1.4: Chloramines
Last updated
Last updated
The possible presence of microbial contaminants in drinking water poses a greater risk to public health than the possible presence of disinfection by-products (DBP). Therefore, disinfection should not be compromised in order to control DBP.
Where the concentrations of DBP consistently exceed associated health-based guideline values, the methods of water treatment, disinfection and distribution should be reviewed.
Chloramines are formed when chlorine and ammonia are added to water:
There are three types of chloramines; monochloramine, dichloramine and trichloramine, with production of the three types dependent on pH and the ratio of chlorine to ammonia. Monochloramine is the preferred choice because it is the most stable and produces the lowest tastes and odours. Dichloramine is a stronger disinfectant than monochloramine, but is less stable and produces distinctive odours. Trichloramine is the least stable and produces offensive odours. Formation and stability of monochloramine is favoured at ratios of 3 to 5 (with 4 typically used) and a pH above 8 (UWRAA 1990, USEPA 1999, Keegan et al. 2012).
The primary reason for using monochloramine rather than chlorine is its much greater persistence. In Australia chloramination has provided persistent residuals through very long drinking water pipelines (>100kms) and provides protection against growth of free-living organisms such as Naegleria fowleri (UWRAA 1990).
Chloramination is used in drinking water supplies where persistence is a key advantage and can be particularly useful in long or complex distribution systems that have extended detention times. It has been particularly effective in reducing the occurrence of Naegleria fowleri in systems incorporating long above ground pipelines, and has also been shown to reduce the occurrence of Legionella in buildings (Flannery et al. 2006).
In conjunction with other disinfectants, such as chlorine or ultraviolet (UV) light, chloramines can also be used as a secondary (or booster) disinfectant to provide persistent residuals within the distribution system.
Where chloramination is used as the primary disinfectant, due to the relatively high C.t values required to inactivate microbial pathogens, it is important to determine the minimum C.t values before the first customer in the drinking water distribution system. This is often done by calculating minimum C.t values at maximum flow rates.
An emerging practice is to dose chlorine followed by ammonia separately, at different points within the water treatment process. The advantage of this practice is that achieves better upfront inactivation of microbial pathogens, whilst still delivering a longer-lasting residual within the distribution system. To maximise the benefits of the practice there needs to sufficient contact time for the chlorine to achieve inactivation, prior to the addition of the ammonia.
Chloramines are formed by the addition of either liquefied anhydrous ammonia or aqueous ammonia (UWRAA 1990, USEPA 1999), either before or after chlorine dosing. The addition of ammonia first reduces the production of chlorinated disinfection by-products, but it also reduces initial inactivation by reducing the contact time between the free chlorine and the treated source water.
Chloramination has a long history of use and was introduced in Brisbane in 1935. Robust and reliable dosing and monitoring equipment is available. Reliable field kits for measuring residuals within the distribution system are also available; these kits generally measure concentrations of chloramines as total chlorine. There have been reports of false free chlorine readings with tablet-based methods (UWRAA 1990). The DPD-Ferrous titrimetric method is less prone to false readings (see monochloramine Fact Sheet).
Table IS1.4.1 presents published C.t values for preformed monochloramine (that is, monochloramine that is formed off-site) that have been demonstrated as achieving a two and four log reduction in the target microorganism. These values are supplied for illustrative purposes only. For chloramine C.t values that achieve a greater log reduction, the cited references should be consulted. The C.t value that is applied at a particular water treatment plant should be based on the microbial risk assessment for that particular water supply system.
Escherichia coli
180 (5°C)
360 (≥5°C)
LeChevallier and Au 2004
Adenovirus 2
1690 (10°C)
≥3100 (10°C)
Keegan et al. 2012
Giardia
1470 (5°C)
2940 (5°C)
USEPA 1999
Naegleria fowleri
320 (30°C)
NA
Robinson and Christy 1984
Notes:
pH is within range of 8-9 for E. coli, pH 7 for Adenovirus 2 and Naegleria fowleri, pH 6-9 for Giardia.
The values in the table are based on published values and should be viewed as the minimum values necessary to achieve effective disinfection.
The important conclusion to draw from Table IS1.4.1 is that chloramines require a much higher C.t value than chlorine to inactivate microorganisms. Therefore, for distribution systems where chloramine is going to be used, the need for much greater C.t values will be an important consideration (i.e. water needs to be held in the pipe network and/or tanks for longer to allow sufficient time for effective disinfection).
The influence of pH on the effectiveness of disinfection appears to be variable (UWRAA 1990, USEPA 1999, Cromeans et al. 2010, Keegan et al. 2012) and could depend on the target microorganism. Varying the pH will influence the species of chloramine present and this could impact on the effectiveness of disinfection. The target range for pH during chloramination is usually 8.5 ± 0.2. Monochloramine stability improves with increased pH, with optimum stability occurring at pH 9. There is no benefit from chloramination at a pH greater than 9 (UWRAA 1990).
As for chlorine, C.t requirements are reduced at higher temperatures (USEPA 1999, Kahler et al. 2011). Monochloramine appears to remain effective at high turbidity but inactivation rates decrease as turbidity increases (UWRAA 1990). This finding was confirmed by more recent work by Keegan et al. (2012), using water that varied from 2 to 20 NTU.
Persistence is the principal advantage of chloramination, and it has been used successfully to produce disinfectant residuals through long pipeline systems exceeding 100km in length.
