Swimming Pool
Water Chemistry
Water Sanitizing
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Contents
As you probably know, the
occasional addition of new water -- or wholesale water replacement
in the case of spas -- isn't enough to keep the water clean and
clear of unwanted and often microscopic contaminants.
But worry not! Pool and spa
chemists have spent years developing a variety of tests and
chemical-treatment methods to keep your pool and spa safe and
sparkling clean.
Your goals here are water
sanitation and water balance. In other words, you want your levels
of sanitizers (such as chlorine or bromine) and your levels of pH,
total alkalinity, water hardness and total dissolved solids to all
fall within acceptable limits.
And learning to keep these
areas in check isn't as complicated as it may seem. To simplify
things, we'll explain the relevant topics one at a time.
| Sanitizers |
Sanitizers,
or disinfectants, are hardworking chemicals. They also have
a killing streak in them -- which is exactly what you want.
The important
thing to keep in mind is, that for them to do their duty,
they have to be present in the pool and spa water at all
times. There always needs to be a certain minimum amount --
called a residual -- of sanitizer in your water.
What we're
saying is, to make sure your pool and spa are protected from
bacteria and their friends, you need your sanitizers on
'round-the-clock duty.
When you add
your sanitizer, therefore, you should be adding enough to
kill any existing contaminants in the water -- and create a
residual that can neutralize any dirt, debris or germs that
may get into the water before you next treat it.
That said,
let's take a look at some of the most common sanitizing
chemicals and equipment available on the market today. |
| Bromine |
Also
available on the market today is a chemical sanitizer called
bromine. Do note, however, that bromine cannot be stabilized
with cyanuric acid. As much as 65 percent of a bromine
residual can be depleted by the sun in a two-hour time
period. Because there is no known way to retain a reliable
level of bromine when exposed to sunlight, many experts
recommend it for indoor pools or spas only.
FYI: The
acceptable range of bromine for your pool or spa is from 2.0
to 4.0 ppm.
Bromine also
has no odor, and dispensing it your spa via a feeder allows
it to dissolve at a slow, constant, desirable rate. Just be
sure to remove the feeder when your spa is in use.
A final note:
When using bromine as your sanitizer, you will need to
occasionally shock the water with large doses of another
chemical to oxidize waste material still in the water.
(We'll explain this in more detail below.) |
| Chlorine |
Chlorine
has been somewhat of a wonder drug for pool and spa
environments. It has the power to kill bacteria and algae --
and works extremely well in aqueous environments.
Chlorine is not invincible,
however. Like most chemicals, it has a threshold -- a point
at which it has used up all its sanitizing power and can no
longer protect your pool and spa water.
Chlorine
levels are also heavily influenced by evaporation, splash
out and destructive UV rays, not to mention a low pH.
(You'll read about this last factor in more detail below.)
Once added to
the water, the "free available" chlorine -- that
portion of the chlorine with the killing power -- will
sanitize and oxidize the water by attacking undesirables
such as bacteria, algae, sweat and oils from your skin,
residual soaps, shampoos, perfume and, yes, urine.
As it uses up
its killing potential, the chlorine becomes ineffective or
it combines with the contaminants and remains in the pool
and spa water in the form of chloramines.
FYI: It is
the chloramines in your pool and spa water -- not too much
"good" chlorine -- that causes a chlorine-like
odor and can irritate your eyes and skin.
Indeed, when
a pool or spa exudes a chlorine odor and you begin to hear
complaints of skin and eye irritation, that is a loud
warning that there is not enough chlorine in the water. If
this is the case, you should test the water and add the
appropriate amounts of sanitizer as soon as possible. (Check
out the Testing Your Water section to learn more about this
process.)
The
recommended level of free available chlorine to keep in your
pool is between 1.0 and 3.0 ppm or parts per million. For
spas, the recommended level of free available chlorine is
between 1.50 and 3.0 ppm.
(For the
definition of parts per million and other pool- and
spa-related terms, see the Glossary of Terms section).
Another note:
We'll address how you measure chlorine and other
water-balance levels in another section.
