When chlorine is added to a water supply containing certain organics, the formation of halogenated organics occurs. Called 'trihalogenated methanes' (THM's) these reaction products are suspected carcinogens and maximum allowable limits in municipal supplies are imposed by the U.S. Environmental Protection Agency (EPA). To reduce the potential for the formation of THM's, many U.S. municipal supplies are converting their chlorine (Cl2) disinfection method to chloramine addition. Chloramines have a low potential to form THM's.
This memo summarizes what chloramines are, their chemical and biocidal characteristics and what is known on how to remove them from supplies.
The effects of chloramines on water chemistry and the equipment/methods used to 'treat' chloraminated streams should be understood as we will encounter this constituent with more frequency. This memo should give you the basics necessary to discuss how Osmonics®' technologies (RO, UP, IX, AC) affect and are affected by chloramines. Many similarities exist between free chlorine and chloramine effects on equipment, and methods of treatment. Yet there exists enough differences to warrant approaching applications from a knowledgeable perspective. The newness of this aspect of water treatment does not, unfortunately, allow all questions to be fully addressed. Indeed, if any conclusion can be drawn it is that too little has been done on how to remove chloramines once they are there (I found only one good article in the volumes of literature I reviewed on the subject, and the two IX experts and one AC expert I contacted knew less than I did!).
WHAT ARE THEY?
Chloramines are biocides. Like Cl2 they are oxidants and kill bacteria by penetrating their cell walls and disrupting their metabolism. Chloramines are, however, much slower to 'react'. They remain as residual ('unused') in water being consumed as they come in contact with bacteria or break down over time as a matter of course. As with chlorine, municipalities aim for I to 2 mg/l (ppm) chloramine residual in the potable water supply.
Several disinfectants ranked by their 'biocidal efficiency' and 'stability' are noted below. (Biocidal efficiency was considered as the effectiveness of the disinfectant against a-number of viruses and bacteria in the pH range of 6 to 9. Stability reflects a lack of reactivity with constituents other than microorganisms and is a measure of persistence in the treated system)
Biocidal Efficiency (best to worst)
ozone > chlorine dioxide > free chlorine > chloramines
Stability (best to worst)
chloramines > chlorine dioxide > free chlorine > ozone
Chloramines are typically generated on-site by the addition of ammonia (NH3) to water containing free C12 (HOCl or OCl depending upon the pH of the water). The optimum reaction pH is on the alkaline side, pH 8.4 (i.e., NH3 (aq) + HOCI « NH2Cl + H2O) Three forms of chloramine can result as well as undesirable but unavoidable interference reactions (#4 below).
Form Name Molecular Weight Predominant
1) NH2Cl Monochloramine 52 >7 best
2) NHCI2 Dichloramine 85 4 to7 some
3) NCl3 Trichloramine 119 1 to 3 mediocre
4) RNHCl Organic Chloramine Varies Unk Insignificant
Organic chloramines cannot be distinguished from the other forms of chloramines with standard methods of chloramine analysis.
Chloramines are not highly disassociated (in other words only minimally ionic). That fact and their low molecular weight make them difficult to remove via RO (discussed in more detail below). The monochloramine form is the best biocide, and as is noted, is the dominant specie at pH 7 and greater. Since slightly alkaline waters are less corrosive, municipalities in many cases maintain the monochloramine form and reduce corrosion potential at the same time. Note that at these alkaline pH's, chlorine exists as the hypochlorite ion (OCl) which has a higher oxidative potential than hypochlorous acid (HOCl), but is 80 to 100 times less effective as a disinfectant.
WHY REMOVE CHLORAMINES?
Like any other molecule, chloramines contribute to the overall total dissolved solids content of the water and like chlorine, are selectively reactive, thus may have deleterious effects on downstream processes. In equilibrium with chloramines are trace amounts of ammonia and/or hypochlorite ions. Their (NH3 and HOCl) presence must also be recognized when one is designing an ultrapure treatment system to remove chloramines.
METHODS OF TREATMENT
Distillation or evaporation does not effectively remove chloramines. During distillation the chloramines would be volatilized and carried over to the product water (distillate). This is especially important to keep in mind in the pharmaceutical, power and laboratory markets due to their heavy use of distillation technology (boilers in the power industry produce steam via evaporation). The effects of reactive chlorinated materials on their products are of special concern.
Chloramine removal by reverse osmosis has not been well documented. Preliminary indications are that HR (CA) membrane will not reject significant percentages of the monochloramine form. Much like chlorine, it will pass through to the permeate side and thus work as a sanitizer on downstream portions of the system. Dichloramine and trichloramine forms would be expected to have greater rejection potential due to their larger mass and higher ionic character, however, precise data is not available.
