Developments in ozone technology have opened new applications for established water treatment technologies. In addition to new, more disinfection-resistant pathogens like Giardia and Cryptosporidium cysts, new government regulations are designed to protect the public health from the hazards of consumption of these microorganisms.
Here is the overview of various byproducts of ozonation:
Organic Oxidation Products
Ozone occurs when organic compounds absorb oxygen. Because of this, by-products of ozonation are generally less toxic and more easily biodegradable than their parent compounds. When chemical oxidizing agents are added to water (such as chlorine, chlorine dioxide, and ozone), organic byproducts are formed, including alcohols, organic acids, aldehydes, aldehyde-acids, keto-acids, alcohol-acids, ketones etc.
Compared to chlorine, ozone has the advantage of being a stronger oxidizer and, therefore, creating more of the oxidized organics. This oxidized organic matter can then readily be “mineralized”. Chlorine present in residual biomass prevents the biomass from forming effectively, so chlorine-oxidized organics cannot be mineralized.
Bromide ions (-OBr) and hypobromous acid (HOBr) are formed by the ozonation of water containing bromide ions (similarly to chlorine). HOBr can produce a wide range of brominated organic compounds. As a result of chlorination of bromide ion-containing waters, halo-organics (such as several trihalomethanes and some haloacetic acids) as well as non-chlorinated bromo-organics (such as bromoform, mono-, di-, and tribromoacetic acids) are formed. When bromide ion is ozonated from water, it forms HOBr, which can undergo further bromination to produce mono-, di-, and possibly tri-bromoacetic acids.
It has been evaluated that -OBr, when exposed to ozone, can be oxidized into bromate ions. These ions are carcinogenic to certain laboratory animals. As a result, BrO3– has been listed by the U.S. Environmental Protection Agency as a probable human carcinogen.
The maximum contaminant level (MCL) for BrO3- has been set at 10 mg/L in the Surface Water Treatment Rule (SWTR). Water treatment plants often treat source waters for their water, and the higher the bromide content, the more bromate will be produced during ozonation, specifically when the pH of the water exceeds 6.5.
Therefore, water treatment specialists should have a solid understanding of the chemistry of bromete formation and of the various chemical techniques to minimize its formation.
Bromide (or chlorine) quickly oxidizes bromide ion into a mixture of HOBr and -OBr during ozonation if raw water contains bromide ion and certain pH and ozone-demanding materials are present. Optimal ozonation around pH 6.5 can slowly oxidize –OBr (not HOBr) to bromate ions, but at pH 6.5 there can be no –OBr, so bromate ion formation significantly is reduced, if not eliminated, under ozonation.
As an alternative to bromate formation during ozonation of potable water supplies, treating water at a pH of 6.5 or less and adjusting pH up later in the process can minimize or eliminate bromate formation.
The ozonation conditions can also be adjusted to minimize residual levels of ozone during bromate formation. As a result, other contaminants in water have a higher chance of out-competing –OBr for ozone. Adding a trace of ammonia to the water before ozonation is another way to minimize bromate ion production from ozone.
Ozone is created during the ozonation process by oxidizing HOBr, which immediately reacts with ammonia to produce monobromamine, which is substantially more slowly oxidized by ozone to form bromide ions again.
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