Chlorine is
extremely susceptible to sunlight and needs to be regularly
monitored. But just as we use sunscreen to protect our skin
from the sun, chlorine uses a sunscreen of cyanuric acid.
Used this way, cyanuric acid is also commonly called a
stabilizer or conditioner.
With its
help, chlorine retains its effectiveness. Without going into
the chemical whys and wherefores, know that acid works to
help keep a fairly consistent chlorine level (a residual) in
the water.
Some
chlorine-based sanitizers are sold with a dose of cyanuric
acid already mixed into the product. One such product is
trichlor tablets, which are usually placed in a floater,
chemical feeder or in the skimmer basket. Another commonly
used product is sodium dichlor, which is a granular
substance usually dispersed directly into the pool or spa
water, or added via the skimmer. |
| Oxidizers |
OK,
you've been reading about oxidizers and oxidizing -- and
still have no idea what they are or what they do. Let's find
out.
Oxidizers work with
sanitizers to rid your pool and spa water of pollutants. By
definition, sanitizers kill things like algae and bacteria
but they work very slowly and inefficiently when trying to
remove waste products such as sweat, skin oil, shampoos,
soap and urine. This is where you need separate oxidizers.
Oxidizers
destroy these undesirables. They do their part by breaking
down the contaminant's structure, rendering them vulnerable.
The sanitizers can then attack and kill the organisms.
Some
products, such as chlorine, act as both a sanitizer and an
oxidizer. Bromine, on the other hand, does not oxidize very
well and therefore needs the aid of a separate oxidizing
chemical to properly clean your pool and spa water.
The marriage
of sanitizers and oxidizers is an almost foolproof method of
keeping invaders out of your pool and spa water. |
| Sanitizers
- Alternative |
Although
chlorine and bromine are the most commonly used sanitizers,
there are a few alternatives available to also consider. One
such alternative is a chlorine generator.
Wait, we just discussed
chlorine! How is this product different?
The liquid or
granular chlorine most people are familiar with is added
directly to pool or spa water or dispensed through a feeder
or similar automated system. Chlorine generators,
conversely, actually create chlorine in the pool or spa
without you having to measure out any chemicals.
These
generators are electrical devices that manufacture chlorine
from salt added to the water. The resulting chlorine gas is
then put directly into the water through the circulation
system.
Chlorine
generators create a residual, which can be tested with a DPD
or OTO test kit. (These test kits are described in more
detail in a later section.)
Another
alternative sanitizing source is ozone. No, it's not just
the atmospheric layer that protects us from the sun's
harmful rays. Rather, the ozone used in pools and spas is a
modified version of that gaseous oxygen.
To its
advantage, ozone works quite well as a sanitizer and an
oxidizer. . Because it is a form of oxygen, ozone doesn't
last long in an aqueous environment. Once it does its job of
ridding the pool and spa water of bacteria and other
unwanted matter, the ozone then reverts back to oxygen and
either dissolves into the water or escapes into the air.
Because it
cannot create a residual, an ozonator must be used in
conjunction with small amounts of chlorine or bromine -- how
much chlorine or bromine depends on how long the ozonator is
run each day.
One other
common kind of sanitizing equipment is an ionizer. This
system introduces silver and copper into your pool or spa
water through the circulation process. It works well as a
sanitizer but does require the addition of an oxidizer. |
This is the most commonly used
method of sanitation today. Bromine and iodine are other members of
the halogen family of chemicals also used to sanitize water. Other
chemicals include ozone, silver and copper compounds. Ultraviolet
light is a nonchemical disinfectant. Each of these methods will be
covered to some extent, but chlorination is presently the most
widely accepted means of treating pool water.
All chlorine - regardless of
whether it is introduced as a gas or as a dry or liquid compound
when added to water, does exactly the same thing: It forms
hypochlorous acid (HOCI) and hypochlorite ions (OCI-).
HOCl is the killing form of
chlorine; OCI- is relatively inactive. However, together, they are
free available chlorine (FAC).
Because each of the many
chlorinating agents produces the same active form of chlorine, we
can ignore the source for now and deal with the process of
chlorination in general.
HOCI is an extremely active,
powerful chemical. It not only destroys such harmful organisms as
bacteria, algae, fungi, viruses, etc., it also destroys impurities
that are not removed by filtration. These two processes are called
sanitation and oxidation.
