Photolysis of Hydrogen Peroxide -Photolysis is a process in which compounds absorb photons and the energy released drives oxidation processes induced by light. The photolysis rate of a compound can be estimated based on its light absorption rate and quantum yield. The extinction coefficient represents the phenomenon that as wavelength decreases, more photons are absorbed. It is the photolysis of hydrogen peroxide that generates the hydroxyl radicals that drive the UV/H2O2 AOP.
Absorption of UV Light by NOM - The natural organic matter (NOM) present in the water can also absorb UV light before it is able to form hydroxyl radicals. This reduces the amount of UV light available in support of the desired reactions in an AOP involving UV light. The extinction coefficient for NOM, which is a measure of the amount of light absorption by NOM, varies widely and is site specific. Certain other constituents (e.g., iron) in the background water matrix can also absorb UV light and reduce the amount of UV light available for the desired reactions in an AOP
UV Lamp Technology - There are two types of lamps commonly applied in the water industry for destruction of target compounds. There are various advantages and disadvantages to each type of lamp and Trussell Tech can help evaluate which type of lamp is appropriate for a given application. For example, LPUV lamps generate UV light more efficiently than MPUV lamps. MPUV lamps can operate at a higher power input so fewer lamps may be needed, but the power requirements are greater for each lamp:
Low Pressure UV (LPUV) Lamps - LPUV lamps may be either low intensity or high intensity lamps. LPUV lamps emit UV light only at a wavelength of 254 nm.
Medium Pressure UV (MPUV) Lamps - MPUV lamps applied in the water industry are high intensity. MPUV lamps emit energy over the 200 through 400 nm range but only the 200 to 300 nm is important in the UV/H2O2 process because hydrogen peroxide only absorbs UV light at wavelengths less than 300 nm.
Electrical Efficiency per Unit Order of Compound Reduction - Photolysis reactions require a large amount of energy so it is important to optimize efficiency on the basis of energy required per amount of compound removed. One measure is electrical efficiency per log order of compound reduction (EE/O). AOP models utilized by Trussell Tech allow for comparison of competing UV technologies in terms of minimizing the EE/O based on varying various factors affecting the process (e.g., type of lamp, hydrogen peroxide dosage, level of pretreatment):
EE/O = P / Q / log-removal
Hydrogen Peroxide Dosage - Relatively high hydrogen peroxide dosages compared to the O3/ H2O2 process are needed for the UV/H2O2 process to generate sufficient quantities of hydroxyl radicals because hydrogen peroxide does a poor job absorbing UV light, especially compared to NOM and iron if they are present. Higher H2O2 dosages will produce significant amounts of residual H2O2 that must be removed from the water. Trussell Tech can provide guidance in dealing with the H2O2 residual. The chemical costs of H2O2 need to be balanced against the energy costs of the UV lamps when evaluating an appropriate UV/H2O2 process (LPUV or MPUV) for a given application.