1) Chloramines
These are very stable antiseptics
that act more slowly than chlorine but remain active for longer
in water. They are generally prepared from chlorine and ammonia
(one-quarter to one-half as much ammonia as chlorine) or ammoniac
salts. These chlorine compounds are not very widely used at
the present time.
2) Chlorine Dioxide (ClO2)
This is a yellow-green gas that
is highly soluble in water and has a characteristic chlorine
type odor. Much like ozone, it cannot be compressed and stored.
It is generated at the point of use. Chlorine Dioxide is an
extremely powerful oxidizing agent and broad spectrum microbiocide,
making it an ideal primary disinfectant for water treatment.
In addition to its disinfection applications chlorine dioxide
is also used for the destruction of taste and odor compounds
while avoiding the formation of trihalomethanes (THM) and
other chlorinated organic.
The keys to the cost-effective
use of chlorine dioxide is its efficient and safe conversion
from the precursor chemicals, the proper placement of the
generator at the most appropriate point (s) in the water treatment
sequence and the use of the lowest treatment level possible
to achieve the desired result. in many cases chlorine dioxide
can be integrated into existing systems to optimizes costs
while improving disinfection and reducing or eliminating undesirable
contaminants in the water.
In a concentration of more than
10% by volume in air it is explosive, but it is quite harmless
in solution in water. It is a highly effective oxidizing agent
with powerful deodorizing and bleaching properties. Its action
on pathogenic substances is at least equal to that of chlorine.
(1) Advantages over Chlorine
- Oxidizes humic substances and
other THM precursors via direct oxidation rather than substitution
reactions characteristic of chlorine. Decreased TOX and
AOX formation when chlorine dioxide is used as a pre-oxidant.
- Does not react with ammonia
or primary amines to form chloramines.
- Unlike chlorine gas, does not
react with water to form hypochlorous and hydrochloric acid.
Less corrosive than chlorine solution.
- Destroys phenolics, chlorophenolics,
sulfides, cyanides, nitrites and other problem contaminants
either present in source waters or formed by pre-chlorination
treatment.
(2) Chemistry of Chlorine Dioxide
The reactions that generate chlorine
dioxide from chlorine and sodium chlorite are:
- In the process of Sodium Chlorite
(2NaClO2) + Chlorine Gas (Cl2) --->
Chlorine Dioxide Gas (2ClO2) + Sodium Chloride
(2NaCl), sodium chlorite is reacted with molecular chlorine
gas prior to its dissolution in water. This reaction occurs
safety under vacuum and offers many benefits over conventional
process.
- In the conventional process
of Cl2 (gas) + H2O ---> HOCl +
H + Cl and 2NaClO2 + HOCl ---> 2ClO2
(gas) + NaCl + NaOH, chlorine gas must be pre-dissolved
in water, with the chlorine rapidly hydrolyzing to form
a mixture of hypochlorous acid (HOCl). This mixture is then
reacted with sodium chlorite (NaClO2) to provide
chlorine dioxide (ClO2), but often in very low
yields.
The pre-dissolution of chlorine
in water and further dissolution of hypochlorous acid produces
hypochlorite ion and other undesirable side reactions which
leads to low yields of chlorine dioxide.
3) Sodium Hypochlorite (NaOCl)
These solutions commonly known
as Javelle Water or Bleach are characterized by their active
chlorine content.
(1) General
In recent year (1970-80) the stress
on safety and fear of a chlorine accident has caused large
metropolitan areas to consider the use of hypochlorite rather
than chlorine gas systems where large amounts of the liquid-gas
chlorine is stored in either stationary tanks or ton containers.
This has occurred in spite of the good safety record of such
installations. Since 1908, when chlorine gas was first used
in the United States, there has been only one fatality from
a chlorine accident at a water or wastewater installation
in the United States. There have been 9 transportation-related
fatalities resulting from massive derailment of tank cars.