Sanitation
Sanitation is the process of
destroying organisms that are harmful to People. These organisms,
referred to as pathogens, include bacteria, fungi, viruses, etc.
Chlorination also controls algae (which are not usually harmful
themselves, but may harbor pathogenic organisms). In addition to
being unsightly, algae can cause the surfaces around the pool to
become slippery and unsafe.
While each of these organisms
may require different amounts of HOCI for control, the required
amount for public swimming pools is often established by local
health officials. Very often, local codes will specify an FAC
residual of 1.0-3.0 parts per million (ppm), but some might vary
from this.
Oxidation
Oxidation is the process of
chemically removing organic debris, such as body waste, particulate
matter and perspiration, from the water. The process is similar to
burning trash in air. It is not important to understand the
chemistry involved; it is sufficient to know that enough chlorine in
water will chemically "burn" impurities.
The use of chlorine to clean
up water is a supplement to filtration, discussed in another
chapter. Filters remove the dirt and debris suspended in water, but
even the best filter cannot remove dissolved impurities because they
are not physically separate from the water. If the water looks dull
or hazy, even though the filter system is operating properly, the
operator should consider a shock treatment to oxidize the organic
impurities and restore the clarity of the water. Although there are
some non-chlorine shock treatments available, the most common method
used to shock water is superchlorination.
Superchlorination
Superchlorination is a term
that describes an extra large dose (usually 8 to 10 ppm) of chlorine
to oxidize organic compounds and kill and remove algae and other
contaminants from the water. This is the same as using three to six
times the normal dosage of a chlorinating agent. For example, a
50,000-gallon pool requires about four gallons of liquid pool
chlorine (12% Available Chlorine) or six pounds of a granular
chlorinating compound such as calcium bypochlorite (65% Available
Chlorine).
As mentioned, HOCI is the form
of chlorine that provides sanitation. Because it is an extremely
active chemical, however, it also reacts with organic impurities.
When there is enough HOCI present, the impurities are completely
oxidized. Combined chlorine is formed when there is an insufficient
supply of HOCI or when there is a very high level of organic
impurities. Combined chlorine compounds can be oxidized by
increasing the HOCI level in the water. The point at which all the
organic impurities are oxidized is called the breakpoint. The
addition of sufficient chlorine to reach this point is known as
breakpoint chlorination.
Combined
Chlorine
Combined chlorine is formed by
chlorine combining with ammonia and other nitrogen-containing
organic compounds. Some sources of these compounds include
perspiration, urine, saliva and body oils. These combined forms of
chlorine, also called chloramines, are still disinfectants, but they
are 40 to 60 times less effective than free available chlorine.
Chloramines kill slowly, so
when they are formed in swimming-pool water, the FAC is no longer
present for "instant kill" sanitation. Free available
chlorine and combined chlorine exist together in many pools. There
are simple tests to measure the levels of each. These will be
described in detail in another chapter.
In addition to reduced
effectiveness against bacteria, chloramines cause eye irritation and
the so-called 'chlorine odor" that swimmers complain about.
Chloramines have a foul, irritating odor; free chlorine in water in
normal concentration has no discernable odor.
Breakpoint
Chlorination
When chloramines are known to
be present, either by test (over 0.2 ppm combined chlorine) or
because of a foul chlorine odor, the continued addition of chlorine
causes a corresponding rise in measurable chlorine residual, but
eventually, a point is reached at which addition of chlorine causes
a sudden drop in residual. This phenomenon is accompanied by a
reduction in eye irritation and chlorine odor.
Investigation reveals that
when the total concentration of chlorine in the water reaches seven
times the amount of combined chlorine, the oxidation of chloramines
and other organic compounds is complete. The point of residual
concentration at which this sudden reaction occurs is called the
breakpoint. Chlorine remaining or added after the breakpoint is
reached exists as free residual chlorine, and all the combined
residual is oxidized. The breakpoint varies in its speed and
amplitude, depending upon the organic matter present. In some
waters, the breakpoint is hardly discernible.
