenance activities.

Cost issues

The costs of full-scale AOP installations are dependent on the pollutant type and concentration, the effluent flow rate, and the reactor configuration4. Since the efficiency of the process is strongly dependent on the raw water quality, it is difficult to compare costs of the different AOPs in broad terms. Few reports have discussed this type of direct comparison.

Costs for photochemical degradation processes are typically compared using the EEO (electrical energy per order,
kWh/m3/order) reactor efficiency parameter. EEO measures the amount of energy required to achieve a 1-log reduction of pollutant per 1 m3 of water, allowing for comparative economic analyses between various AOPs. ⁵ However, EE/O values will again be specific to the water and contaminant combination for which they were determined, as they will depend on several water quality parameters 6.

Due to the number of influencing factors, full-scale costs should be estimated based on bench and/or pilot scale studies on a case by case basis. The cost of residuals management for each process should also be considered.

Taste and odour

In many cases, advanced oxidation processes may prove to be more effective than conventional water treatment processes at removing taste and odour compounds as well as several trace organic contaminants. There are typically two reaction pathways involved: one is a direct reaction with the original mechanism (e. g. oxidation by ozone or photolysis by UV), and a second is oxidation by the hydroxyl radical. For the most part, AOPs are advantageous over conventional treatment when the hydroxyl radical significantly dominates the degradation reactions for the contaminants of concern. There are several design and implementation factors to consider and thus AOPs should be assessed on a case-specific basis.

Linda Wojcicka, M. A. Sc. and Elia Edwards, M. A. Sc., P. Eng. are with Associated Engineering’s water division in Toronto. Quinn Crosina, M. A. Sc. is in Associated Engineering’s Burnaby, B. C. office.

1 Parsons, S., and M. Williams. 2004. Advanced Oxidation Processes for Water and Wastewater Treatment. Parsons, S. (ed). IWA Publishing, London, UK.

2 Legrini, O., E. Oliveros, and A. M. Braun. 1993. Photochemical processes for water treatment. Chemical Reviews. 93: 671-698.

3 Mills, A., and S. K. Lee. 2004. Semiconductor Photocatalysis. In: Advanced Oxidation Processes for Water and Wastewater Treatment. Parsons, S. (ed). IWA Publishing, London, UK.

4 Andreozzi, R., V. Caprio, A. Insola, and R. Marotta. 1999. Advanced oxidation processes (AOP) for water purification and recovery. Catalysis Today. 53: 51-59.

5 Stefan, M. I., and C. T. Williamson. 2004. UV light-based applications. In: Advanced Oxidation Processes for Water and Wastewater Treatment. Parsons, S. (ed). IWA Publishing, London, UK.

6 Huck, P. M., W. A. Anderson, S. A. Andrews, G. Pereira, C. L. Lang. 1996. Evaluating the feasibility of advanced oxidation processes for removal of geosmin. WQTC Conference Proceedings. (1996) AWWA.