Chlorine demand can be defined as the inability to maintain a chlorine residual. An almost infinite number of substances can
contribute to demand. These include bacteria, algae, ammonia,
urine, sweat, health and beauty products, bather waste, and animal
waste. Such contaminants can enter the water in a number of
different ways — through source water, rain water, bathers,
animals, fertilizers, plants/leaves and industrial pollution. Thus,
determining the cause of chlorine demand in a particular pool can
sometimes seem like an insurmountable task.
Hypochlorous acid, or free available chlorine, can be a great tool
in tackling chlorine demand problems. That’s because
hypochlorous acid reacts easily with many different types of
materials. By looking at the chemical structure of some
contaminants, one can predict whether there will be an interaction
with hypochlorous acid — which will determine whether that
particular contaminant contributes to demand.
All atoms have what is referred to as a preferred “oxidation
state,” or “oxidation number.” This is simply a
number that is assigned to a particular atom based on its chemical
properties. For example, the preferred oxidation number for
chlorine is -1. The preferred oxidation number for nitrogen and
phosphorous is +5. Atoms in an oxidation state that is not
preferred are very reactive, while atoms in their preferred
oxidation state are stable and are much less reactive. It is not
important to know how the oxidation numbers are determined, but
knowing what they are is very helpful.
In hypochlorous acid (HOCl), chlorine actually has an oxidation
number of +1, which is not preferred. Because chlorine is
constantly trying to reach its preferred state of -1, hypochlorous
acid is very reactive. This is why it’s such a great
oxidizer. When hypochlorous acid oxidizes other materials, the
chlorine atom ends up where it wants to be, at -1.
In the ammonia (NH3) molecule, the nitrogen atom has an oxidation
number of -3. As mentioned earlier, it prefers +5. This makes it
very reactive as well. Because the chlorine in hypochlorous acid
needs a decrease in oxidation number (from +1 to -1), and the
nitrogen in ammonia needs an increase in oxidation number (from -3
to +5), these chemicals are perfect to react with one
Ammonia is removed from pool water through the application of
chlorinating products, and hypochlorous acid is used up during this
process. A similar process happens with bather waste contaminants
such as urea and creatinine. However, the oxidation process is much
longer for these compounds because they are more complex.
Just as some compounds readily react with hypochlorous acid, some
compounds do not tend to react with it. For example, the nitrogen
in nitrate (NO3-) already is where it wants to be at +5. The same
is true for the phosphorous atom in phosphate (PO4-3). In the
orthophosphate molecule, the phosphorous atom also is where it
wants to be at +5. This makes them quite stable and unlikely to
react with hypochlorous acid. If material does not react with
hypochlorous acid, then it does not contribute to chlorine demand.
If phosphate or nitrate reacted with chlorine, then these compounds
would be removed when shocking the pool, but this does not
Unfortunately, there is no easy cure for many chlorine demand
situations. In most cases, there are still only two options.
The first is to apply the appropriate amount of chlorinating
product; the second is to replace some of the water with fresh
water that has no chlorine demand.
In situations where the chlorine demand is accompanied by very
cloudy water, a flocculant treatment may reduce the demand by
physically removing some of the contaminants from the water. Keep
in mind that a floc treatment or water replacement does not
completely cure the demand — it only lessens it. It will be
necessary to re-test and apply the newly recommended amount of
Treating chlorine demand can seem overwhelming, especially if the
demand is large. During treatment it is important to recognize that
there are two categories of contaminants being treated. The first
is “fast reacting,” which includes small molecules such
as ammonia. The second type is “slow reacting,” which
includes the larger molecules such as amino acids and proteins.
Typically more complex than the smaller molecules, they require
more time to oxidize. These larger, slower-reacting contaminants
are usually the result of swimmer waste such as sweat or urine, and
can take up to 36 hours to completely oxidize. Chlorine demand is
likely caused by a combination of different types of contaminants,
so treatment time and difficulty can vary.
Of course, the best course of action is always prevention. Doing
routine oxidation as well as application of a maintenance algaecide
will help keep water clear and free from contaminants that can
contribute to chlorine demand. It's also important to know when
additional oxidation is needed. This includes heavy bather loads,
rain, warmer-than-normal temperatures and any time there is
suspected contamination of the pool water. Designing a maintenance
program specific to the characteristics of each facility will help
prevent problems before they begin.