The practice of periodic
superchlorination is actually an attempt to pass the breakpoint to
rid the water of an accumulation of combined chlorine and potential
chlorine-consuming compounds.
Figure 1 - Graph of
breakpoint phenomenon
Inorganic
and Organic Chlorine
Inorganic chlorine compounds
such as calcium hypochlorite, sodium hypochlorite and lithium
hypochlorite, as well as chlorine gas, are greatly affected by
direct sunlight. They require the addition of cyanuric acid to be
stabilized when used in outdoor pools. These products are
recommended for indoor swimming pools.
Organic chlorine compounds are products
combined with cyanuric acid. Sodium dichloro-striazinetrione ("dichlor')
and trichloro-s-triazinetri-one ('trichlor) are both popular
stabilized chlorines for water treatment of outdoor pools.
Calcium hypochlorite will
support combustion and must never be mixed with carbonated drinks,
oils of any type or an organic chlorine product such as "dichlor'
or 'trichlor' - explosions can result.
Figure 2 -
Disinfection time for free and combined chlorine residuals at
various pH values.
Effect
of pH on Chlorine
The pH of water has a definite effect on the
efficiency of chlorine as well as on the corrosive properties of
water (covered later in this chapter.) For now, we will consider
only the effect of pH on sanitation.
It can be seen in Table 2.a
that free chlorine is most efficient in pH ranges below the ideal
range of 7.2-7.6. Some pool operators do, however, maintain pH
levels higher than the ideal range. They should also maintain
appropriately higher FAC levels to provide the same concentration of
the active HOCL form.
For example, at a pH of 8.0,
21% (about 1/5 of the FAC is in the active form. At that pH level,
it would take 2.5 ppm of FAC to provide about 0.5 ppm of HOCI. At a
pH of 7.5, about 1/2 (50%) of the FAC is in the active HOCI form. At
that pH level, it would take only 1.0 ppm of FAC to provide the same
0.5 ppm of HOCI. For this reason, many authorities recommend that
the pH of pools be maintained in the range between 7.2 and 7.6 and
as close to 7.5 as practical. These conditions are also considered
to be most comfortable for the swimmers' eyes and skin.
| HOCI |
H* |
OCI- |
| Hypochlorous
Acid
Killing Agent
Active, but unstable form |
Hydrogen
Ion |
Hypochlorite
Ion
Inactive, but stable form |
| %
Chlorine as HOCI |
pH |
%
Chlorine as OCI- |
| 90 |
6.5 |
10 |
| 73 |
7.0 |
27 |
| 66 |
7.2 |
34 |
| 45 |
7.6 |
55 |
| 21 |
8.0 |
79 |
| 10 |
8.5 |
90 |
Chlorine
Gas (Cl2)
Chlorine gas has 100%
available strength, is very toxic and can be lethal if an operator
is overcome by it. This form of chlorine is the most economical,
pound for pound, but the laws regarding safety practices and the
intricate feeding equipment it requires make it a feared method of
water sanitation. Chlorine gas for pool use is contained under
pressure in steel tanks as large as one ton. The gas is green in
color and heavier than air.
Strict adherence to the
following practices is required for minimum safety:
-
Chlorine tanks should
always be stored indoors in a fire-resistant building.
-
Tanks and chlorinator must
be kept in a separate room with a vent fan capable of complete
air exchange in one to four minutes.
-
Tanks must be chained or
strapped to a rigid support to prevent accidental tipping.
-
A self-contained air
supply gas mask must be immediately available in case of
emergencies.
-
The chlorinator and all
tanks should be checked daily for leaks. A small amount of
ammonium hydroxide (household ammonia) on a piece of cloth
produces white vapor in the presence of chlorine.
Sodium
Hypochlorite (NAOCl)
Sodium hypochlorite is a
clear, slightly yellow liquid solution used in dilute form as common
household bleach. In its commercial form, it provides 12% to 15%
available chlorine. The chemical is usually introduced to pool water
through a chemical feeder, but it can be poured directly into the
pool for a quick increase in chlorine residual. It has no sediment
or precipitate and raises Total Dissolved Solids (TDS). This
chemical has a pH of 13 and causes a significant increase in the
pool's pH. The occasional addition of muriatic acid or sodium
bisulfate can correct the increased pH, however.
