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Copyright 2005, Board of Trustees, University
GULF COAST RESEARCH AND EDUCATION CENTER
n WATER MANAGEMENT SERIES
institute of Food and A tural Sciences, University of Florida ....
S. /.\ \ \ \ .......
Institute of Food and Agricultural Sciences, University of Florida
GCREC Bradenton Research Report
Treating Irrigation Systems Wit. Chlr1ine
Gary A. Clark and Allen G. Smajstrla
TREATING IRRIGATION SYSTEMS WITH CHLORINE
Gary A. Clark and Allen G. Smajstrla'
Chlorine is used in many water supply systems and home
swimming pools to keep algae and other microorganisms from
growing. Chlorine is also used for cleaning and maintaining
irrigation systems. Proper injection methods and amount of
chemical must be used to provide an effective water treatment
program without damaging any part of the irrigation system or the
crop being produced.
Irrigation systems can become partially or completely
clogged from biological growths of bacteria, fungi, or algae.
Bacteria, fungi, and algae are present in surface and ground
water sources and use chemical elements such as nitrogen,
phosphorus, sulfur, or iron as nutrient sources to grow and
develop. Generally, filtration alone cannot effectively remove
these microorganisms. Chlorination can be used to minimize the
growth of microorganisms within the pipes and other components of
irrigation systems. Without proper water treatment, clogging of
pipes, fittings, and emission devices (sprinklers, drippers,
spray jets, etc.) can occur, resulting in decreased growth and
development of the irrigated crop because of reduced water
application amounts, uniformity, and efficiency. This
publication provides information on the sources of chlorine and
the amounts required for treating irrigation water and systems to
control the growth of microorganisms.
SOURCES OF CHLORINE
Chlorine is available in gas, liquid, and solid (granular or
tablet) forms. However, only the liquid form (liquid sodium
hypochlorite) is labeled for use in irrigation systems in
Each of the three different chlorine forms reacts
differently with the irrigation water, depending on the other
chemicals or elements in the water. Reactions may be an
1 Associate Professor of Agricultural Engineering and Extension
Irrigation Specialist, Gulf Coast Research and Education Center,
University of Florida, Bradenton, FL; and Professor of
Agricultural Engineering, Dept. of Agricultural Engineering,
University of Florida, Gainesville, FL.
alteration of the pH of the water, or may involve precipitation
of some element which could result in clogging of microirrigation
Chlorine Gas: Chlorine gas (Cl,) is commonly used in municipal
water treatment systems. As chlorine gas reacts with water,
hypochlorous acid (HOC1), hydrogen (H), and chloride (Cl-) are
formed. This reaction lowers the pH of the irrigation water.
The change in pH depends on how much chlorine gas is injected and
on the buffering capacity of the water.
Chlorine gas is used in municipal water treatment systems
because it provides chlorine in the most concentrated and
economical form. Basically, 1 lb of chlorine gas will provide a
1 ppm concentration of Cl2 to 1,000,000 gallons of water.
Similarly, an injection of 1 lb of chlorine gas per hour will
provide a 1 ppm concentration of C1, to a water supply with a
flow rate of 2000 gpm.
Chlorine gas is a respiratory irritant and affects the
mucous membranes. It can be detected as an odor at a
concentration of 3.5 ppm and can be fatal after a few breaths at
1000 ppm. Therefore, the user of chlorine gas must exercise
extreme caution and safety. Maximum air concentrations should
not exceed 1 ppm for prolonged exposure. This form of chlorine
is limited to water treatment systems or other applications by
Solid Chlorine: Granular (powered or tabular) forms of chlorine
are commonly used to chlorinate swimming pools. Calcium
hypochlorite is the form that is typically used and found at
local pool supply stores. Dissolving calcium hypochlorite in
water will result in the formation of hypochlorous acid (HOC1)
and hydroxyl ions (OH") a reaction that raises the pH of the
water. The calcium hypochlorite form may react with other
elements in the irrigation water to form precipitates which could
clog microirrigation emitters and thus counteract the purpose for
Calcium hypochlorite is used to treat swimming pool water
because the solid chlorine form is inexpensive, easy to store,
and easy to use. It generally has 65 to 70 percent of available
chlorine. Thus, approximately 1.5 lb of calcium hypochlorite
will treat 1,000,000 gallons of water with a 1 ppm concentration
Liquid Chlorine: Liquid sodium hypochlorite is most commonly
used as laundry bleach. However, it is also labeled for use in
irrigation systems. Mixing liquid sodium hypochlorite in water
results in the formation of hypochlorous acid (HOC1) and hydroxyl
ions (OH'1), a reaction that raises the pH of the water. Unlike
the calcium added in the solid chlorine form, the sodium added in
this liquid form does not contribute to clogging problems.
Neither the sodium nor the chlorine added to the water should
detrimental to crops or soils at the typical concentrations used.