Despite this record and the considerable
additional cost of hypochlorite over chlorine gas (two - four
times) and its inherent unwidely and cumbersome handling problems,
some of wastewater treatment plants in San Francisco where
changed from gas to hypochlorite; This move prompted three
of largest wastewater treatment plants to evaluate the situation
of chlorine gas versus hypochlorite. After separate and independent
investigations these plants decided not to change hypochlorite
for the following reasons. (a) the reliability and safety
procedures of the chlorine storage system were satisfactory;
(b) the amount of chlorine delivered to these plants was less
than 10 percent of the total amount of chlorine moving into
the area; and (c) the cost of hypochlorite was too great compared
to chlorine.
In the last decade (1970-80) some
of the large users have switched back to chlorine gas. After
several years of trial some power plants have given up hypochlorite
because of the inherent difficulties in handling it in large
amounts. Others have done so to save money.
(2) Chemistry of Hypochlorite
The application of hypochlorite
achieves the same result as does that of chlorine. The active
ingredient is the hypochlorite ion (OCl-), which
hydrolyzes to form hypochlorous acid. The only difference
between the reactions of the hypochlorites and chlorine gas
is the side reaction of the end products. The reaction with
the hypochlorites increases the hydroxyl ions by the formation
of sodium hydroxide; the reaction with chlorine gas and water
increases the H+ ion concentration by the formation of hydrochloric
acid. There is reason to speculate that a chlorine gas solution
at pH 2 to 3 will always be somewhat more effective than a
solution of hypochlorite at pH 11 to 12 at the immediate area
of the point of application, simply because there is more
of the active ingredient HOCl and possibly some extremely
active molecular chlorine on account of the low pH of the
chlorine gas solution. It is a well-known fact that at pH
11 to 12 the HOCl is almost completely dissociated to the
ineffective hypochlorite ion as follows:
HOCl <---> H+
+ OCl-
This high pH condition will exist
only momentarily at the interfaces of the hypochlorite solution
and the water to be treated.
(3) Stability of Solutions
Sodium hypochlorite solutions are
vulnerable to a significant loss of available chlorine in
a few days. This is a major problem with this type of chlorination
system. The user must dedicate laboratory time to monitoring
the decay rate in available chlorine. This serves two purposes:
(a) it establishes an understanding with the supplier to arrive
at the optimum cost for a given trade strength of solution
and (b) it will establish the most cost-effective quantity
per delivery and frequency of delivery to minimize loss of
chlorine in the stored hypochlorite solution.
The stability of hypochlorite solutions
is greatly affected by heat, light, pH, and the presence of
heavy metal cations. These solutions will deteriorate at various
rates depending upon the following factors.
- The higher the concentration
the more rapid the deterioration.
- The higher the temperature the
faster the rate of deterioration.
- The presence of iron, copper,
nickel, and cobalt catalyzes the rate of deterioration of
hypochlorite.
Iron is the worst offender. In
minute quantities it causes rapid deterioration of these solutions.
The source of iron is usually the caustic used in the making
of these solutions. Iron in quantities as low as 0.5 mg/l
will cause rapid deterioration of a 15% solution in a few
days.
Copper should be kept as low as
possible not in excess of 1 mg/l in the finished solution.
It is generally present because of the copper flexible connections
and brass body chlorine line valves used in the chlorine supply
system. Great care must be taken by the product to prevent,
insofar as is possible, active corrosion of these parts. This
can be done by keeping them internally free of chlorine. This
is a different task.
The most stable solutions are those
of low hypochlorite concentration (10%), with a pH 11 and
iron, copper, and nickel content less than 0.5 mg/l, stored
in the dark at a temperature about 70oF.
(4) Hypochlorite Quantities Required
To get an idea of quantities involved,
let us examine the chlorine requirement for disinfection of
a secondary treated effluent discharging into a receiving
water. Proper disinfection to maintain the receiving waters
safe for water contact sports is usually about 100-125 lb
chlorine per million gallons of treated effluent.
