Controlling Disinfection By-Products: Trihalomethane Formation

DAW Controlling Disinfection By-Products.pdf

Trihalomethane Formation

Trihalomethanes (THM) are common by-products formed when water containing organic matter, usually natural organic matter (NOM), is disinfected with free chlorine. Considered to be a long-term health hazard, allowable THM levels are limited by federal and state drinking water regulations.   The formation of THM is dependent on several factors, such as temperature, time of reaction, water pH, and nature and concentration of organic matter present.

Options for Controlling
Disinfection By-Products

There are several potential options for controlling the excessive formation of THM.  These options are explained in several water industry publications, one of which is the Environmental Protection Agency publication entitled Controlling Disinfection By-Products and Microbial Contaminants in Drinking Water.  The following list of possible control methods was compiled from such publications.

1. Source water selection.  This method involves choosing a high-quality source water that is low in naturally occurring organic matter (NOM), which reacts with certain disinfectants to form THM.

2. Removal of NOM from source water.  If the raw water has significant NOM present, sometimes treatment for removal of the NOM can be effective in minimizing the potential for forming THM.  This approach is part of the basis for drinking water regulations for surface waters and influenced ground waters requiring enhanced coagulation.

3. Modifying the location of chlorine application.  Eliminating pre-chlorination of raw or partially treated water with high NOM levels has been found to be useful in avoiding THM formation.  This modification allows for NOM removal to occur in the treatment process before chlorine is introduced into the flow stream.

4. Use of alternative disinfectants.  Application of free chlorine for disinfection is a major factor in the formation of THM when NOM is present.  Other disinfectants, such as chlorine dioxide, ozone, and chloramine have been used to varying degrees of success in achieving necessary disinfection without creating excessive THM concentrations.

5. Minimize reaction time for the formation of THM.  The reaction of free chlorine and NOM to form THM is dependent on temperature and holding time.  In an existing system, there is little one can do to significantly modify either water temperatures or water age in the distribution system.

Chloramination as a Strategy for
Controlling Disinfection By-products

Chloramination is a widely accepted practice for controlling disinfection by-products.  Chloramine generally does not disinfect as rapidly as free chlorine; however, while free chlorine readily reacts with NOM to form THM, chloramines do not. 

Chloramines are created when ammonia is contacted with free chlorine.  The ammonia may be naturally occurring or may be added in the treatment process.  The reaction is quite rapid, occurring in a matter of just seconds.

In surface water plants and plants treating ‘influenced’ groundwater, use of chloramination as the primary disinfection method is not generally feasible because of the significantly longer contact times required to meet drinking water rules.  In such cases, though, it is common to use free chlorine as the primary disinfectant with sufficient contact time to provide good microbial kill.  The free chlorine disinfection is then followed by dosing with ammonia to form chloramines as the secondary disinfectant that carries out into the distribution system for combined chlorine residual maintenance.

For groundwater systems containing naturally occurring ammonia in the raw water, the control of the proper ammonia dosage can be somewhat complicated. One strategy is to closely monitor the naturally occurring ammonia, and add supplemental ammonia to reach the optimum level (usually a chlorine/ammonia ratio of 3:1 to 5:1) needed to react with the desired chlorine dose to achieve a targeted residual chloramine concentration.  If the naturally-occurring ammonia concentration is too high, the resulting residual disinfectant level will be higher than acceptable. Reducing the chlorine dose would result in sending water containing free ammonia out into the distribution system, with the attendant water quality issues related to nitrification in the distribution system.

To simplify process monitoring and control, another strategy is to “breakpoint” chlorinate first, adding enough chlorine to convert the naturally-occurring ammonia to chloramines and then to destroy the resulting various species of chloramines, leaving free chlorine as a residual.  Then in a subsequent step more ammonia can be added in the correct proportion to form the desired combined chlorine residual (mostly monochloramine). 

Ammonia may be purchased in several forms – anhydrous ammonia, aqua ammonia, ammonium chloride, or ammonium sulfate.  Because of problems with some of the other forms creating safety or room-environment problems, use of ammonium sulfate is often preferred.  The chemical can be purchased as a premixed solution or as a dry material for mixing into solution at the treatment plant. 

Special Attention Needed When
Changing to Chloramination

When a water system chooses to adopt chloramination as a disinfection by-products control strategy, there are several aspects that must be given special attention, including the following:

 Dialysis patients and aquarium owners must be notified, since chloramines in water used by these groups of people can be detrimental; they will need to take measures to remove or counteract the chloramines prior to use.

 Biological nitrification in the distribution system must be monitored and controlled or else nitrite concentrations could become elevated to levels that would violate drinking water standards.

 Disinfection by-products, such as nitrosamines, may be formed in waters treated with chloramines.  Presently these other by-products are not regulated, but research on health effects and technical developments may result in future regulation of maximum allowable levels.

 Some water customers may notice a different taste imparted to the water when switching from chlorine to chloramine disinfection.  Many people prefer the taste of chloraminated water over chlorinated water, but taste can be a very subjective matter affecting customer satisfaction.

As explained in this brief article, chloramination is but one of several strategies for dealing with disinfection by-products.  Each water system is unique in its requirements and constraints, and a thorough understanding of the particular system is necessary to develop and implement the best solution for a community.

Contact:
FOX Engineering Associates, Inc.
Dale A Watson, Ph.D., P.E., BCEE
800.433.3469   www.foxeng.com