Introduction to Water Disinfection and Chlorination
It has been long understood that drinking water could be a source of illness and disease, and humans have been using various means to disinfect water for millennia. However, it was not until the late 1890’s that chlorine and chlorine-containing products were evaluated and demonstrated to be effective disinfectants. Waterborne diseases such as typhoid, dysentery, and cholera occurred with regularity in the 1800’s and into the early 1900’s due to the condition of US water systems. However, with the implementation of chlorination throughout the US, a dramatic drop in illnesses and fatalities occurred (for instance, typhoid deaths in the US dropped from 25,000 in 1900 to less than 20 in 1960).
Chlorine became, and remains to this day, the disinfectant of choice for drinking water because of its effectiveness, efficiency, economy of operation, and convenience, and because of its ability to remain in water as a residual, so that it can continue providing protection throughout the water distribution system. In treatment, the chlorine can be provided by chlorine gas, sodium hypochlorite or chloramines.
The goal of water disinfection is the destruction or inactivation of disease-producing microorganisms. It should be noted that water disinfection as practiced does not intend the complete destruction of all living organisms, or sterilization of the water.
With respect to disinfection, there is a major distinction between water sourced as groundwater/well water and surface water. Disinfection of groundwater is far easier to achieve because water quality changes little over long periods of time, while surface water sees seasonal changes and can see rapid quality changes due to rainfall events, etc. Also, surface water has not benefited from the natural filtering process of water and can contain greater quantity and variety of pathogens, including some resistant to disinfection.
In water, chlorine hydrolyzes to the weak acid hypochlorous acid (HOCl), and it is the HOCl and the hypochlorite ion (ClOˉ) that are the actual oxidizing agents. Free residual chlorine is the combined concentration of HOCl and ClOˉ.
Substitutes for chlorine gas are sodium hypochlorite (NaClO), and calcium hypochlorite (Ca(ClO2)) . These systems provide the same hydrolysis agent (the hypochlorite ion), without the hazards of chlorine gas. However, the use of sodium hypochlorite, or liquid chlorine, is more expensive than chlorine gas.
Chloramine, the combination of chlorine and ammonia, had some limited use historically but has become more popular with the discovery of health problems associated with chlorine disinfection byproducts (such as trihalomethane (THM)) in some water systems. Chloramination produces less disinfection byproducts than free chlorine under most conditions. When chlorine is added to water containing ammonia, the ammonia reacts with HOCl to form chloramines (monochloramine NH2Cl, dichloramine NHCl2); these chloramines have weaker oxidizing capacity compared to the HOCl.
To ensure proper disinfection, a level of free residual chlorine is maintained in the treated water. This is a level of chlorine (actually hypochlorous acid) that remains in water after reaction with the water contaminants. A free residual chlorine level of 0.1 to 0.2 ppm is typically targeted. The EPA requirement for free residual chlorine is that it be detectable; the maximum for chlorine and chloramines is 4 ppm (as Cl2).
Chloramine is often used as a secondary disinfectant to establish a total residual chlorine for the system. The use of chloramine for residual instead of chlorine should reduce the disinfection byproduct formation/concentration in the distribution system.
Bacterial kill by chlorination is influenced by pH, temperature, time of contact, residual chlorine type, and residual chlorine concentration. At pH values of 5 or 6, the kill is more rapid than at higher pH. The rate of kill increases as temperature increases. The kill by free residual chlorine occurs within a matter of minutes while combined residuals (such as chloramines) require 1 to 4 hours to be effective.
For primary disinfection, free chlorine is usually added post‐filtration to inactivate target pathogens at a dose chosen to exceed a specified concentration‐time product before water reaches the first consumers. In secondary disinfection with chlorine, a free chlorine residual concentration is maintained throughout the distribution system, ideally from the initial dosing point until water reaches the system extremities. In addition to countering accidental contamination, secondary disinfection is intended to control bacterial regrowth and the possible harboring of pathogens in the distribution system.
As well as inactivating pathogens, free chlorine reacts with contaminants in the water, particularly dissolved organic matter. Consequently, chlorine concentration continually decreases as the water’s travel time increases, so there can be issues with maintaining chlorine residuals throughout the entire distribution system.
Because the groundwater water source for individual municipal systems typically shows relatively constant levels of impurities, chlorine for groundwater is typically injected on basis of lb/day or lb/ mmGal.
Chlorine (Cl2), in the gaseous state, is greenish-yellow in color and has a pungent odor. It is neither explosive nor flammable, but is a strong oxidant and reacts violently with many substances. Chlorine is stored and shipped as a liquified gas under pressure, typically in cylinders.
