High chlorine demand is the inability to keep adequate
chlorine in pool water, even though the water is balanced
and properly maintained. Various contaminants increase
oxidation levels, consuming chlorine faster than it can
be replaced by automatic feeders or normal shocking. Symptoms
can include slimy or slick pool walls and cloudy water,
although water can be clear and still have a high chlorine
demand. A sudden drop in cyanuric acid can be an indicator
that there is a chlorine demand problem, as the CYA reading
is often masked by the demand. If this occurs, do not
Another form of chlorine demand is a high level of combined
chlorine that cannot be broken by successive shock applications.
Often this is caused by the presence of ammonia in the
water. This can come from fertilizers, pesticides, and
even fill water.
especially common during spring start-up. For outdoor
pools subject to many environmental factors, it may not
be possible or feasible to isolate the exact cause. However,
this is less important than addressing the problem immediately.
NOTE: A zero chlorine reading does not necessarily indicate
that there is no chlorine in the pool. The chlorine level
may be so high that it is bleaching the color out of the
reagent. See Lab Troubleshooting for more information.
of Chlorine Demand
Chlorine resistant algae, fungus or bacteria
An infestation can exhaust normal chlorine levels and
require a specially formulated algicide to solve the problem.
Often, the growth is not visible on the pool surface itself.
Pink slime and water mold are notorious for growing first
inside lines, skimmers, and behind light niches before
becoming visible. Growth in these areas can deplete chlorine
Lawn fertilizers and other nitrogen products in pool water
produce a high level of chloramines, which require larger
amounts of chlorine for oxidation.
Lake water, which is usually contaminated with algae,
metals, and other debris, should not be used to fill pools.
Well water may have some of the same problems. Even a
local municipal water supply can create high chlorine
demand, especially if it contains chloramines. In these
cases, every time fill water is added to the pool, chloramine
and nitrogen levels rise, requiring more chlorine. Shocking
after fill water addition helps prevent excessive chloramine
levels in this situation, as does avoiding the addition
of large amounts of fill water at a time.
Rain and Pollution
Contaminants from factories, highways, airports, and other
sources may be deposited in pool water, especially during
rainfalls. Clouds sometimes transport pollution over long
distances. Rain and wind also carry algae spores, leaves
and other debris which raise chlorine demand. During the
winter, stagnant water in uncovered pools exposed to air,
rain and snow often develop chlorine demand problems.
High bather loads
A large crowd using the pool over a few days can boost
bacteria and oxidizable compounds in the water to unusually
high levels. Requiring all swimmers to shower before swimming
can help prevent this. Since this isn't going to be likely
most of the time, shock immediately following parties
or other occasions where many people have enjoyed the
The best way by far to break a chlorine demand is to
perform a chlorine demand test. This test is contained
in a separate lab available to BioGuard Dealers. (For
lab test instructions, see Chlorine Demand Test Instructions.)
This test can give the exact amount of chlorine needed
to break the demand.
In some cases, this may be quite high. It may be a more
feasible option to perform a partial drain and refill
with fresh water to reduce the demand. (Check environmental
factors such as water tables etc. before proceeding with
a drain. Consult the pool manufacturer or builder before
draining significant amounts of water from the pool.)
Performing this test can prevent a great deal of frustration
by allowing you to make an educated decision on how to
proceed. If a chlorine demand test is performed, you can
enter the test results in the encyclopedia tab titled
"Chlorine Demand" and it will automatically
calculate the proper amount of Burn Out or Burn Out 35
If you do not have the Chlorine Demand Test station,
call BioGuard and place your order. In the meantime, you
can use the following procedure to try and eliminate the
demand until it comes in!
Shock the pool with 3 lb. of Burn
Out per 10,000 gallons or 3 lb. of Burn Out 35 per 6,000
gallons all at one time.
- Circulate the water continuously. Three hours after
adding the Burn Out or Burn Out 35, test the water for
chlorine. If it is not greater than 3 ppm, repeat step
- Continue shocking and retesting every three to four
hours until you can maintain a 3 ppm free chlorine reading
for 24 hours.
NOTE: There is no way without the Demand test station to
determine how many of these shock applications the demand
Prevention of Chlorine Demand
The best way to prevent chlorine demand
is to follow one of the Three Step Programs.
Consistent sanitation, regular shocking, and the application
of an algae preventative are the best weapons against any
kind of problem, chlorine demand included.
To minimize the risk of chlorine demand
from outside contaminants, shock the pool after periods
of heavy use and avoid getting chemicals in the pool, such
as lawn sprays, fertilizers, pesticides and other pollutants.