Chlorate, Perchlorate, and
Bromate in Sodium Hypochlorite
Disinfection System
- A summary of formation, regulations and solutions
By Jeny Shah and Naeem Qureshi (Progressive Consulting Engineers, Inc.)
30 MRWA TODAY Spring 2012
Almost all water systems in the United States that disinfect drinking water use some type of chlorine-based process, either alone or
in combination with other disinfectants. With the prospect of a terrorist attack and the passage of the Public Health Security and
Bioterrorism Response Act of 2002, many utilities have started
using sodium hypochlorite in place of chlorine gas. These utilities
are now finding that they have elevated levels of chlorate, perchlorate and bromate when they had none when they were using chlorine gas.
Proposed federal regulation (i.e. the Drinking Water System
Security Act of 2009, HR 3258) may impose significant burdens
on the continued use of chlorine gas at drinking water utilities.
This development could potentially result in the increase in the
use of hypochlorite for disinfection. This will lead to introduction
of perchlorate into a system that currently does not contain perchlorate. Utilities may be faced with a situation in which they are
required to maintain perchlorate, bromate and chlorate concentration below federal and/or state MCL and at the same time be
under the pressure to switch to sodium hypochlorite. As a result
additional steps to reduce chlorate, bromated and perchlorate will
be required.
Sodium hypochlorite as a disinfectant has the following advantages: it can easily be stored and transported or it can be produced
on-site and the dosage is simple. It is as safe and effective as chlorine gas for disinfection and maintaining chlorine residual in the
distribution system. However, Sodium hypochlorite is a corrosive
substance and when it comes in contact with air it disintegrates.
Also it is not effective against Giardia Lambia and
Cryptosporidium. As previously noted, sodium hypochlorite solutions contain many regulated and unregulated contaminants
including bromate, chlorate and perchlorate (Asami et al, 2009;
Greiner et al, 2008; Weinberg et al, 2003; Gordon et al, 1993).
NSF/ANSI Standard 60 covers the chemicals used in water treatment including the disinfectant chemicals. These standards are
developed to ensure that the treatment chemicals do not add
unsafe levels of chemicals or contaminants to drinking water.
Changes to Standard 60 are constantly under consideration. New
changes that can impact the disinfection process are anticipated to
go into effect in 2013. Although there may be other changes to
Standard 60, those affecting sodium hypochlorite are of great
importance to the water industry since it is the most widely used
disinfectant after chlorine gas. The details about how these contaminants are formed and regulated is summarized in this article.
e
r
SodiumHypochloriteFeedSystem
New Jersey proposed maximum contaminant level (MCL) of 5
ppb. Minnesota does not have a standard for perchlorate at this
time. The USEPA has issued an advisory limit of 15 ppb of chlorate per litre of water. NSF Standard 60 is expected to have a perchlorate limit established by January 2013.
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Water Treatment Plant
City of Saint Peter, Minnesota
Perchlorate
Perchlorate ion is an endocrine-disruption chemical that can affect
the human thyroid system. Perchlorate is a product of decomposition of sodium hypochlorite. The levels of perchlorate increase
with the age of the sodium hypochlorite solution. The perchlorate
can also be found in raw water supplies due to improper disposal
of wastes from the manufacturers of the rocket propellants.
There is no regulation for perchlorate in drinking water at this
time, however, several states have established regulatory limits for
perchlorate in drinking water. Examples of the regulatory limits
are: California 6 parts per billion (ppb); Massachusetts 2 ppb; and,
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Chlorate
Chlorate is formed when the bleach solution of sodium hypochlorite decomposes. Thermal decomposition of bleach is the primary
source of chlorate. The concentration of chlorate increases during
storage as a function of time, temperature, and a suite of chemical
factors (Adam & Gordon, 1998; Gordon et al, 1995). There is no
current federal regulation for chlorate, however, a regulatory standard is being considered for addition to Standard 60 by January
2013. Chlorate is included in the contaminant candidate list and
will probably be included in the unregulated contaminant mandatory rule.
Bromate
Bromate is potent human carcinogen. It results from the oxidation
of bromide via mechanism analogous to that of chlorate (Asami et
al, 2009), although its concentration remains constant once the
available bromide has been converted to bromate (Snyder et al,
2009). The maximum contaminant levels (MCL) standard for bromate in potable water plant effluents is 10 parts per billion (ppb).
Bromate in drinking water comes from two different sources:
1. Hypochlorite manufacturing process –
a. Bulk generation - Bromide ions are found in the salt used
to make chlorine and sodium hydroxide. The two raw materials
react to form sodium hypochlorite. Virtually all of the bromine in
chlorine and the bromide in the sodium hydroxide quickly convert
to bromate at the pH of sodium hypochlorite. As a result the addition of hypochlorite solution to water adds bromate to the finished
water.