Sodium hypochlorite is not
stable in storage and gradually loses strength, especially in
sunlight. If stored in a dark, cool room, it has a one-month shelf
life.
Dilute solutions of sodium
hypochlorite can be used for poolside sanitation and for
disinfecting and cleaning decks. The chemical should not be spilled
directly on clothes and should be immediately washed off if it gets
on the skin. However, its safety and low cost has made this a very
popular chlorinating agent.
Calcium
Hypochlorite (Ca(OCl)2)
Calcium hypochlorite is
available in granular or tablet form. It provides 65% available
chlorine by weight and remains stable if stored in a dry, cool area.
The chemical can be dissolved and introduced to the pool as a
liquid, or it can be added in dry form. When applied directly to the
pool, it may cause a temporary cloudiness. Direct applications
should be broadcast evenly over the water surface to avoid bleaching
the pool bottom as a result of having a concentrated amount in one
spot. This must be done when no bathers are in the pool.
This chemical, when
contaminated by or mixed with an organic compound, can produce a
fire. A good rule is never to mix calcium hypochlorite with another
chemical or store it in anything but the original container. Mix the
chemical into water not water into the chemical. Calcium
hypochlorite should not be handled with bare hands and must be kept
off the operator's clothes.
As a chlorinating agent,
calcium hypochlorite will slightly increase pool pH. It has a pH of
11.8. Operators of gas-chlorinated pools often keep a supply on hand
for emergency use or for a quick charge when superchlorinating.
Dissolved in water, calcium hypochlorite can be used as a sanitizing
agent for decks and locker rooms.
| |
GAS
CHLO-
RINE |
SODIUM
HYPO
CHLORITE |
CALCIUM
HYPO
CHLORITE |
LITHIUM
HYPO
CHLORITE |
DICHLOR |
TRICHLOR |
| %AVAILABLE
CHLORINE |
100% |
12.15% |
65-75% |
35% |
56% OR 62% |
90% |
| pH EFFECT |
LOWERS
(pH < 1.0) |
RAISES
(pH 13.0) |
RAISES
(pH 11.8) |
RAISES
(pH 10.7) |
NEUTRAL
(pH 6.9) |
LOWERS
(pH 2.9 |
| LOST TO
SUNLIGHT |
YES |
YES |
YES |
YES |
NO |
NO |
| PHYSICAL
APPEARANCE |
GAS |
LIQUID |
GRANULAR &
TABLETS |
POWDER |
GRANULAR |
GRANULAR&
TABLETS |
Lithium
Hypochlorite (LIOCl)
Lithium hypochlorite is a
newer entry in the field of chlorinating agents. Its cost is greater
than other hypochlorites, and it provides only about 35% available
chlorine. However, the chemical is fast dissolving and totally
soluble in water, and pH tends to rise more slowly with its use than
with other chemicals. It has a pH of 10.7. Lithium hypochlorite
should not be mixed with organic compounds, but it is safer to store
and use than calcium hypochlorite.
Chlorinated
lsocyanurates
(Stabilized Chlorine)
This family of chemicals is in
wide use for swimming pool chlorination. The family is composed of
sodium dichloro-s-triazinetrione and trichloro-s-tria zinetrione -
chlorine compounds that contain cyanuric acid (stabilizer). The
dichlor is more soluble and provides 56% or 62% available chlorine,
depending on formulation. dichlor provides 90% available chlorine
and is used when a slow release of chlorine over a period of time is
desired. The dichlor compound has little effect on pH, while
trichlor is extremely acid (pH 2.8-3.0). Dichlor can be added
directly to the pool; trichlor is generally fed through an
erosion-type feeder, but never through the skimmer basket.
The major effect of cyanuric
acid on bypochlorous acid (HOCI) is to keep it from being decomposed
by ultraviolet (LTV) light such as contained in sunlight.
Because it is readily
decomposed by LTV light, the dosage of a chlorinating agent that is
sufficient for an indoor pool is dissipated rapidly in an outdoor
pool. Cyanuric acid bonds with the available chlorine in a manner
that does not use up the chlorine. At high stabilizer levels (over
100 ppm), chlorine's efficiency may be reduced. The operator should
consult the local codes and manufacturers' recommendations on the
proper use of stabilizers.
