A slime invasion might make for a good horror movie, but no one wants to find one in their drip emitters.

Iron-loving slime bacteria can cause severe clogging in drip irrigation systems. Iron bacteria is a filamentous amorphous gelatinous type of reddish-brown slime. When the bacteria, the most common is Gallionella Sp., reacts with iron through an oxidation process, iron is changed to an insoluble form. In this form, the iron is surrounded by the filamentous bacteria colonies creating the sticky iron slime that is responsible for clogging the dripper.

Sarge Green, interim director of Center for Irrigation Technology at Fresno State University, said the slime/iron problem that is causing problems in drip irrigation systems might be caused by water quality or the age of the well. It is likely that on the west side of the San Joaquin Valley, where wells are pumping water from historic depths, iron in the water is easily oxidized and converted by the bacteria species. Older wells are more common the east side of the valley, and the well casings may be the source of iron in the water as casings degrade over time.

Dropping the bowls and pumping water from deeper depths can make the slime problem worse, Green said. Cleaning the well can help, but that can be an expensive option. Chlorination can kill the bacteria but over time it will return.

According to the irrigation system company Netafim, concentrations of ferrous iron as low as 0.15 to 0.22 parts per million (ppm) are considered as a potential hazard to drip systems. Concentrations above 1.5 are considered severe. Any water system that contains concentrations higher than 0.5 ppm iron cannot be used in drip systems unless it is treated chemically to dissolve the bacteria.

Recently retired University of California Cooperative Extension irrigation specialist Blake Sanden said the worst clogging he encountered was southwest of Buttonwillow where the water turned the reservoir red. He said the pistachio grower at the site tried aeration- which made the rust-colored reservoir, and acidification. In the end, he said the best tactic to stall off clogging is to finish the irrigation with fresh, non-iron canal water to flush out the high iron well water.

Sanden said the worst clogging issues in drip systems occur when iron-loving slime bacteria invade a well and irrigation system to create long strings of slime that even clog the louvers in the well casing.

University of California (UC) on maintaining micro irrigation systems provides guidelines for predicting if there is a need to treat for biological clogging.

Visual evidence of organic contamination, such as greenish color to the water or algae indicates water treatment may be necessary to prevent biological clogging. Filters in the system should be monitored and flush water should be filtered for evidence of a build up of organic contaminants. Sand media filters should be watched for build up of organic matter. Periodic biocide treatments of the filters via the access port on each of the media tanks can be made.

There is also the simple diagnostic step of running a finger around the inside of a drip tubing at the head or tail end of lateral lines to see if they feel slimy.

Treatment Options

Netafim offers two basic treatment options. The first is stabilization or precipitation inhibitors. The stabilization treatments keep the ferrous iron in solution by chelating it with sequestration agents (poly phosphates and phosphonate.)

The other option is oxidation, sedimentation and filtration. This treatment oxidizes the soluble or invisible ferrous iron into the insoluble or visible ferric iron. It will then precipitate so it can be physically separated from the water by filtration.

According to the UC micro irrigation management program, treating irrigation water with a biocide will minimize the growth of organic contaminants and often destroy the organic material, minimizing the clogging hazard.

Three biocide methods are commonly used to treat irrigation water.  In addition, use of acid is sometimes recommended for biological clogging problems. There are also a number of water treatment devices on the market that claim to control biological clogging problems.

Chlorine is often added to irrigation water to oxidize and destroy biological microorganisms such as algae, fungi, and bacteria.

Forms of Chlorine

Common sources of chlorine are chlorine gas, sodium hypochlorite (a liquid), and calcium hypochlorite (a powder or granules).

Adding chlorine to water produces mainly hypochlorous acid and hypochlorite, both referred to collectively as free available chlorine. Hypochlorous acid is the most effective agent for controlling biological growths. Its concentration depends on the pH of the water. Maintaining a pH of 7 or less means that at least 75 percent of the chlorine in the water is hypochlorous acid, while at a pH of 8 only about 25 percent of the chlorine is hypochlorous acid. At a pH of less than 3, chlorine gas predominates.

While using chlorine gas is generally considered the least expensive method of injecting chlorine, it is the most hazardous and requires extensive safety precautions. Trained personnel are needed for installing and using chlorine gas injection systems.

Sodium Hypochlorite

Sodium hypochlorite (liquid bleach) is usually available with up to 15 percent available chlorine. Household bleach is sodium hypochlorite with 5.25 percent available chlorine. Most liquid fertilizer injection equipment is capable of injecting liquid chlorine. If the injection point is downstream of the filters, it may be necessary to manually treat the filters with chlorine.

Adding sodium hypochlorite to water produces hydroxyl ions, which raises the pH of the water and in doing so may decrease the effectiveness of chlorination. Acid injection may be necessary to reduce the pH and increase the chlorine’s effectiveness.

Calcium Hypochlorite

Calcium hypochlorite normally contains 65 to 70 percent available chlorine. Note that 12.8 pounds of calcium hypochlorite dissolved in 100 gallons of water forms a 1 percent chlorine solution. A 2 percent chlorine solution therefore requires adding 25.6 pounds of calcium hypochlorite to 100 gallons of water. Any chlorine stock solution can be mixed following the same pattern. Use caution when dissolving calcium hypochlorite in water because of the possible formation of chlorine gas.

Desired Chlorine Concentrations

Continuous injection of chlorine should be used if the irrigation water has high levels of algae and bacteria, and biological clogging is a serious problem. The recommended level of free chlorine is 1 to 2 ppm at the end of the irrigation system. It is important to check the concentration at the end of the lateral line since chlorine is consumed when it reacts with organic constituents and any iron and manganese in the water. The chlorine concentration can be determined with a good quality swimming pool or spa chlorine test.

Super-chlorination (injecting chlorine at high concentrations) is recommended for reclaiming drip irrigation systems clogged by algae and bacterial slimes. Super-chlorination requires special care to avoid damage to plants and irrigation equipment.