Chlorine has been traditionally used as a disinfectant in municipal water systems to control bacteria, viruses and other microorganisms.

In 1979, the EPA enacted the Disinfection By-Products Rules as part of the Clean Water Act. These rules set lower limits on the by-products formed when free chlorine reacts with the organics in a water source.

These reacted chlorinated compounds are called trihalomethanes (THMs) and haloacidic acids (HAAs). The primary chlorinated by-product is chloroform, which is a known cancer-causing compound.

The stricter limits on chlorine by-products have forced municipalities to find alternative disinfectants. In the near future many more municipalities will be converting from chlorine to chloramines.

Businesses requiring high purity water should be aware of the treatment methods, equipment and costs of removing chloramines from their municipal water supply.

Chloramines have become a suitable alternative for several reasons:

• Chloramines form only minimal amounts of THMs or HAAs when in contact with organics.
• Chloramines offer advantages as a more stable disinfectant and better resistance to bacterial re-growth.

These are important characteristics in keeping disinfectant residuals in a large distribution system.

However, while chloramine residuals are important in keeping potable water safe, many industrial and medical processes require very pure water and chloramines can cause problems in these situations.

Chloramines contribute to the overall total dissolved solids (TDS). They are selectively reactive and have negative effects on many industrial processes.

Chloramines can foul cation exchange resin in demineralized water systems used in power generation and chemical manufacturing. In addition, they can interfere with cooling and boiler water chemistries, resulting in corrosion.

In kidney dialysis, relatively large volumes of water come in contact with blood across a permeable membrane. Chloramines or any other disinfectant would prove toxic to the patient.

A cost effective technology to degrade and remove chloramines from potable water is activated carbon filtration. Activated carbon removes impurities by transferring the impurity from a liquid phase (water) to the solid phase (carbon).

There are two methods of transfer:

• Physical adsorption; and
• Chemi-sorption.

Carbons also have reduction-oxidation (redox) functionality and act as catalysts to enhance certain chemical reactions. This catalytic functionality is a necessary ingredient in the removal of chloramines by activated carbon.

Physical adsorption holds impurities within the pores of the carbon particle by weak van de Waals forces called London Dispersion Forces. London Forces are similar to gravitational and magnetic forces that pull and trap the impurities on the carbon pore.

Adsorption is the primary method for removing trace amount of organic impurities.

Redox and other chemi-sorption reactions occur on the surface of the activated carbon. Unlike physical adsorption, redox and chemi-sorption change the impurity’s chemical structure.

For example, the result of chemi-sorption on chlorine is its reduction to chloride.

Chloramines are degraded by a combination of redox and chemi-sorption reactions with the activated carbon’s surface oxides chemistries.

To produce activated carbon, raw materials such as bituminous coal, lignite, coconut shell or wood are heated to temperatures of 600-650°Celcius. This removes volatile organics from the raw material, leaving an intermediate carbon product with little porosity.

The intermediate is then reacted with steam at a temperature of 800-1100°Celcius. The high temperature burns off part of the carbon structure and reorders the carbon atoms in a graphitic form.

Steam enhances the development of the graphitic pore structure and forms very reactive oxides on the external surface of the carbon particle. The surface oxides are divided into acidic and basic groups.

Acidic surface oxides enable steam activated carbons to remove chlorine and chloramines and include the carbonyl, carboxyl, phenol and benzoquinone groups.

These surface oxides degrade and remove chloramines by the following reactions:

Chlorine removal and chloramine degradation are chemical reactions with acidic surface oxides. The reaction mechanism involves surface attraction, followed by reduction-oxidation.

Redox reactions involve the transfer of electrons from one atom to another.

In the case of chlorine removal, chlorine is held on the carbon in the form of hypochlorous acid. Hypochlorous acid degrades in the presence of surface oxides on the carbon to form hydrochloric acid and carbon dioxide.

Degradation of chloramines to chloride and ammonia is very similar, only slower.

These redox reactions are external surface area dependent. To increase the reaction rate, surface area can be enlarged by a smaller mesh particle size, more irregular particle shape or larger carbon bed volume.

The rate of this reaction can also be increased with surface enhanced or catalytic carbons.

There is much confusion on choosing the proper activated carbon for chloramines degradation and removal. Below are the main factors to consider:

1. The empty bed contact time using a general-purpose carbon should be at least 15 minutes (2 gpm per cubic foot) and the superficial flow rate should not exceed 2 gpm per square foot.

The empty bed contact time (EBCT) measures contact between carbon particles and water as the water flows through the vessel. Fifteen minutes is the minimum EBCT required to maximize the carbon capability to remove chloramines.

EBCT is calculated from the following:

For this equation to result in minutes, the flow rate typically expressed in gallons per minute (gpm) must be converted to units such as cubic feet/ minutes (ft3/min). The conversion is 1gpm = .13368 ft3/min.

Below is an example calculation of EBCT:

2. If pressure drop across the vessel is not a limiting factor, use a finer mesh, change from an 8 x 30 to 12 x 40 or from a 12 x 40 to 20 x 50. Consider upflow operation of the vessels instead of downflow to enable smaller mesh size.

The activated carbon’s internal pore structure has only a minimal effect on chlorine removal or chloramine degradation. Chloramines are larger molecules and their degradation on activated carbon is slow.

Activated carbons with a slightly larger pore size distribution are best for this application because chloramines are held on the carbon surface for a longer period of time, compared to very microporous carbons.

A coal-based carbon with a 900-iodine number would be the best suited and most economical.

In treatment systems that have limited contact time, a catalytic carbon will provide effective chloramines degradation and removal.

The chemi-sorption potential or reactivity of the carbon surface is dependent on two factors:

• Oxygen content of the starting raw material
• Steam concentration during the activation process.

Raw materials with high oxygen contents would have greater surface reactivity. However, the activated carbons produced from these raw materials would be structurally weak, extremely dusty and could not withstand handling.

Surfaced-enhanced activated carbon or catalytic carbon is also available. Catalytic carbons have been manufactured in environments that increase formation of surface oxides or have been impregnated with metal oxides.

Enhancing the reduction-oxidation potential by surface treatments gives the activated carbon greater selectivity, capacity and reaction kinetics for chloramines degradation and removal. Catalytic carbon allows for smaller carbon volumes, smaller adsorption vessels and slightly greater removal capacity.

Figure 1 is a comparison between catalytic and general-purpose carbon.

Depending on the application and operating parameters, catalytic carbons can provide an effective and cost efficient choice for chloramine removal.

Phil Adams is a technical representative with ResinTech Inc., West Berlin, NJ.