Ozone-bromide interactions in water treatment.

Abstract

Ozonation of drinking waters, particularly as a preoxidant, is becoming a widespread practice. Ozone is a powerful oxidant and reacts with many of the natural constituents present in water. The presence of bromide ion in water can lead to the formation of brominated disinfection by-products upon ozonation. The existence of brominated by-products in a public water supply could be of public health concern since some of them have been shown to be mutagenic. Production of both organic and inorganic by-products upon ozonation of waters containing bromide ion was investigated. Organic by-products identified include bromoform, dibromoacetonitrile (DBAN), and 1,1(DBAA); inorganic by-products identified include bromate, hypobromous acid and hypobromite ion. Formation of by-products is a function of bromide ion concentration, the source and concentration of humic substances, pH, ozone dose, temperature, alkalinity and reaction time. Bromoform concentration ranged from 5 to 60 $\mu$g/L and total organic bromine (TOBr) concentration varied from 15 to 150 $\mu$g/L for a bromide concentration ranging from 0.5 to 1.5 mg/L. TOBr concentrations were much higher than bromoform indicating that bromoform constitutes only a fraction of the pool of brominated DBP material. Bromate threshold levels were shown to vary according to precursor source and pH level. At pH 7.5 bromide threshold concentrations for bromate were higher than at pH 8.5; the exact opposite case was observed for bromoform. Ionic strength had no significant effect on the formation of by-products whereas an increase in alkalinity resulted in decreased amounts of by-products. Temperature effects are manifested in two different ways: (i) the water temperature at which ozonation was carried out versus (ii) the subsequent incubation temperature. While enhanced precursor oxidation was observed at higher ozonation temperatures, the partial oxidation by-products varied in their reactivity in forming brominated by-products upon incubation. An increase in incubation temperature from 20 to 30$\sp\circ$C produced about a 30% increase in bromoform where as an increase in ozonation temperature resulted in a 20% increase in bromoform. Results of this research indicate that control of by-products can be effected by using PEROXONE (hydrogen peroxide plus ozone) or ammonia. PEROXONE produced 55% less organic by-products at the cost of a 25% increase in bromate. Ammonia addition resulted in a 30% decrease of both organic and inorganic by-products.