When chlorine is added to water, it generally forms hypochlorous acid, the powerful killing form of chlorine, and a hypochlorite ion, a relatively weak form of chlorine. The percentage of hypochlorous acid and hypochlorite ions is determined by the pH of the water. As the pH goes up, less of the chlorine is in the killing form and more of the chlorine is in the weaker form . The total of hypochlorous acid and hypochlorite ions is the free available chlorine.
Chlorine can combine with ammonia and nitrogen compounds in the water to form chloramines, also called combined chlorine. By combining with ammonia and nitrogen, free chlorine in the water is disabled. Chloramines are 60 to 80 times less effective than free chlorine and are formed any time ammonia and nitrogen are in the water. Some of the ammonia and nitrogen compounds are introduced into the water by bathers in the form of perspiration, urine, saliva, etc. The average person sweats three pints per hour in a heated spa. Ammonia and nitrogen compounds are also introduced into the water by rain. Each drop of rain has some dissolved nitrogen from the atmosphere.
Not only do chloramines smell bad, but they are eye and skin irritants, and they can cloud the water.
Chloramines can be removed from the water by the following three methods:
- By adding a high dose of chlorine and raising the levels to 10 times the level of combined chlorines (5 to 10 ppm) for a minimum of 4 hours. This is called super chlorination. To remove chloramines, the ratio of chlorine to chloramines must be at least 7.6 to 1. If this ratio is not obtained more chloramines will be produced.
- By adding a non-chlorine shock to the water. The most common chemical used for this is potassium peroxymonosulfate (MPS). This “shocking” requires the addition of 1 oz. per 625 gallons of water.
- By adding ozone to the water. If an ozone generator is installed and wired so that it comes on each time the pump comes on, then oxidation of the ammonia and nitrogen compounds will take place on a continuous basis. This reduces, and can even eliminates the need for shocking. Each time ammonia and nitrogen enter the ozonated water, they are oxidized by the ozone.
Generating activated carbon
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:
NH2Cl + H2O + GAC —> NH3 + HCl
2NH2Cl + GAC —> N2 + 2HCl + H2O The removal process
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.
Making the right choice
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:
|EBCT =||Volume of Activated Carbon in the Vessel|
|Flow Rate of the Water|
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:
Carbon vessel: 10 cubic feet
Flow rate: 8 gpm
|EBCT =||10 ft3||=18.7 minutes|
|4 gpm x .13368|
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.