A New Weapon in the Fight for Clean Water

Recent studies show that activated carbon can effectively reduce bromate in drinking water

Activated carbon is well known for its ability to remove organic compounds from water through a process known as adsorption, remove chlorine and chloramine through various chemical reactions and serve as a general filter media; however, its use for reduction of bromate is unclear. Various authors have studied the use of activated carbon and for the most part have concluded activated carbon is not a cost-effective solution. However, these authors have failed to realize the limitations of carbon validation methods or the fact that not all carbons are alike, especially when chemical reactions control the process.

Regulations and Reactions
Bromate (BrO3-) is a disinfection by-product formed by the reaction of ozone and naturally occurring bromine in drinking water. Although bromate is unlikely to be formed using standard chlorination disinfection, there is some evidence that commercially available sodium hypochlorite solutions may contain bromate as a contaminant1. Bromate is a highly toxic substance that causes irreversible renal failure, deafness and death and has been linked to renal tumors in rats. As such, the American, Canadian and European environmental protection agencies have designated 10 micrograms per liter (m g/L) as the maximum acceptable concentration level (MCL) in drinking water.

The important precursor to bromate formation in drinking water is bromide. In the United States, the average bromide concentration in drinking water is ~100 m g/L. Since bromate is 63 percent bromide, only 6.3 m g/L of bromide needs to be converted to bromate upon ozonation to exceed the MCL. Natural sources of bromine in groundwater are saltwater intrusion and bromide dissolution from sedimentary rocks. Bromine is usually present in drinking water as either hypobromous acid (HOBr-) or hypobromite (OBr-). When exposed to ozonation, the bromide ion is readily oxidized to aqueous bromine. In addition to bromate, aqueous bromine can cause various types of brominated disinfection by-products such as bromoform and brominated haloacetic acids.

In order to understand the formation of aqueous bromate, a corollary understanding of ozone decomposition is needed. Ozone can play a direct (molecular ozone pathway) or indirect (hydroxyl radical pathway) oxidative role in forming by-products. Ozone reacts directly with the bromide ion to form hypobromite and oxygen.

O3 + Br- à O2 + OBr-

Two ozone molecules then react directly with the hypobromite to form bromate and oxygen. Alternatively, the hypobromite can react with multiple hydroxyl radicals created by the destruction of ozone.

2O3 (or OH*) + OBr- à 2O2 + BrO3-

These reactions are generalized and not necessarily balanced, but they give a good overview of the mechanisms at work in bromate formation.

While the 10 m g/L MCL is anticipated to impact a limited number of utilities currently using ozone as the primary disinfectant to inactivate Giardia and viruses, a greater number of utilities will be impacted by this MCL when compliance with the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) is required. Compliance will mean continuance of meeting filtration avoidance criteria of two log cryptosporidium inactivation and overall inactivation requirements (three log giardia, four log viruses and two log crypto) using a minimum of two disinfectants.

Activated Carbon Research for Bromate Reduction
The use of activated carbon has been investigated by various authors for the removal or reduction of bromate.2,3,4 The data to date has been inconsistent and in some cases misleading due to the techniques used to determine the applicability of activated carbon.5,6 It also is apparent that the carbon selection process was overlooked, which has led to generalizations concerning the use of activated carbon for this application.

One paper however has focused on the effect of surface properties on bromate removal.7 The data shows surface properties can and do affect bromate removal performance. Other applications such as chloramine removal in the liquid phase and hydrogen sulfide and sulfur dioxide oxidation in the vapor phase have also been shown to be affected by surface properties. Commercially available activated carbons produced for catalytic properties as well as adsorptive properties do exist, and have been investigated for bromate reduction, but again, the data is misleading due to testing conditions.

Activated Carbon for Bromate Reduction ? Reaction Kinetics
Testing was conducted using a differential reactor to determine the reaction rate for a standard bituminous coal-based granular activated carbon (GAC) and a catalytically enhanced carbon. Typical carbon properties are shown in Table I. Reaction rate data, Figure I, show the reaction follows a first order reaction, and more importantly, data show the reaction rate for the catalytically enhanced carbon is 3.4 times faster than the standard carbon.

Test Procedure

Catalytically Enhanced GAC

(8x30 mesh)

Standard Bituminous GAC

(8x30 mesh)

Mean Particle Diameter (mm) (Used for Testing)

1.44

1.44

Peroxide Number

(Used for Testing)

6.6

53.0

Iodine Number

(Specification)

825 minimum

900 minimum

Table I : Carbon Properties

Figure I.,Figure II.,Figure III. are below:


Analysis of the water confirms the reaction product from bromate destruction is bromide, Figure II. The faster reaction rate for the catalytically enhanced carbon would allow shorter contact time systems to be designed for full-scale use. Experimental design studies show typical properties such as iodine number cannot be used to predict bromate reduction performance, however, catalytic activity as measured by the peroxide number is useful in determining the more applicable carbon.

Activated Carbon -- Real World Application
Differential reactor studies are useful for the determination of reaction rates; however, full scale testing is required to verify the data. Studies published concerning the reduction of bromate have utilized the Rapid Small Scale Column Test (RSSCT) procedure, which was designed for adsorption applications and may not translate well to applications where a different removal mechanism such as oxidation/reduction or ion exchange exists. Data from the literature as well as the differential reactor work conducted for this paper show the reaction to be a reduction of bromate; therefore, the RSSCT column study procedure may not be accurate.

Column studies were conducted using actual particle size carbons and full-scale contact times to verify performance. A column study using a 30-minute contact time and catalytically enhanced 8 x 30 mesh carbon showed bromate could be successfully reduced from an average of 110 parts per billion (ppb) bromate to an average of less than 5 ppb (see Figure III).

In conclusion, these recent studies have provided useful information concerning effective approaches to reducing bromate levels in drinking water such as:

  • Differential reactor studies indicate the bromate reduction reaction to bromide to be first order.
  • Data show standard carbon properties such as iodine number cannot be used to indicate bromate reduction performance.
  • Catalytic activity as measured by the peroxide number does give some indication of bromate reduction performance.
  • Column study data show activated carbon can be utilized to reduce bromate to acceptable levels.

References

  1. American Water Works Association Research Foundation Report #90714, Formation and Control of Brominated Ozone By-Products.
  2. Meijers, R.T. and Kruithof J.C., "Potential Treatment Options for Restriction of Bromate Formation and Bromate Removal," Water Supply, Vol. 13, No.1, Paris, pp.83-189, 1995.
  3. Asami, M, et. al., "Bromate Removal During Transition from New Granular Activated Carbon (GAC) to Biological Activated Carbon (BAC)," Water Research, Vol. 33, No. 12, pp. 2797-2804, 1999.
  4. Kirisits, M.J., Snoeyink, V.L., "Reduction of Bromate in a BAC Filter," Journal AWWA, Vol. 91, No. 8, pp. 74-84, 1999.
  5. Siddiqui, M., et. al., "Bromate Ion Removal by Activated Carbon," Water Research, Vol. 30, No. 7, pp. 1651-1660, 1996.
  6. Siddiqui, M., et. al., "Alternative Strategies from Removing Bromate," Journal AWWA, pp. 81-96, October 1994.
  7. Miller, J., et. al., "The Effect of Granular Activated Carbon Surface Chemistry on Bromate Reduction," Disinfection By-Products Water Treatment, pp. 293-309, (1996) CA 124-241548U.

This article originally appeared in the 07/01/2003 issue of Environmental Protection.

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