32 MRWA TODAY Spring 2012
b. On-site generation – The electrolysis process of the brine
solution used in the on-site generation of sodium hypochlorite
produces hypochlorite that contains bromate. The concentration
of bromide varies tremendously in different salt sources.
2. Ozonation - Bromide ions can be present in both surface water
and ground water supplies. When water containing bromide ions
is exposed to disinfection using the Ozonation process, the reaction of bromide with ozone will produce bromate.
At present the current maximum contaminant level of bromate
allowed in the sodium hypochlorite is 69 parts per million (ppm),
but is expected to reduced to 39 ppm by January 2013.
Solution
A quick solution to keep perchlorate and chlorate levels low is to
quickly turnover the sodium hypochlorite storage. The graph
below shows the effect of age on perchlorate in sodium hypochlorite based on the NSF study of 60 chemicals.
Graph 1: Aging Effect of Sodium Hypochlorite Solution on
Formation of Perchlorate.
4. Use fresh hypochlorite solution when possible. For utilities
using an on-site generation system: use low-bromide salt to minimize the amount of bromide present in the brine.
The conversion of a hypochlorite system to a chlorine gas system
would eliminate the concern since perchlorate and chlorate are not
present in chlorine gas.
References
Adam, L.C. & Gordon, G., 1999. Hypochlorite Decomposition:
Effects of Temperature, Ionic Strength, and Chloride Ion.
Inorganic Chem., 38:6:1299.
Asami, M.; Kosaka, K.; & Kunikane, S., 2009. Bromate, Chlorate,
Chlorite and Perchlorate in Sodium Hypochloroite Solution Used
for Water Supply. Jour. Water Supply Res. & Technol.-Aqua,
58:2:107.
Source: Best Management Practices for Sodium Hypochlorite by
Dave Purkiss, March 23, 2011 – PowerPoint presentation.
Frequent cleaning of the storage tank can prevent old sodium
hypochlorite from remaining in the tank. Temperature reduction
and/or dilution of the solution on receipt from the supplier can
also reduce the formation of chlorate.
An AWWA research team developed a plan to investigate the factors impacting the formation of perchlorate, bromate and other
contaminants in hypochlorite solutions. They then developed a
set of guidelines or recommendations to assist utilities in minimizing the formation of such contaminants. The AWWA report
(Snyder et al., 2009) provides the following recommendations to
reduce or lower the formation of chlorate and perchlorate in the
system:
1. Store hypochlorite solutions at a lower temperature as higher
temperatures speed up the chemical decomposition of hypochlorite and results the subsequent formation of chlorate and perchlorate. Every 5oC reduction in storage temperature will reduce the
rate of perchlorate formation by a factor of approximately 2.
Gordon, G.; Adam, L.C.; & Bubnis, B.P., 1995. Minimizing
Chlorate Ion Formation in Drinking Water When Hypochlorite
Ion is the Chlorination Agent. Jour. AWWA, 87:6:97.
Gordon, G.; Adam L.C.; Bubnis, B.P.; Hoyt, B.; Gillette, S.J.; &
Wilczak, A., 1993. Controlling the Formation of Chlorate Ion in
Liquid Hypochlorite Feedstocks. Jour. AWWA, 85:9:89.
Greiner, P.; McClellan, C.; Bennett, D.; & Ewing, A., 2008.
Occurrence of Perchlorate in Sodium Hypochlorite. Jour. AWWA,
100:11:68.
Snyder, S.A.; Stanford, B.D.; Pisarenko, A.N.; Gilbert, G.; &
Asami, M., 2009. Hypochlorite – An Assessment of Factors That
Influence the Formation of Perchlorate and Other Contaminant.
AWWA
and
Water
Research
Foundation
Rept.
www.awwa.org/files/GovtPublicAffairs/PDF/HypochloriteAssess.
pdf (accessed May 23, 2011).
Weinberg , H.S.; Delcomyn, C.A.; & Unnam, V., 2003. Bromate
in Chlorinated Drinking Waters: Occurrence and Implications for
Future Regulation. Envir. Sci. & Technol., 37:14:3104.
2. Control the pH of stored hypochlorite solutions
at a pH of 11-13 even after dilution.
3. Dilute stored hypochlorite solutions upon delivery since the decomposition of hypochlorite and
subsequent formation of chlorate and perchlorate is
dependent upon hypochlorite concentration and
ionic strength. A higher ionic strength and
hypochlorite concentration will lead to greater production of chlorate and perchlorate. For example:
diluting a 2 molar hypochlorite solution by a factor
of 2, the rate of perchlorate formation decreases by
a factor of 7; a four-fold dilution will decrease the
rate of formation by factor of 36; and a ten-fold
dilution will decrease the rate of perchlorate formation by a factor or 270.
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Spring 2012 MRWA TODAY 33