Sodium hypochlorite (NaOCL), often referred to as liquid bleach, is available as a solution with 5 to 20% available chlorine. Unlike elemental chlorine, sodium hypochlorite is subject to decomposition. The rate is dependent on conditions such as concentration, temperature and pH, but is approximately 0.5% per day.
Issues with Disinfection Byproducts (DBP’s)
As mentioned above, one issue with the use of chlorine is the formation of disinfection byproducts (DBP’s) that can result from chlorine disinfection. The DBP’s are typically monitored by measuring total organic halogens (TOX) in the treated water. Much of the TBP formation occurs in the distribution system, and is associated with residual chlorine.
Of the identified DBP’s, the two classes of compounds of most concern are the trihalomethanes (THM), and haloacetic acids (HAA). However, a significant fraction of the TOX in drinking water still cannot be accounted for by known specific DBPs.
Some members of these two groups of DBPs (THM’s and HAA’s) are suspected human carcinogens. The EPA’s Stage 2 Disinfectants/Disinfection Byproducts Rule sets the US maximum contaminant level for four THMs (chloroform, bromodichloromethane, dibromochloromethane and bromoform) and five HAAs (monochloro‐, monobromo‐, dichloro‐, dibromo‐, and trichloroacetic acids) at 80 μg/L and 60 μg/L, respectively, on the basis of a locational running annual average (as opposed to a system wide average).
While disinfectant type and residual content are important considerations, DBP formation depends on many variables involved with water quality, the treatment plants and the distribution systems. These factors include reaction/exposure time, the level of organic matter (the dissolved organic carbonDOC); the composition and structure of the organic matter; water temperature and pH; and the concentration of chloride, bromide, and iodide ions. Disinfection conditions have equivocal effects on DBP formation. Important variables include contact time, pH, dosage and residual concentration, and temperature. Therefore, the optimal strategy to minimize DBP formation will require likely require bench and pilot testing and some trial and error.
There are strategies to reduce TOX, THM, and HAA formation. As there is potential for significant DBP formation in the distribution system, practices to optimize chlorine residual levels are important, including the use of booster locations/dosing. Alternate practices, including use of chloramines as a secondary disinfectant to establish residual chlorine, and use of chlorine dioxide for peroxidation, are also increasingly being used.
System Blending and Nitrification
There are potential issues when blending waters from chlorinated systems and systems using chloramines. System blending is relatively common; for instance, a smaller water system may draw on another larger water system to meet peak seasonal demands. As mentioned above, chloramine is often used as a secondary disinfectant to establish a total residual chlorine for the system. However, exposure of such a chloramine system with a system using chlorine (hypochlorous acid) as residual poses issues in chloramine decay and nitrification, which can result in problems maintaining proper chlorine residuals.
Observations indicate that distribution system mixing can be reasonably successful if chloramine flows are limited to about 30 percent of total water entering the system, chloramines entering the system are stable, and chloramines come in contact with moderately strong free‐chlorine residuals. If not, problems will be more pronounced within areas with greater water age; the worst case could be a major nitrification episode.
In nitrification, nitrite and nitrate compounds react with chlorine and chloramines, increasing chlorine demand and decreasing chlorine residuals. The nitrite and nitrate compounds are formed by bacteria reacting with free ammonia. This situation can have regulatory consequences because low or absent disinfectant can promote coliform bacteria growth, which can result in a Total Coliform Rule violation. The following information on nitrification is taken from the EPA’s October 15, 2002 paper entitled “Nitrification”.
Nitrification can have the adverse impacts of increasing nitrite and nitrate levels, reducing alkalinity, pH, dissolved oxygen, and chloramine residuals, and promoting bacterial regrowth. Various potential health impacts have been associated with these issues.
Nitrification is a microbial process by which reduced nitrogen compounds (primarily ammonia) are sequentially oxidized to nitrite and nitrate. Ammonia is present in drinking water through either naturally-occurring processes or through ammonia addition during secondary disinfection to form chloramines. The nitrification process is primarily accomplished by two groups of autotrophic nitrifying bacteria that can build organic molecules using energy obtained from inorganic sources, in this case ammonia or nitrite. Under the Safe Drinking Water Act (SDWA), primary MCLs have been established for nitrite-N, nitrate-N, and the sum of nitrite-N plus nitrate-N.
Environmental conditions that can increase nitrification include temperature (the ideal temperature range for nitrifying bacteria regrowth is 77°‐86°F); pH (nitrification occurs rapidly in the pH range of 8.5‐8.9); and water age.
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