A Viable Option for Water Utilities
Over the past 20 years, many water utilities have turned away from chlorination and toward chloramination for the disinfection of their water supplies. Thanks to new ammonia feed systems featuring more efficient and less costly technologies, that trend is likely to continue.
All drinking water suppliers using surface water are required by the U.S. EPA to use disinfectants to eliminate pathogenic microorganisms in drinking water supplies. Chlorination has played a critical role in protecting the U.S. drinking water supply from waterborne infectious diseases for nearly 100 years.
One of the first known uses of chlorine for water disinfection was by John Snow in 1850, when he attempted to disinfect the Broad Street Pump water supply in London after an outbreak of cholera.
In 1897, Sims Woodhead used “bleach solution” as a temporary measure to sterilize potable water distribution mains in Maidstone, Kent, England following a typhoid outbreak.
Continuous chlorination of drinking water began in the early years of the last century in Great Britain, where its application sharply reduced typhoid deaths. Shortly after this dramatic success, chlorine use spread to Jersey City, N.J., in 1908. Adoption by other cities and towns across the U.S. soon followed and resulted in the virtual elimination of waterborne diseases such as cholera, typhoid, dysentery and hepatitis A.
Today, chlorine-based chemicals are the disinfectants of choice for more than 95% of systems that treat water.
Chlorine’s most important attributes are its broad-spectrum germicidal potency and persistence in water distribution systems, providing residual protection against microbial regrowth. It also is used to control taste and odor problems by oxidizing many naturally occurring substances such as algae secretions, decaying vegetation, hydrogen sulfide and ammonia.
Increased use of chloramines
In the late 1970s and early 1980s, it was discovered that when some of the components of natural organic matter in water come in contact with chlorine, they form low concentrations of trihalomethanes (THMs) and other disinfection byproducts (DBPs), including haloacetic acids (HAAs).
It is suspected that exposure to THMs at high concentrations over a lifetime may statistically increase the rates of some cancers. Because of this finding, the EPA began regulating THMs in 1979, with a maximum contaminant level (MCL) of 100 ug/L (100 ppb). The MCL has since been reduced to 80 ug/L, and further reductions are expected.
Chloramines, a combination of chlorine and ammonia, also have been used for drinking water disinfection since the early 1900s. When it was discovered that THMs and other DBPs were forming in chlorinated water, chloramines increased in usage and were found to reduce the formation of these potentially carcinogenic THMs.
Chloramines are the result of combining chlorine and ammonia at a weight ratio of approximately 5:1. Monochloramine is the dominant compound formed and is generally considered to be a suitable “residual” disinfectant, appropriate for maintaining an effective disinfectant level in the distribution system. Such a residual effect is an advantage that both chloramines and chlorine share over chlorine dioxide, ozone and UV, which can be used only for primary disinfection at a treatment plant because they provide limited or no residual disinfectant.
The use of chloramination in the pretreatment of surface water greatly reduces the production of THMs and HAAs. In this application, the chlorine will readily combine with the free ammonia before uniting with the organic and inorganic THM and HAA precursors.
The benefits of chloramines used as a residual disinfectant for the distribution system are:
- Persistence and ability to reach remote areas in the distribution system;
- Effectiveness as a residual disinfectant and ability to penetrate biofilms in the distribution system;
- Tendency to form lower levels of THMs and HAAs; and
- Ability to minimize chlorinous or other objectionable tastes and odors.
Chloramines are more stable and persist longer in the distribution system because they are less reactive than free chlorine. The water agencies that have converted to chloramines report that customers note an improvement in the taste and odor of the water.
Inorganic chloramines may consist of up to three chemicals that are formed when chlorine and ammonia are combined in water: monochloramine (NH2Cl), dichloramine (NHCl2) and trichloramine (NCl3). Inorganic chloramines, free chlorine and organic chloramines are chemically related.
When chlorination of freshwater occurs in the presence of ammonia, NH2Cl usually forms. Formation of NHCl2 is discouraged by optimizing ratios of chlorine to ammonia. Conditions favoring the formation of NCl3 in drinking water are rare. In general, almost all chloramines are NH2Cl with insignificant amounts of NHCl2 and NCl3 under conditions of water treatment and distribution. Organic chloramines may also be produced if certain organic nitrogen compounds, including amino acids and nitrogen heterocyclic aromatics, are present.
Advantages of pressure feed systems
In the chloramination process, two different methods can be used to feed gaseous ammonia from cylinders or bulk storage supply containers directly into a water or wastewater treatment process: vacuum feed and pressure feed. Vacuum feed systems traditionally have been required for larger water distribution systems because of their ability to handle greater quantities of ammonia.
Increasingly, however, utilities are turning to pressure feed systems for various reasons.
New system designs. New positive pressure ammonia feed systems designed with more rugged, all stainless steel construction can feed larger quantities of ammonia than previous pressure feed systems.
Reduced clogging. In areas where hard water is prevalent, mineral deposits can form in the solution distribution system when vacuum feed systems are used, causing clogging of the transport system.
Lower costs. Pressure feed systems do not utilize the ejectors, solution lines and booster pumps that are required with vacuum feed systems. Eliminating the need for this equipment reduces capital costs, attendant maintenance and repair costs. Operating costs with pressure feed systems also are considerably lower than with vacuum feed systems.
Increased backpressure capabilities in some of today’s pressure feed systems make this technology even more attractive in ammonia feed applications. For example, a positive pressure feeder from Severn Trent Services has a pressure rating of 25 psi. Constructed of stainless steel, the system was designed to feed capacities of ammonia gas ranging from 3.5 to 2,000 ppd. The system features a self-cleaning pipeline-mounted diffuser that is designed to flex under variations in pressure, effectively breaking up the mineral deposit buildup that can obstruct the flow of ammonia gas.
Just as chloramination is gaining acceptance as an alternative to chlorination for the disinfection of water supplies, pressure feed systems are being considered as replacements for vacuum feed systems in the chloramination process. While the switch to a pressure feed system represents a capital expense, the system’s operating costs, reduced clogging due to mineral deposits, and long-term ROI make the switch a viable option for many water utilities.