Chlorination
Summary
-
Chlorination both
sanitizes and cleans the water by oxidizing organic impurities.
-
A free chlorine residual
of 1.0-3.0 ppm is preferred. Combined chlorine should not exceed
0.5 ppm and may be destroyed by breakpoint chlorination.
-
Proper pH control
(7.2-7.6) provides better chlorine efficiency.
-
When cyanuric acid is used
in an outdoor pool, chlorine consumption is reduced, because the
chlorine degradation caused by LTV light is reduced.
-
There are many
chlorinating agents available but each provides HOCI, the active
chemical specifically for disinfecting.
Other members of the halogen
family - bromine and iodine - are used for treating pool water. New
developments in the use of ozone, ultraviolet light, ionization of
salts and other chemical compounds continue to provide new
challenges and techniques in water treatment.
Bromine
Although bromine in its
elemental form is a liquid, it is not available for swimming pool
disinfection in that form. Elemental bromine is a heavy, dark brown,
volatile liquid with fumes that are toxic and irritating to eyes and
respiratory tract. For pool sanitation, bromine compounds are sold
in two solid forms - a two-part system that uses a bromide salt
dissolved in water and activated by addition of a separate oxidizer;
and a one-part stick or tablet that contains both bromine and an
oxidizer and is dispensed by an erosion-type feeder.
The chemistry of bromine is
similar in many respects to the chemistry of chlorine; however,
bromine cannot be used for shock treating.
Bromine has a pH of 4.0-4.5.
When bromine is added to water and an oxidizer is present, the
bromine forms hypobromous acid (HOBR) and bypobromite ions (OBr).
Like chlorine, the percentage of each is affected by pH. However,
the effect is not as dramatic as it is with chlorine. Table 2.c
displays the effect of pH on bromine. Like chlorine, bromine
combines with organic impurities to form combined bromine or
bromamines. However, combined bromine is still an effective
sanitizer, and it does not smell. Because of this, bromine is
popular for spas.
HOBr
Hypobromous Acid
% Bromine as HOBr |
pH |
OBr-
Hypobromite Ion
% Bromine as OBr- |
| 100.0 |
6.0 |
0.0 |
| 99.4 |
6.5 |
0.6 |
| 98.0 |
7.0 |
2.0 |
| 94.0 |
7.5 |
6.0 |
| 83.0 |
8.0 |
17.0 |
| 57.0 |
8.5 |
43.0 |
Ozone
Ozone was first used as a
water sanitizer in France in the early 1900s. It is the most popular
method of treating drinking and pool water throughout Europe. Ozone
is one of the strongest oxidizers available for treating swimming
and spa water and is growing in popularity in the U.S.A. It kills
bacteria and oxidizes organic compounds including chloramines,
soaps, oils and bather wastes and does not alter the water's pH. Due
to limitations of the amount of ozone that can be economically
introduced to pool and spa water, algae growth is not eliminated and
may possibly increase.
Two systems have been
developed for the production of ozone for pool and spa water. The
most common European method - and the most expensive - is the Corona
Discharge. This method generates ozone by exposing pressurized,
dried air to high-voltage electricity. The ozone gas is then
directed into the bottom of the pool and seen as very small bubbles
rising to the surface. The ozone can be introduced into a separate
chamber or directly to the pool.
The Ultraviolet (LTV) or
photo-chemical method of ozone production passes the pressurized,
dried air next to a UV bulb within a chamber, where the UV rays
bombard the oxygen molecules and produce ozone. A single LTV lamp is
capable of treating approximately 10,000 gallons of water. The UV
method is more common for spas or private small pools and is less
expensive to operate. The LTV method cannot match the output of the
Corona Discharge method.
Once it has done its job as an
oxidizer, ozone reverts to oxygen and improves both smell and taste
of the water. This factor is worth noting, especially for indoor
pools. Ozone has to be continually generated, because its
effectiveness is about 22 minutes while the gas remains in the
water. Ozone offers no continuous residual, and because it is not
effective against algae growth, most manufacturers recommend that
ozone be introduced with a halogen such as chlorine or bromine.