Using a high-strength sodium hypochlorite
of 10 percent by weight chlorine would require the following
amount of sodium hypochlorite:
% available Cl2 by weight
= 10% / (Specific Gravity = 1.14) = 8.8%
Each gallon NaOCl contains 9.5
x 8.5% = 0.84 lb of chlorine. If the dosage is 100 lb/mg,
then 100/0.84 = 119 gallon of 10% NaOCl/24 hr. Assuming peak
rate of 2-1/2 times average = 500 mgd x 119 = 59,500 gpd or
59,500/1,440 = 41 gpm. So the metering equipment should be
sized to handle a maximum of 50 gpm of 10% hypochlorite solution.
Comparing the half-lives of various
strength of hypochlorite, it appears that 10% strength is
the most economical. Large installations are probably suited
for a maximum storage of one week. There would be very little
deterioration in the strength of a 10% solution in this length
of time. Manufacturers of sodium hypochlorite are able to
provide strength as high as 15 percent.
The choice of one over the other
is primarily a matter of economics. The 10 percent solution
has a greater stability than the higher strengths, and so,
other things being equal, it should be favored. However, storage
facilities are such a large cost factor in the overall installation
that the economy of the 15% solution must be considered as
well as the deterioration due to age.
(5) Hazards of hypochlorite
The use of hypochlorite as an alternative
to liquid or gaseous chlorine in reasonably large quantities
is primarily for safety reasons. However the hazard due to
presence of hypochlorite must not be overlooked. These hazards
derive from storage accidents.
One such accident occurred in Knoxville,
Tenn. April 8, 1983. A lethal cloud of chlorine gas escaped
from sodium hypochlorite tanks used in the disinfection system
for water treatment plant. They also use ferric chloride as
a coagulant which is normally shipped in railcars. When railcars
are not available, ferric chloride is shipped by tank trucks.
The hypochlorite is always delivered by tank trucks similar
to those used for shipping ferric chloride.
In this instance the ferric chloride
was shipped by truck, and since the truck connections were
compatible, the driver, who have never made a delivery to
the plant before, made the connections, pressurized the truck,
and unloaded approximately 600 gallons of ferric chloride,
which mixed with approximately 3,000 gallons of 10-12% sodium
hypochlorite.
Owing to the low pH of FeCl3
and the concentration of reactants, molecular chlorine was
released instantaneously from the hypochlorite. A cloud of
Cl2 began rolling out of the hypochlorite tank
vent. Fortunately an emergency response plan that had been
worked out by the city was implemented as soon as the cloud
was sighted. This included local evacuation and rerouting
of all mobile traffic near the area.
Precaution must be taken to make
certain that only hypochlorite can be put into a hypochlorite
storage tank. Any acidic chemical will generate the release
of molecular chlorine from a sodium hypochlorite solution.
Also precaution must be taken in the design systems to prevent
the possibility of unloading bisulfite into a hypochlorite
tank or vice versa. This mixture products a heat of reaction
sufficient to cause disinfection of a fiberglass tank. The
heat generated is so great that an explosive force would surely
be produced.
(6) Operating Cost
The operating cost of any imported
hypochlorite system will depend entirely upon the amount of
chemical to be delivered at one time and the total amount
to be consumed over a contract period. The cost of chlorine
gas and hypochlorite varies considerably depending upon the
locality, demand, and availability. The price spread between
hypochlorite and chlorine gas increases significantly as the
distance from the source of chlorine gas manufacture and user
increases. The most optimistic estimate is that imported hypochlorite
will cost at least three, or more likely, four times that
of liquid-gas chlorine.
Cost comparisons of the chlorination
facility between liquid chlorine and hypochlorite should include
storage and supply facilities, metering equipment, instrumentation,
and monitoring equipment. Generally speaking, the metering
and feeding equipment for chlorine gas is more expensive than
that for hypochlorite, but the expenses of storage facilities
for hypochlorite are far greater and more than offset the
equipment difference. Maintenance of a hypochlorite system
requires more man-hours than the gas system.
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