Even more limited is experience on PA-type RO membranes. Historically very sensitive to oxidants such as chlorine, PA membrane use has been limited to water free of any such disinfectants. However, chloramines have a significantly lower oxidative potential than the hypochlorite ion or hypochlorous acid. PA-type tolerance t9 water containing chloramines would be expected to be much greater compared to chlorine. This would certainly be even more true for newer generation TFC membranes purported to have greater chlorine tolerance.
We are not making any claims for continuous low level disinfection with chloramines on PA RO systems. At this time no PA-type system should be exposed to >0.2 ppm chloramines without R&D approval. The effects of chloramines on PA membrane are, however, of interest to us as they may have potential as a sanitizing agent once the development of a moderately oxidant tolerant PA membrane has been realized. One must keep in mind that in the absence of free ammonia, a minute amount of free chlorine is in equilibrium with chloramines.
Due to tighter pore structure, TFC membranes would be expected to reject a higher percentage of chloramines than cellulosic membranes. Indeed, one report of up 90% rejection of the monochloramine form has been heard of in R&D.
For the moment it appears that RO's utility in removing chloramines is in removing water impurities that would otherwise be competitors or provide interference in downstream ion exchange (IX) or activated carbon (AC) technologies. As we will see, RO's primary utility is removing chloramine breakdown products as a result of AC treatment.
IX resin has a certain affinity for cations and anions. The more highly ionized species (such as sulphates, chlorides, etc.) are preferentially adsorbed to the resin over less strongly charged molecules such as chloramines. With RO as pretreatment, competition for exchange sites would be practically absent. Hence, some chloramines would be removed by 'fresh' strong base IX resin, but this is not a reliable mode of treatment. Another portion of chloramines may decompose via oxidation in an IX system to the chloride ion as happens with Cl. Again, this is not a reliable reaction. Feed water quality and resin characteristics are likely to provide unique performance for each application.
Some degradation via oxidation of the cation resin could also expected. Though not nearly as severe as with free chlorine, life of the resin would be reduced a slight degree. While IX effect some chloramine removal, it has limitations
Activated Carbon (AC) is proven to reduce chloramine presence from 1 to 2 ppm to less than 0.1 ppm (a USP WFI requirement). The mode is similar to free aqueous chlorine destruction, however, with chloramines one encounters 'by-products' of ammonia, chloride and nitrogen gas. Remember that AC does not adsorb C12 or NH2Cl like organics. Bear with me as I present the generally accepted reactions:
1. NH2Cl + H2O + C* => NH3 + Cl- + H+ + CO*
2. 2NH2Cl + CO* => N2 (g) + H2O+ 2H+ + 2Cl¯+ C*
(C* and CO* represent carbon and carbon oxide surface (of activated carbon) respectively)
Note that in the reduction of free aqueous chlorine by AC only H+and Cl¯ ions are generated:
3. C* + HOCI => CO* + H++ Cl¯
4. C* + ¯OCl => CO* + Cl¯
For USP WFI requirements, ammonia nitrogen must also be less than 0.1 ppm in the product water. AC will not remove NH3. At pH 7.5 or lower, both cellulosic and noncellulosic RO membranes would reduce the NH3 and Cl concentrations to less than 0.1 ppm from AC feed waters up to 2 ppm NH2Cl.
Õ Clinoptilolite, a natural-occurring ammonia selective zeolite (resign) was not found to be effective in reducing NH3 levels to USP criteria. Strong base cation would probably be effective in removing NH3, but only RO and distillation are acceptable as the final form of treatment in the production of WFI grade water.
Activated carbon is a viable method to reduce chloramines. The literature notes some important facts in designing AC beds for chloramine removal:
1. Chloramine reacts more rapidly with finer GAC particle sizes (CECA brand 12 x 40 mesh was found significantly better compared to Darco 12 x 40 and Witco 12 x 30 mesh).
2. Two gpm per square foot and 4 foot deep for an empty bed contact time of 15 minutes provides over one year run time with 1-2 ppm chloramine feed with effluent of less than 0.1 ppm.
3. The removal efficiency of GAC is much greater for free chlorine than for chloramines. Therefore, if one can first oxidize chloramines to free chlorine and N2 the GAC bed can be sized smaller because GAC can handle Cl2 much quicker.
Activated carbon followed by RO (or IX and RO, depending on purity required) appears to be the best non-chemical-intensive method to treat chloramines.
Your comments/questions are invited. R&D would appreciate any field experience a data involving chloramines on especially RO. For further information I suggest reviewing the following articles from which I drew much of the above information:
- Water Chemistry by V. Snoeyink, D. Jenkins 1980, Pages 396-399
- Proceedings of the 47th Annual IWC 1986 'Innovative Design for Chloramine Removal...' by Jones et al, Pages 440-448
Journal AWWA - Research and Technology June 1986; 'A Review of Chlorine Dioxide in Drinking Water' by E. M. Aieta and J. D. Berg, Page 70
These are located in the Osmonics library or R&D satellite library.