EFFECTS OF CHLORINE
Hypochlorous acid is the effective agent that controls
bacterial growths. The amount of HOC1 that will be present in
solution, and thus active, will be present at greater
concentrations at lower pH levels (more acidic conditions). At
extremely low pH levels (or high acidity) chlorine gas (Cl1) will
form. Therefore, for safety it is very important to store
chlorine and acid sources separately.
Hypochlorous acid will react with iron in solution to
oxidize the ferrous form to the ferric form which will then
become the insoluble ferric hydroxide as a precipitate. This
reaction should take place prior to the irrigation filters so
that the precipitates may be trapped. Chlorine will also react
with hydrogen sulfide to form elemental sulfur. Because some of
the chlorine is used up by reacting with the sulfide or ferrous
ions, additional chlorine must be provided to supply enough
residual to control the microorganisms which can clog
microirrigation systems such as sulfur or iron slimes, or algae.
Most microorganisms will be inactivated and controlled at
free residual chlorine concentrations of 1 ppm. However, higher
injection levels are needed due to the inherent chlorine demand
of different water sources. As a start, use 2 ppm of chlorine
for each ppm of hydrogen sulfide, plus 0.6 ppm of chlorine for
each ppm of ferrous iron. A water test can be used to determine
the levels of hydrogen sulfide or ferrous iron present in
solution. Surface water sources such as lakes, ponds, or canals
should be treated with approximately 5 to 10 ppm of chlorine.
Higher levels may be needed for water with high amounts of
microbial activity such as during the warmer months of the year.
The chlorine injection rate should be checked by testing the
treated water at the most distant part of the irrigation system
using a test kit designed to measure "free" residual chlorine.
Residual concentrations of 1 to 2 ppm indicate that active
chlorine still exists and that the water and system parts have
been appropriately treated. Active chlorine may be tested using a
color indicating test kit (D.P.D.) that measures "free" residual
chlorine. Do not use a test kit that only measures total
chlorine. While levels of total chlorine may appear appropriate,
the "free" residual form may not. Therefore, ask for a D.P.D.
test kit from either a pool or irrigation supplier.
CHLORINE APPLICATION AMOUNTS
After determining the desired chlorine concentration, the
proper application amount must be determined. The amount of
chlorine to apply per unit of irrigation water will depend on the
desired concentration in the irrigation system and the
concentration or strength of the chlorine source.
Liquid sodium hypochlorite is the most convenient and
generally safest form of chlorine available to inject into
irrigation systems. Stock solutions are available in
concentrations of 5, 10, or 15 percent of available chlorine.
Table 1 or the following equations may be used to determine the
chlorine solution injection rate in gallons per hour (gph) for
different desired ppm injection levels and irrigation system flow
rates. Equations 1, 2, and 3 are specific for liquid chlorine
injection and are designed for stock solution chlorine
concentrations of 5, 10, and 15 percent, respectively.
For a 5% available chlorine stock solution:
(ppm)*(Irrigation Flow Rate, gpm)
Injection Rates, gph = ---------------------------------- (1)
For a 10% available chlorine stock solution:
(ppm)*(Irrigation Flow Rate, gpm)
Injection Rate,,, gph = -------------------------------- (2)
For a 15% available chlorine stock solution:
(ppm)*(Irrigation Flow Rate, gpm)
Injection Rates,, gph = ---------------------------------- (3)
For example, an irrigation system has a flow rate of 450 gpm
and the water is to be treated with 8 ppm of available chlorine
using a stock solution with 10% available chlorine. Using
Equation 2, the injection rate of the stock solution should be
approximately 2 gph [(8 ppm)*(450 gpm)/1850 = 1.95 gph]. If the
stock solution had just 5% available chlorine, the injection rate
should be about 4 gph.
Table 1 may be used for smaller irrigation system flow
rates. For example, consider a microirrigation system with a
flow rate of 80 gpm. The water is to be treated with a liquid
chlorine stock solution with 5% available chlorine and a 6 ppm
treatment level is desired. Using Table 1, at a 6 ppm treatment
level and a 5% stock solution concentration, the injection rate
should be about 0.7 gph per 100 gpm of irrigation system flow
rate. Since the actual flow rate is 80 gpm, the injection rate
should be 80/100 or 80% of 0.7 gph which is equal to 0.56 gph.
If this injection rate was too small for the injector, the stock
solution could be diluted with fresh water. Thus, if the stock
solution was diluted with 4 parts fresh water and 1 part 5%
chlorine solution, the new stock solution would have 1% available
chlorine, assuming that the additional water did not tie up any
of the available chlorine. From Table 1, the new injection rate
would be 80% of 3.3 gph which equals 2.6 gph.
