Southeastern U.S. Vegetable Crop Handbook - 2022 - 39

to keep drip systems from clogging. Further information on drip
irrigation systems can be obtained from manufacturers, dealers,
and your local Extension office.
Chlorination. Bacteria can grow inside drip irrigation tapes,
forming a slime that can clog emitters. Algae present in surface
waters can also clog emitters. Bacteria and algae can be effectively
controlled by chlorination of the drip system. Periodic
treatment before clogs develop can keep the system functioning
efficiently. The frequency of treatments depends on the quality
of the water source. Generally, two or three treatments per season
are adequate. Irrigation water containing high concentrations
of iron (greater than 1 ppm) can also cause clogging problems
due to a type of bacteria that " feeds " on iron. In consuming the
dissolved (ferrous) form of iron, the bacteria secrete a slime
called ochre, which may combine with other solid particles in
the drip tape and plug emitters. The precipitated (ferric) form of
iron, known commonly as rust, can also physically clog emitters.
In treating water containing iron, chlorine will oxidize the iron
dissolved in water, causing the iron to precipitate so that it can
be filtered and removed from the system.
Chlorine treatment should take place upstream of filters
to remove the precipitated iron and microorganisms from the
system. Chlorine is available as a gas, liquid, or solid. Chlorine
gas is extremely dangerous, and caution should be exercised if
this method of treatment is chosen. Solid chlorine is available as
granules or tablets containing 65% to 70% calcium hypochloTABLE
11. HOURS REQUIRED TO APPLY 1 " WATER
BASED ON ROW SPACING.
Drip Tube Flow Rate
gph/100 ft.
11.4
13.2
20.4
27.0
40.2
80.4
gpm/100 ft.
0.19
0.22
0.34
0.45
0.67
1.34
4
21.9
18.9
12.2
9.2
6.2
3.1
Row Spacing (Ft.)
6
32.8
28.3
18.3
13.9
9.3
4.7
5
27.3
23.6
15.3
11.5
7.8
3.9
8
43.7
37.8
24.4
18.5
12.4
6.2
10
54.7
47.2
30.6
23.1
15.5
7.8
rite but might react with other elements in irrigation water to
form precipitates which could clog emitters. Liquid chlorine is
available in many forms, including household bleach (sodium
hypochlorite), and is the easiest and often safest form to use for
injection. Stock solutions can be bought that have concentrations
of 5.25%, 10%, or 15% available chlorine. Use chlorine only if
the product is labeled for use in irrigation systems.
Since chlorination is most effective at pH 6.5 to 7.5, some
commercial chlorination equipment also injects buffers to maintain
optimum pH for effective kill of microorganisms. This type
of equipment is more expensive, but more effective than simply
injecting sodium hypochlorite solution.
The required rate of chlorine injection is dependent on the
concentration of microorganisms present in the water source, the
amount of iron in the irrigation water, and the method of treatment
being used. To remove iron from irrigation water, start by
injecting 1 ppm of chlorine for each 1 ppm of iron present in the
water. For iron removal, chlorine should be injected continuously.
Adequate mixing of the water with chlorine is essential. For
this reason, be certain to mount the chlorine injector a distance
upstream from filters. An elbow between the injector and the filter
will ensure adequate mixing.
For treatment of algae and bacteria, a chlorine injection rate
that results in the presence of 1 to 2 ppm of " free " chlorine at the
end of the furthest lateral will assure that the proper amount of
chlorine is being injected. Free, or residual, chlorine can be tested
using an inexpensive DPD (diethyl-phenylene-diamine) test
kit. A swimming pool test kit can be used, but only if it measures
free chlorine. Many pool test kits only measure total chlorine.
If a chlorine test kit is unavailable, one of the following
schemes is suggested as a starting point:
For iron treatment:
* Inject liquid sodium hypochlorite continuously at a rate of
1 ppm for each 1 ppm of iron in irrigation water. In most
cases, 3 to 5 ppm is sufficient.
For bacteria and algae treatment:
* Inject liquid sodium hypochlorite continuously at a rate of
5 to 10 ppm where the biological load is high.
TABLE 12. MAXIMUM APPLICATION TIME IN MINUTES
FOR DRIP IRRIGATED VEGETABLES.
Available Water Holding
Capacity1
Inch Of
Water/Inch Soil Depth
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
Drip Tubing Flow Rate (gpm/100 ft.)
0.2
0.3
20
41
61
82
102
122
143
163
183
14
27
41
54
68
82
95
0.4
0.5
8
0.6
Maximum Number of Minutes
per Application2
10
109
122
20
31
41
51
61
71
82
92
16
24
33
41
49
57
65
73
Assumes a 10-inch deep root zone and irrigation at 25% soil moisture
depletion.
7
14
20
27
34
41
48
54
61
1 Refer to Table 9 for Available Water Holding Capacity based on soil texture.
2
* Inject 10 to 20 ppm during the last 30 minutes of each irrigation
cycle where the biological load is medium.
* Inject 50 ppm during the last 30 minutes of irrigation cycles
two times each month when biological load is low.
* Superchlorinate (inject at a rate of 200 to 500 ppm) once
per month for the length of time required to fill the entire
system with this solution and shut down the system. After
24 hours, open the laterals and flush the lines.
The injection rates for stock solutions that contain 5.25%, 10%
and 15% can be calculated from the following equations:
FOR 5.25% STOCK SOLUTION:
Injection rate of chlorine, gph = [(Desired available chlorination
level, ppm) x (Irrigation flow rate, gpm)] divided by
875.
2022 Vegetable Crop Handbook for Southeastern United States
39
Southeastern U.S. Vegetable Crop Handbook - 2022

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