Chloramines

Chloramines
In order to understand the mechanism of chloramine removal, a little background information on the chemistry of chloramines is necessary. Chloramines are formed by the reaction of ammonia and chlorine gas. Chloramines can exist as three chemical species: monochloramine (the predominant species found in tap water), dichloramines and trichloramines. The chloramine species depends upon the pH of the water and the ratios of chlorine to ammonia. At tap water pH levels of 7 to 8.5, the formation of monochloramines is favored. Of the three species, monochloramine is the most stable and difficult to remove, as well as the most damaging to aquatic life.

A “ppm-hour†is defined as the exposure of 1 ppm chlorine/chloramine water for 1 hour.
Film-Tec quotes 300,000 ppm-hours (six years at 1 ppm) of chloramine resistance for their TFC polyamide (PA) membrane material, but only 200 to 1000 ppm-hours of free chlorine resistance. This indicates that chloramines will not damage Film-Tec membranes, while free chlorine levels must be held below 0.1 ppm to prevent oxidation damage. The easiest test for chloramine is with a Total Chlorine Test Kit (SpectraPure Part # TK-CL-10). The TK-CL-10 tests for a combination of free chlorine and chloramines. A sample of the wastewater stream from the RO membrane should show no signs of chlorine.

The most important purpose of a sediment filter is to protect the downstream carbon block filters from plugging with sediment. A properly designed sediment filter will have a micron rating smaller or equal to the closest downstream filter element. It will have a gradient density structure such that the outer layers capture the larger particles and the inner layers capture the finer particles. This will maintain a large dirt holding capacity and prevent the finer particles from plugging downstream carbon filters. Using a 5 micron carbon block followed by a 0.5 micron carbon block, maximum chlorine and volatile chemical removal can be achieved without premature filter failure. If a sediment filter is used that passes particles larger than the next downstream filter, that filter will plug, blinding off the active carbon surfaces, reducing its ability to remove chlorine and organic chemicals.

Trade-offs exist in almost any circumstance and carbon filtration is no exception.
The smaller the micron rating, the better the removal capacity due to greater surface area. Carbon block filters made with bituminous carbon are more effective than coconut shell carbon filters for removal of monochloramine. On the other hand, in water supplies with chlorine only, the coconut shell carbon may have higher capacities for the removal of free chlorine and low molecular weight volatile organic compounds such as trihalomethanes (chloroform).

Multi-carbon block pre-filtration is not always necessary, especially in smaller flow rate systems when adequate pre-filtration and sub-micron carbon block filters are used.
Activated carbon will break the chloramine bond and remove the chlorine component leaving free ammonia (NH3+). RO membranes are transparent to dissolved gases that will pass freely through the membrane concentrating in the RO product water.

Generally, reverse osmosis water is slightly acidic, due to the higher ratio of free CO2 to bicarbonate alkalinity. The exception to this rule is the presence of high pH “soda-lime softening†used by some municipalities. Free CO2 dissolved in water forms carbonic acid that lowers the pH to the range of 5 to 6 pH. In low pH RO product water, the ammonia is converted to the ionized ammonium ion NH4+. Downstream de-ionizing resins can then easily remove this charged species. It is cationic and removed by strong acid cation resins (in the hydrogen form) in either mixed bed or separate bed systems. Aquarists can be certain that when salt is properly added to RO or RO/DI water, the expected salinity and pH will be realized.

Charles Mitsis
President SpectraPure Inc.

 

Excellent info Charles

I edited the title of the thread to include the 'L'.

 

Probably the biggest environmental factor in removal of chloramine is the pH of the tap water. At a pH of 8.3, almost all of the chloramine is in the monochloramine form, which is much harder to remove. As the pH level is lowered, the ratio of dichloramines to monochloramines is increased. Dichloramine is very easy to remove by bituminous activated carbon.

The combination of soda lime softening or sodium hydroxide (to prevent piping corrosion) with chloramines is the worst possible condition. The pH is then often in the 9 to 10 range and at that pH, chloramine is totally converted to monochloramine. The reverse osmosis membrane pores will swell by the combination of high pH and free ammonia. This causes very poor rejection of silica and phosphates, and passage of ammonia through the membrane. Hydroxide ions that are present are also very poorly rejected by the membrane so the pH of the RO product water will be high and ammonia laden, creating additional load to the downstream ion exchange resins.

So,there are almost no universal solutions to water treatment. Environmental factors will weigh heavily in deciding how to best treat those individual water sources.

Charles Mitsis
SpectraPure, Inc.