Persistence can be reduced by nitrification, particularly at the ends of distribution systems. Nitrification is caused by bacterial oxidation of ammonia to nitrite, and nitrite to nitrate (Cunliffe 1991). Nitrifying bacteria are naturally-occurring sediment and biofilm organisms. Nitrification can accelerate chloramine decay, and at the ends of distribution systems can lead to complete loss of residual and the replacement of chloramines with oxidised nitrogen (nitrate and nitrite). A number of factors have been associated with nitrification including detention times, excess ammonia (low chlorine:ammonia ratios) and low chloramine residual. The common practice to reduce the likelihood of nitrification is maintaining minimum chloramine residuals of 1.5-2 mg/L (Cunliffe 1991, USEPA 1999).
Work in South Australia indicates that the risk of nitrification can be mitigated by ensuring that the concentration of free ammonia after chloramination does not exceed 0.3 mg/L. Additionally, the concentrations of nitrate and nitrite can be monitored within the distribution network to given an early indication of the onset of nitrification.
The replacement of chlorination with chloramination improves the microbiological quality of the water in large distribution systems (UWRAA 1990). The general reason for this is that biocidal residuals of monochloramine travel further into the system, providing effective disinfection. If nitrification occurs within the system, then a return to chlorination for about a week may be necessary in order to control nitrifying bacteria.
Chloramination generally reduces chlorinated DBP, including trihalomethanes (THMs), but does not eliminate them. A similar range of DBP will be produced as when chlorine is used, particularly when chlorine is added before the ammonia (WHO 2000). Dichloroacetic acid can be formed from monochloramine and cyanogen chloride formation is greater than with free chlorine (USEPA 1999).
In addition, chloramination can be associated with the production of nitrosamines, including NDMA (see NDMA Fact Sheet). Factors that influence the formation of NDMA include the chloramine dose, the concentrations and types of organic nitrogen-containing compounds that are present, pH and detention time (WQRA 2013).
Health-based guideline values have been developed for a number of DBP (see Chapter 10 and associated Fact Sheets). Whilst every effort should be taken to minimise the formation and concentration of DBP this should never be done in a manner that compromises disinfection as poor microbiological quality represents a greater and more immediate risk to human health than short term exposure to DBP (Hrudey 2009).
Given that the chloramination process will be a critical control point (CCP), other important issues that will need to be considered to ensure the effectiveness of the process are:
establishing target criteria and critical limits for the chloramination process (section 3.4.2);
preparing and implementing operational procedures (section 3.4.1) and operational monitoring (section 3.4.2) for the process;
preparing corrective action procedures (section 3.4.3) in the event that there are excursions in the operational parameters; and
undertaking employee training (section 3.7.2) to ensure that the chloramination process operates to the established target criteria and critical limits.
The table below summarises the operational monitoring that should be undertaken for chloramine.
pH
Online monitoring
Turbidity
Online monitoring
Chlorine-to-ammonia ratio
Calculated
Contact time
Calculated
Chlorine residual (total)
Online monitoring
Chloramine concentration
Field kit
Cunliffe DA. (1991). Bacterial nitrification in chloraminated water supplies Applied and Environ. Micro 57: 3399-3402.
Cromeans TL, Kahler AM, and Hill VR. (2010). Inactivation of Adenoviruses, Enteroviruses, and Murine Norovirus in Water by Free Chlorine and Monochloramine. Applied and Environ. Micro 76: 1028-1033.
Flannery B, Gelling LB, Vugia DJ, Weintraub JM, Salerno JJ, Conroy MJ, Stevens VA, Rose CE, Moore MR, Fields BS, and Besser RE. (2006). Reducing Legionella colonization of water systems with monochloramine. Emerging Infectious Diseases 12: 588-596.
Hrudey S. (2009). Chlorination disinfection by-products, public health risk tradeoffs and me. Water Research 43: 2057-2092.
Kahler AM, Cromeans TL, Roberts JM, Hill VR. (2011). Source water quality effects on monochloramine inactivation of adenovirus, coxsackievirus, echovirus, and murine norovirus. Water Research 45:1745-51.
Korich DG, Mead JR, Madore MS, Sinclair NA and Sterling CR (1990). Effects of ozone, chlorine dioxide,chlorine, and monochloramine on Cryptosporidium parvum oocyst viability. Applied and Environ Micro.56, 1423-1428.
Keegan A, Wati, S, Robinson B. (2012). Chlor(am)ine disinfection of human pathogenic viruses in recycled waters. Smart Water Fund Project SWF62M-2114, Smart Water Fund, Melbourne, Australia.
LeChevallier MW and Au K-K. (2004). Water treatment and pathogen control. World Health Organization,Geneva, Switzerland.
Robinson BS and Christy PE. (1984). Disinfection of water for control of amoebae. Water 11: 21-24.
United States Environmental Protection Agency (USEPA) (1999). Alternative disinfectants and oxidants guidance manual. Washington DC.
Urban Water Research Association of Australia (UWRAA) (1990) Chloramination of water supplies. UWRAA, Melbourne Australia.
Water Quality Research Australia (WQRA) (2013). Guidance Manual for the Minimisation of NDMA and other Nitrosamines in Drinking and Recycled Water. Research Project 1018, Water Quality Research Australia Limited, Adelaide, Australia.
World Health Organization (WHO) (2000). Disinfectants and disinfection by-products. Environmental Health Criteria 216. WHO, Geneva.