The sources of chlorine used to treat water for
microorganisms include chlorine gas, powder or tablets of calcium
hypochlorite (pool bleach), and liquid sodium hypochlorite
(laundry bleach). However, only liquid sodium hypochlorite is
labeled for use in irrigation systems in Florida. The
concentration of available chlorine ranges from 5-15% in liquid
sodium hypochlorite. Therefore, the amounts of these products
used to treat water will be very different. The user should
check with the chlorine supplier to ensure that the material is
labeled for injection into irrigation systems. In addition,
safety and proper backflow prevention are always required when
injecting materials into an irrigation system.
Clark, G. A., A. G. Smajstrla, D. Z. Haman, and F. S. Zazueta.
1990. Injection of chemicals into irrigation systems: Rates,
volumes, and injection periods. Bulletin 250. Fla. Coop.
Ext. Ser. Univ. of Florida, Gainesville, FL.
Ford, H.W. 1979. The Use of Surface Water for Low Pressure
Irrigation Systems. Fruit Crops Mimeo Rep. FC79-1 Univ. of
Ford, H.W. 1979. The Present Status of Research on Slimes of
Sulfur in Low Pressure Irrigation Systems and Filters.
Fruit Crops Mimeo Rep. FC79-2. Univ. of Fla., Gainesville.
Ford, H.W. 1979. The Present Status of Research on Iron Deposits
in Low Pressure Irrigation Systems. Fruit Crops Mimeo Rep.
FC79-3. Univ. of Fla., Gainesville.
Ingram, D., and B. Hoadley. 1986. Chemical Injection for Drip
Irrigation in the Woody Ornamental Nursery. Ornamental
Horticulture Commercial Fact Sheet OHC-6. Fla. Coop. Ext.
Ser. Univ. of Florida, Gainesville, FL.
Kovach, S.P. 1984. Injection of Fertilizers into Drip Irrigation
Systems for Vegetables. Circular 606, Fla. Coop. Ext. Ser.,
Univ. of Florida, Gainesville, FL.
Nakayama, F.S., and D.A. Bucks. 1986. Trickle Irrigation for Crop
Production: Design, Operation, and Management. Elsevier.
Amsterdam. 383 p.
Pitts, D.J., D.Z. Haman, and A.G. Smajstrla. 1990. Causes and
Prevention of Emitter Plugging in Micro Irrigation Systems.
Bul. 258. Fla. Coop. Ext. Ser., Univ. of Fla., Gainesville.
Smajstrla, A.G., D.S. Harrison, W.J. Becker, F.S. Zazueta, and
D.Z. Haman. 1985. Backflow Prevention Requirements for
Florida Irrigation Systems. Bulletin 217. Fla. Coop. Ext.
Ser., Univ. of Florida, Gainesville, FL.
Smajstrla, A.G., D.Z. Haman, and F.S. Zazueta. 1986. Chemical
Injection (Chemigation): Methods and Calibration. Agric.
Engr. Ext. Report 85-22 (revised). Fla. Coop. Ext. Ser.,
Univ. of Florida, Gainesville, FL.
Yeager, T.H. 1986. Fertigation Management for the Wholesale
Container Nursery. Bulletin 231. Fla. Coop. Ext. Ser., Univ.
of Florida, Gainesville, FL.
Yeager, T.H. and R.W. Henley. 1987. Techniques of Diluting
Solution Fertilizers in Commercial Nurseries and
Greenhouses. Circular 695. Fla. Coop. Ext. Ser., Univ. of
Florida, Gainesville, FL.
Liquid chlorine (sodium hypochlorite) injection rates
in gallons per hour (gph) per 100 gallons per minute
(gpm) of irrigation system flow rate for different
levels of stock solution concentrations of available
chlorine (%) and the desired water treatment level
Treatment Concentration of available chlorine in stock solution
1 2 3 4 5 10 15
(gph of injection per 100 gpm of irrigation flow rate)
2 1.1 0.6 0.4 0.3 0.2 0.14 0.09
4 2.2 1.1 0.7 0.6 0.4 0.22 0.15
6 3.3 1.7 1.1 0.8 0.7 0.3 0.2
8 4.4 2.2 1.5 1.1 0.9 0.4 0.3
10 5.5 2.8 1.8 1.4 1.1 0.6 0.4
15 8.3 4.1 2.8 2.1 1.6 0.8 0.6
20 11.0 5.5 3.7 2.8 2.2 1.1 0.7
25 13.8 6.9 4.6 3.4 2.8 1.4 0.9
30 16.5 8.3 5.5 4.1 3.3 1.6 1.1
40 22.0 11.0 7.3 5.5 4.4 2.2 1.5
50 27.5 13.8 9.2 6.9 5.5 2.8 1.8
75 20.6 13.8 10.3 8.3 4.1 2.8
100 27.5 18.3 13.8 11.0 5.5 3.7
150 27.5 20.6 16.5 8.3 5.5
200 27.5 22.0 11.0 7.3
* These are commercially available concentrations.
Other concentrations are obtained by diluting with water.