3.2 UV-based oxidation processes

UV-based Advanced Oxidation Processes (AOPs) are cutting-edge wastewater treatment methods. They use UV light and oxidants like hydrogen peroxide or ozone to create powerful hydroxyl radicals, which break down tough pollutants in water.

These processes are super effective at removing a wide range of contaminants, from pharmaceuticals to pesticides. They work through direct photolysis or indirect oxidation, making them versatile tools for cleaning up our water resources.

UV-based Advanced Oxidation Processes (AOPs)

Principles of UV-based AOPs

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• UV-based AOPs harness the power of highly reactive hydroxyl radicals (OH·) to effectively oxidize and break down contaminants in wastewater
  • UV light efficiently photolyzes oxidants such as hydrogen peroxide (H2O2) or ozone (O3), generating OH· radicals
  • OH· radicals are non-selective and extremely powerful oxidants, boasting an impressive oxidation potential of 2.8 V, making them highly effective in degrading a wide range of pollutants
• UV-based AOPs employ two primary mechanisms to degrade contaminants
  • Direct photolysis occurs when UV light directly interacts with and breaks down contaminants, without the need for additional oxidants
  • Indirect oxidation relies on the generation of OH· radicals by UV light, which then react with and oxidize contaminants, effectively degrading them into less harmful compounds
• The UV/H2O2 process utilizes the photolysis of hydrogen peroxide to generate OH· radicals
  • When exposed to UV light with wavelengths shorter than 280 nm, H2O2 undergoes photolysis, producing two OH· radicals per molecule of H2O2
  • The generated OH· radicals then react with and oxidize contaminants, converting them into less harmful substances
• The UV/O3 process leverages the photolysis of ozone to produce OH· radicals
  • UV light with wavelengths shorter than 320 nm photolyzes ozone (O3), generating molecular oxygen (O2) and excited atomic oxygen (O(1D))
  • The excited atomic oxygen then reacts with water molecules to form two OH· radicals, which subsequently react with and degrade contaminants

UV/H2O2 vs UV/O3 processes

• UV/H2O2 offers several advantages in wastewater treatment
  • Hydrogen peroxide is relatively stable, easy to handle, and readily available commercially, making it a convenient oxidant for UV-based AOPs
  • The UV/H2O2 process is effective in treating a wide range of contaminants, including organic micropollutants (pharmaceuticals, pesticides) and taste and odor compounds (geosmin, 2-methylisoborneol)
  • Unlike UV/O3, the UV/H2O2 process does not form bromate, a potential carcinogen, making it a safer option for wastewater treatment
• However, UV/H2O2 also has some limitations
  • Heavily contaminated wastewaters may require higher doses of H2O2 to achieve desired treatment goals, increasing operational costs
  • At high concentrations, H2O2 can act as a scavenger of OH· radicals, reducing the overall efficiency of the process
• UV/O3 also offers several advantages in wastewater treatment
  • Ozone itself is a strong oxidant and can directly react with some contaminants, providing an additional pathway for pollutant degradation
  • The UV/O3 process has a higher quantum yield of OH· radicals compared to UV/H2O2, potentially leading to more efficient contaminant removal
  • Like UV/H2O2, UV/O3 is effective in treating a wide range of contaminants, including organic micropollutants and taste and odor compounds
• However, UV/O3 also has some limitations
  • Ozone is unstable and must be generated on-site, increasing the operational complexity and maintenance requirements of the treatment system
  • In waters containing bromide, UV/O3 can lead to the formation of bromate, a potential carcinogen, requiring additional control measures
  • The UV/O3 process typically consumes more energy compared to UV/H2O2 due to the energy-intensive ozone generation process

Factors affecting UV-based AOPs

• pH plays a crucial role in the efficiency of UV-based AOPs
  • The speciation of H2O2 and the formation of OH· radicals are influenced by pH
  • At higher pH values, the formation of HO2- is favored, which has a higher molar absorption coefficient than H2O2, leading to increased OH· generation
  • For UV/H2O2, the optimal pH range typically falls between 7 and 8, balancing OH· generation and other competing reactions
• UV dose is another critical factor in the performance of UV-based AOPs
  • The UV dose is determined by the intensity of the UV light and the exposure time of the wastewater to the UV source
  • Higher UV doses lead to increased generation of OH· radicals and improved contaminant removal
  • However, the UV dose must be carefully optimized to balance treatment efficiency and energy consumption, as excessive UV doses can lead to diminishing returns and higher operational costs
• Water quality characteristics significantly influence the effectiveness of UV-based AOPs
  • The presence of natural organic matter (NOM) can compete with target contaminants for OH· radicals and UV light, reducing the overall treatment efficiency
  • Inorganic constituents, such as carbonates, bicarbonates, and chlorides, can scavenge OH· radicals, reducing the available OH· for contaminant oxidation
  • High levels of turbidity and color in the wastewater can reduce UV transmittance, requiring higher UV doses or pretreatment steps to ensure adequate UV penetration and OH· generation

Case studies in wastewater treatment

1. A municipal wastewater treatment plant implemented a UV/H2O2 system for tertiary treatment to remove organic micropollutants and improve water quality for reuse applications
   • The plant optimized the H2O2 dose and UV intensity based on the specific water quality characteristics and treatment goals
   • The UV/H2O2 process resulted in a significant reduction in micropollutant concentrations and improved the overall water quality, making it suitable for various reuse applications (irrigation, industrial processes)
2. A chemical manufacturing plant employed a UV/O3 system to treat its effluent, aiming to degrade recalcitrant organic compounds and meet stringent discharge regulations
   • The UV/O3 system was customized based on the unique wastewater characteristics and treatment objectives of the plant
   • The application of UV/O3 effectively removed the target contaminants, ensuring compliance with discharge limits and minimizing the environmental impact of the plant's effluent
3. A drinking water treatment plant incorporated a UV/H2O2 process to mitigate seasonal taste and odor issues caused by algal blooms in the raw water source
   • The plant integrated the UV/H2O2 process with its existing treatment train and optimized the dosing based on fluctuations in raw water quality
   • The UV/H2O2 process significantly reduced the concentration of taste and odor compounds (geosmin, 2-methylisoborneol), resulting in improved customer satisfaction and a more reliable supply of high-quality drinking water