Testing of ozone residual is
in parts per billion (ppb). Test kits are available that use DPD as
the reagent and color calibrations that indicate readings from 0 to
100 ppb. Testing with the use of an Oxidation Reduction Potential (ORP)
meter is effective, and a minimum level of 650 millivolts is
recommended. Because ozone of the smallest residual is effective and
adequate for treating water, when combined with chlorine or bromine,
a test reading of either is an indication of the effectiveness of
the ozone.
Electrical devices (chlorine
generators) have been developed that produce chlorine from salt
dissolved in the pool water or within a separate salt-solution
(brine) chamber. This method is becoming popular in Australia and
some European countries. Because of their use on spacecraft,
development of metals that are used for electrodes has been rapid.
There is a real future for this concept in sanitizing water.
Brine
Method
The brine type of chlorine
generator passes 4 to 16 D.C. voltage of about 20 amps through two
chambers. One chamber houses the positive (anode) electrode within a
brine solution of salt and distilled water, while the second chamber
houses the negative (cathode) electrode, also in water. Chlorine is
formed as a gas at the anode and sodium hydrox ' ide (caustic soda)
at the cathode. Chlorine gas is drawn off through a tube and
introduced into the pool or spa water. Some systems use the caustic
soda as a neutralizer for pH balance. This design is similar to
those used for producing chlorine gas commercially and can be
designed for any size pool.
In-Line
Method
The in-line type of chlorine
generator is located within the pool or spa circulation system, past
the filters. A low concentration of salt is added directly to the
pool. The electrolytic cell has been designed for smaller pools or
spas and requires a current of 5 to 15 amps with 5 to 35 volts D.C.
The production of chlorine gas, caustic soda and hydrogen gas passes
directly into the pool or spa water.
The water passing the
electrodes is superchlorinated and thereby reduces the build-up of
chloramines. The caustic soda aids in balancing pH. The control unit
is capable of reversing the charge on the electrodes to eliminate
scale build-up. The chlorine generated from salt will, after use,
revert back to salt, and the process starts all over again. The
addition of salt is reduced greatly by this recycling.
Automation
Chlorine generators activated
by ORP sensors have produced a system that reacts to fluctuations in
swimmer load and oxidizing demands for algae prevention. In-line
units in large pools have been coupled with electronic control
systems to offer a means of continuously treating the water.
The bactericidal properties of silver nitrate
and Argyrol are well known in medical practice. Silver ions are
introduced to water by electrolysis or by passing a current through
a silver electrode. The primary limiting factors in its use in
swimming pools are the high cost of silver and the fact that its
bactericidal action is quite slow. Silver is also photo-sensitive
and may cause a black deposit to form on the walls.
Products are available to
oxidize body wastes and organic materials and to reduce combined
chlorine or chloramines. These products are not sanitizers and are
used as an alternative to superchlorination for oxidizing only.
Non-chlorine oxidizers are useful for clearing cloudy water in
heavily used pools located in areas where health codes place upper
limits on free available chlorine readings.
The greatest use of chemical
products is in the bactericidal treatment of pool water. Many
special conditions, however, create requirements for additional
chemical treatment.
Flocculants
Aluminum sulfate (Al2(SO4)3),
commonly called alum, is used as a filter aid and as a coagulant and
settling agent for water turbidity. Alum "floc' is a white,
gelatinous substance that attaches to freefloating matter in water
to form larger, heavier-than water particles, which settle to the
bottom of the pool. Alum floc is especially effective on sand filter
beds. The floc partially fills the voids in the sand bed and holds
organic debris in its suspended gelatinous coating.
Alum is introduced as a filter
aid at the most convenient entry point ahead of the filter. The
chemical feed, hair and lint strainer or skimmer are effective
points of introduction. However, to coagulate particles in pool
water, a powdered alum is broadcast over the pool surface at the
rate of two ounces per square foot of surface area and is permitted
to stand overnight or for a minimum of two hours. After standing,
the pool should be vacuumed with minimal agitation to prevent the
floc from breaking apart. It is recommended when using alum as a
water clarifier or a filter floc that the pool water be adjusted to
pH of 7.6-7.8.
