|Figure 1: Ozone application as a pre-oxidant.|
|Figure 2: Ozone application prior to filtration.|
Ozone (O3) is one of the strongest disinfectants and oxidants available in drinking water treatment. Ozone must be generated onsite and used immediately. Due to its short half-life, typically less than 30 minutes, a residual is not maintained in downstream processes; therefore, it can only be used as a primary disinfectant. A secondary disinfectant such as chlorine or chloramine must be added to maintain a disinfectant residual within the distribution system. Ozone can be applied at various points in the treatment train, although it is usually applied prior to coagulation (reduces coagulant demand) (Figure 1) or filtration (causes micro-flocculation which improves filterability) (Figure 2).
Ozone is generated onsite by an ozone generator that uses either dried air (requiring air dryers and compressors) or liquid oxygen (LOX). The LOX system is preferred as it produces higher a percent weight concentration of ozone (%wt as O3) than the dry air system. The solubility of ozone in water depends on temperature and its concentration in the feed gas. Ozone contactors (diffused bubble or in-line injection systems) are used to dissolve ozone in water. Diffused bubble systems, commonly used in drinking water treatment, are typically composed of several enclosed consecutive chambers. In the first chamber, water flows downward against rising bubbles (countercurrent). Additional chambers are added to ensure sufficient contact time between ozone and water. These chambers may be countercurrent, cocurrent (water and rising bubbles flowing upward) or flow through (no ozone bubbles introduced in the chamber). A sampling port located in each chamber is used to measure ozone residual. The ozone contactor off-gas must be recycled or destroyed to minimize exposure to unhealthy ozone levels. Ozone destructors usually use heat or a combination of heat and a catalyst to remove ozone from the air. Ozone in the off gas results when all the applied ozone is not transferred into the water.
When ozone is added to water, a complex chain of reactions results in the formation of radicals, such as hydroxyl radicals (·OH). The hydroxyl radical is stronger than ozone itself. Oxidation with molecular ozone occurs slowly in contrast to oxidation with hydroxyl radicals which occurs very rapidly. Water quality parameters, such as pH have a significant impact on ozonation. Different ozone dosages are required for different pH levels. Higher pH facilitates ozone decomposition due to increased hydroxyl radical formation; whereas, lower pH (less than 7.0) slows down ozone decomposition resulting in higher concentrations of molecular ozone. The rate of ozone decomposition increases significantly (due to ·OH formation) when the pH is grater than 8.0. Ozone residuals are difficult to maintain at pH levels greater than 9.0. While molecular ozone is easily measured, hydroxyl radical is difficult to measure and typically measured in research efforts.
In addition to pH, other water quality parameters can impact ozonation and maintenance of ozone residuals. Higher alkalinity affects pH control. Turbidity, organic matter and color all increase ozone demand. Inorganics like iron and manganese also increase ozone demand. Disinfecting and oxidative properties are relatively independent of temperature; however, as temperatures increase, the solubility of ozone in water decreases. The major challenge with higher temperatures is the ability to transfer an adequate ozone dosage to the water. This can be accomplished by increasing the ozone concentration in the feed system and/or by providing adequate design for ozone transfer.
The product of ozone concentration (C) and contact time (T) determines CT which is an important measure ability of ozone to disinfect and inactivate microbes.
Ozone organic disinfection byproducts (DBPs) are numerous and include aldehydes, ketones and carboxyl acids. Ozone also converts a portion of the total organic carbon (TOC) into biodegradable dissolved organic carbon (BDOC). If untreated (typically by GAC filter or by a biological filter), BDOC may cause biological growth in the distribution system. Ozonation of water containing bromide can lead to the formation of the inorganic DBP bromate (BrO3), which must be maintained below the regulated 10 µg/L level. Bromate formation depends on water quality conditions including bromide levels, pH, temperature, alkalinity, ammonia concentration and TOC levels. Bromate levels can be controlled while achieving effective Cryptosporidium inactivation by using bromate mitigation strategies such as pH depression, ammonia addition, and/or chlorine-ammonia processes.