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Ultraviolet Irradiation + Hydrogen Peroxide

Overview
 

The UV/H2O2 system is an advanced oxidation process (AOP) in which hydrogen peroxide (H2O2) is added in the presence of ultraviolet (UV) light to generate hydroxyl radicals (·OH). The oxidation potential of a hydroxyl radical (2.8V) is much greater than other oxidizing agents such as ozone (2.07V) and chlorine (1.39V) and thus has the capability of oxidizing a variety of organic and inorganic contaminants. See the Ultraviolet Irradiation and Hydrogen Peroxide overviews for descriptions of those treatment processes. In the UV/ H2O2 process, H2O2 is applied ahead of UV so that H2O2-treated water is irradiated.

The UV/ H2O2 process is accomplished through two stages of oxidation. In the first step, UV light catalyzes the dissociation of hydrogen peroxide into hydroxyl radicals through chain reactions. These oxidants then react with the contaminants of interest in the water. This technology has been shown to be effective in destroying many micropollutants present in the groundwater (e.g., MTBE, perchlorate, pesticides, 1,4-Dioxane, etc.) and surface water (pharmaceutical and personal care products, taste and odor compounds such as MIB and Geosmin) through direct chemical oxidation. The hydroxyl radical can be generated using either low-pressure (LP) or medium-pressure (MP) continuous UV light in the presence of H2O2. Low pressure high output (LP-HO) lamps are preferred when continuous contaminant removal is required (e.g., MTBE and perchlorate removal). MP lamps are recommended for periodic events (such as taste and odor (T&O) or pesticides). LP-HO lamps use less power and have lower operating temperatures (i.e., lower temperatures produce less scaling of sleeves) than MP lamps.

The Electrical Energy per Order (EEO) is used to determine the removal efficiency of organic contaminants in a UV/ H2O2 system. EEO is defined as the number of kilowatt-hours (kWh) of electrical energy required to reduce the concentration of a pollutant by one order of magnitude (90%) in one cubic meter of contaminated water. The EEO has units of kWh/1,000 gal/order of removal. The EEO is dependent on water quality and is measured at the optimum H2O2 dose. It enables a direct comparison of the effectiveness of removing different organic compounds using UV radiation. Contaminants with greater EEO values are more difficult and costly to remove in comparison to those with lower EEOs. For example, the EEO values for MTBE and benzene are 10 and 2, respectively, indicating that MTBE is more difficult to treat than benzene. An EEO of 10 for MTBE means it would take 10 kWh of energy to reduce MTBE from 1,000 µg/L to 100 µg/L (90% reduction) in 1,000 gallons of water. It would take another 10 kWh to reduce the MTBE from 100 µg/L to 10 µg/L, and so on.

The key design and operating parameters for UV/ H2O2 systems include: peroxide dose, UV lamp type and intensity, reactor contact time, and control system (pH and temperature). The hydroxyl radical generated in the UV/ H2O2 process is nonselective and thus can be 'used up' by organic and inorganic scavenging compounds. Water quality parameters, such as organic matter, alkalinity, and nitrite play an important role for the UV/ H2O2 system because they are considered hydroxyl radical 'scavengers', which reduce system effectiveness during oxidation of contaminants. Optimizing the peroxide dose is also critical because excess peroxide can act as a scavenger, which can limit system effectiveness.

The most common point of application for an UV/ H2O2 system is in the last step of the conventional treatment process, typically after filtration (lower turbidity, reduced obstruction/shielding of UV light, and fewer scavengers). Closed vessel UV reactors are generally preferred for drinking water applications with hydrogen peroxide feed due to their smaller footprints, reduced airborne material pollution, reduced exposure to UV light, and installation simplicity. Some concerns of UV/ H2O2 systems include lamp fouling and subsequent lamp sleeve cleaning. Lamp breakage and aging are other potential problems since UV intensity output decreases with time.





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