MAINTAINING DRIP IRRIGATION SYSTEMS TO PREVENT CLOGGING
Franklin Laemmlen
 Applying irrigation water via tape is being used by more and more growers on the Central Coast. The following article was recently published by Blaine Hanson, Irrigation and Drainage Specialist at UC Davis. The article is an excellent review for the "old pros" of drip irrigation as well as a must read for "new users" of drip irrigation technology.
Drip irrigation systems have the potential of applying water uniformly and precisely. However, achieving this potential requires good system maintenance to prevent clogging of emitters.
Causes of clogging include:
- Suspended materials in irrigation water
- Chemical precipitation caused by:
Irrigation water quality, fertilizer injection, or evaporation in emitters
- Biological growth in drip lines and other pipelines
- Root intrusion (buried laterals)
- Soil ingestion (buried laterals)
SUSPENDED MATERIALS
Sand, silt, plant material, algae, fish, and other materials suspended in the irrigation water must be removed before water enters the drip line. Removal of these materials is accomplished by filtering the water. Any particles larger than about one-tenth the size of the emitter orifice or flow passage should be filtered out to prevent particles from bridging.
The degree of filtration is expressed as the mesh size or equivalent mesh size. The mesh size is the number of openings per inch. A 200-mesh filter has 200 openings per inch. A common recommendation for filtration is a 200-mesh filter.
Two sets of filters are recommended. The primary filters remove suspended material from water. A secondary filter installed downstream from the primary filter is recommended to protect the irrigation systems if the primary filter should fail.
Generally, sand-media filters are used for surface water containing algae and other suspended organic matter, while screen/disc filters are used for ground water, which may contain suspended sand, silt, and clay. Screen/disc filters can be used for surface water containing suspended organic matter, but they may require frequent backflushing compared to sand-media filters.
Filters remove some but not all of the suspended matter. Fine particles of algae, silt, and clay can pass through filters and settle in the drip lines. So a periodic flushing should be done to prevent excessive accumulation of settled-out material along the lower part of drip lines.
CHEMICAL PRECIPITATION
Precipitation of calcium carbonate (a white precipitate) and iron (a reddish-brown precipitate) in emitters is a problem with ground water. Iron precipitation is not a usual problem in the Central Valley, but does occur in some coastal areas.
Calcium carbonate precipitation can occur in emitters due to changes in the pH of the water (sometimes a result of pumping ground water), temperature increases as the water flows throughout the irrigation system, evaporation, and chemical reactions with some types of fertilizers.
The potential for carbonate-clogging exists when the bicarbonate concentration in the water exceeds 2 meq per liter and pH exceeds 7.5, based both on theoretical approaches and on the experience of the drip irrigation industry. Recommended treatments to prevent carbonate clogging are:
- Inject acid continuously to maintain the pH between 5 and 7.
- Inject acid intermittently to maintain the pH at 4 or less for 30-60 minutes.
BIOLOGICAL GROWTH
Microorganisms, such as algae, bacteria, and fungi can grow in drip lines and cause clogging problems. Chlorine is often added to irrigation water to oxidize and destroy biological microorganisms.
Dissolving chlorine in water produces hypochlorous acid and hypochlorite — referred to collectively as the free available chlorine. Hypochlorous acid, however, is by far the most effective chlorine form for controlling bacteria, algae, etc. The amount of hypochlorus acid depends on the pH of the water. The lower the pH, the more the hypochlorous acid. Maintaining the pH at about 7 results in about 70 percent of the chlorine being in the form of hypochlorous acid.
Acid may need to be injected at the same time as the chlorine injection for pH control. Use separate injection ports for each chemical. CAUTION: Do not mix chlorine compounds and acid in the same container unless you want to die. The most common sources of chlorine are sodium hypochlorite (liquid), calcium hypochlorite (powder or granules), and chlorine gas.
ROOT INTRUSION
Root intrusion can be a serious problem for drip systems. Plant roots tend to grow in the soil areas with the highest water content, which in subsurface drip systems is near the emitters. Because of this tendency, roots can grow into and clog emitters.
Various strategies have been used to combat root intrusion, including water management, chemical treatment, and drip tape emitter design, but very little research has been conducted to evaluate their relative effectiveness. Some practices used by growers to prevent or correct root intrusion problems include:
Weekly acid injections to discourage root intrusion by lowering the pH of the irrigation water to less than 4 (which may damage some irrigation system components), and
Superchlorination (injecting chlorine at very high concentrations, such as 100 to 400 ppm after harvesting).
DOES IT MAKE A DIFFERENCE?
Does maintaining a drip system make a difference? Emitter discharge measurements made along a buried drip line showed little emitter clogging for a system with periodic maintenance after three years of use (see Fig. 1). (Note: The trend of decreasing discharge rate with distance was caused by pressure losses along the drip line.) In a field, where root intrusion severely clogged emitters along the lower end of the field, yield was about 10 tons/acre less at the lower end compared to the upper end of the field.
Figure 1. Emitter discharge rates along a drip line after three years of use. Sampling locations were about 100 feet apart. At each sampling location, discharge rates of two emitters were measured. The distribution uniformity was 88% - considered good.
Information adapted from: Vegetable Notes, Special Edition #2, September 2002.
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