Flocculant aids, with a
combination of ingredients, sold under various trade names, have
been used to produce a heavier or more stable floc. Colloidal
silica, a clay called bentonite and a new family of organic
polyelectroloytes are available.
Algaecides
The chemistry of algaecides is
complex, because 46 species of clean-water algae exist. Some
algaecides work better on one kind of algae than on another.
Planktonic clean-water algae
float on the surface. Other types attach themselves to rough spots
on the pool walls and floor and are very difficult to remove.
Clean-water algae may be blue-green, red, brown or black and can
cause tastes, odors, turbidity and slippery spots, as well as
increased chlorine demand.
Sunlight, temperature, pH,
bacteria, chlorine residual and the mineral content of the water
affect the presence and growth rate of algae. Algae can be
introduced to a pool by wind-borne debris, rain and falling leaves,
or they may be present in the source from which the pool is filled.
Preventing algae growth by
chlorination is usually not a problem, but removing existing algae
from a pool can be difficult. If algae get a firm start on the side
or bottom of a pool, draining the pool is sometimes more practical
if the local water table is not too high to allow it. The pool
should be thoroughly washed down with a chlorine solution.
Sunlight is necessary to the
growth of algae, so it is a much greater problem in outdoor pools.
If not controlled, algae can spread rapidly, turning an entire pool
dark green in as little as a day or two.
Pools that consistently
maintain a high freechlorine residual concentration are seldom
troubled by algae. Maintaining free available chlorine and
superchlorinating are the best preventative measures. Combined
chlorine is not as effective as free chlorine in preventing algae
growth, and bromine and iodine are even less effective.
An algae inhibitor is a
commercial product that acts as a penetrating or wetting agent to
allow the chlorine to be more effective. Algae inhibitor is said to
control all types of algae growth and provide a stable backup system
to chlorine. It is not pH sensitive, does not evaporate,
concentrates on surfaces of the algae and is a powerful wetting
agent.
Chelating
or Sequestering Agents
Many stains around main drains
and inlets have to be cleaned by hand, but the stains can be kept
from returning. Sequestering agents increase the ability of water to
hold metals in solution instead of precipitating out to form stains.
Chelating agents remove iron and other metals from the water and the
pool walls. Pools with high iron content use a chelating agent as
part of routine water treatment.
Degreasers
There are commercial acids and biodegradable
detergents that effectively clean D.E. filter bags and filter sand.
Each product has its own ability to degrease and rejuvenate filters.
Each filter system has its own individual solution for doing the
job.
Defoamers
Foam or sudsing is a chronic
problem for most spas. Occasionally, a box of detergent is thrown
into the water as a prank, and a defoamer made specifically for
pools and spas is necessary to remove the foam. A bottle of defoamer
is handy to have on hand as a safeguard.
Cyanuric
Acid
Chlorine in outdoor pools must
be shielded from the degrading effects of the sun's ultraviolet (UV)
rays. Cyanuric acid is used in outdoor pools with the inorganic
chlorines such as calcium hypochlorite, sodium hypochlorite, lithium
hypochlorite and chlorine gas.
It is recommended that the
cyanuric-acid level be 30-50 ppm with a maximum of 100 ppm and a
lower limit of 10 ppm. The level of cyanuric acid is reduced by
dilution due to filter backwashing, bather dragout or dumping. To
meet health codes, pools above 100 ppm need to be drained of about
20% of volume, and fresh water added. Four pounds of cyanuric acid
will offer a reading of 50 ppm in 10,000 gallons of water. Cyanuric
acid has a pH of 4.0, and if added to a gas-chlorinated pool, the
addition of one-third of a pound of soda ash per pound of cyanuric
acid is recommended.
'Chlorine Lock,' a term given
to a condition once thought to be produced from high cyanuric-acid
levels tying up free available chlorine, has been proven false by
the industry. Generally, high cyanuric-acid levels of 400 ppm or
higher are associated with excessive Total Dissolve Solids (TDS) or
combined chlorine or cbloramines and not "Chlorine Lock". |
