Calcium Carbonate Clogging of Irrigation Emitters

 

A crucial aspect to microsprinkler and dripper performance is maintaining the size of the orifice as it was delivered from the factory. Even small changes in the size of the orifice can have significant effects on the volume of water distributed in the orchard.  One of the most common causes of a decrease in orifice size or even clogging is a result of the high lime content of our waters.  Calcium carbonate precipitation can readily be observed by the whitish deposits that form on emitters and microsprinklers.  With the drought it's important to make that water go farther.

Reasons for carbonate precipitation include the following:

            1.  Change in the pH of the groundwater.  This can occur when groundwater is pumped.  Pumping reduces the pressure of the groundwater as it flows into the well.  This reduction releases dissolved carbon dioxide gas causing the pH of the groundwater to increase.  The pH increase can result in carbonate precipitation.

            2. Evaporation of water in the dripper or microsprinkler.  Evaporation increases the concentration of chemicals dissolved in the water remaining in the emitter.  Because of its low solubility in water, calcium carbonate readily precipitates during evaporation.

            3.  Increase in water temperature.  The solubility of calcium carbonate is reduced as water temperature increases.

            4,  Injection of certain chemicals, such as bleach or some fertilizers that interact with the water.

The problem of lime precipitation depends primarily on the pH of the water. At pH values less than 6, mostly dissolved carbon dioxide and a small amount of carbonic acid exist in the water.  At pH values between about 6.5 and 10, bicarbonate is the dominant species.  When water evaporates from the irrigation system the bicarbonate precipitates as lime if there is adequate calcium in the water.  The potential for carbonate clogging is highest when bicarbonate concentration in the water exceeds 2 milliequivalents per liter (meq/L) and the pH exceeds 7.5.

This relationship of bicarbonate to water pH indicates that lowering the pH will prevent or reduce carbonate clogging of the system. Lowering the pH, dissolves any existing carbonate precipitation and prevents the formation of lime deposits.

A water's pH is lowered by injecting acids. The common acids, such as sulfuric and hydrochloric (muriatic) have been used, as well as the more expensive citric and nitric acids.  An acid/fertilizer compound of a combination of urea/sulfuric acid (N-pHURIC ®) has proven to be useful and much safer than straight acids.  This acidifying material is most commonly used for water treatment rather than as a source of nitrogen.  If the material is used in any significant amount, its nitrogen contribution to the fertilizer program needs to be considered.

Determining frequency and amounts of acid to prevent clogging can be fairly matter of fact. Depending on the rate at which carbonate precipitation occurs, acid injection may only need to occur intermittently during the irrigation cycle to.  It might only require 30-60 minutes to maintain a pH of 4.  With more problematic waters, continuous acid injection to maintain a pH between 5 and 7 may be necessary.

The amount of acid needed to lower the pH to a desired level depends on the bicarbonate/carbonate concentration in the water and the target pH. The water can be sent to a lab for determining the acid amount or a trial and error approach can be used.  Acid can be added to water in increments while measuring the pH until the desired pH is reached.  Water pH can be measured with pH test strips or a hand held pH meter.  Test kits are also readily available at swimming pool supply stores. 

Other than acid for correcting lime clogging, there are several other amendments being sold on the market. Sodium hexametaphosphate has also been used and works against iron and manganese clogging.  The small amount of sodium is not a problem. It is safer than an acid, but costs a bit more.

emitter clogging

Chemical Treatment to Prevent System Clogging

Chemical treatment of water for microirrigation systems is required when the water may cause chemical precipitate or biological clogging of the microirrigation drippers or microsprinklers. The chemical treatment varies depending on the clogging source.

Biological Clogging

Biological clogging problems are most often associated with surface waters—waters that have been stored in reservoirs or ponds, or transported in canals, rivers, etc. While it is often difficult to identify the biological contaminant, algae and biological slimes are often major contributors to biological clogging.

Groundwaters high in iron may also be a biological clogging hazard for microirrigation systems. The dissolved iron in the water provides an energy source for the iron bacteria. The gelatinous slime produced by the iron bacteria can clog emitters, often in conjunction with particulates (silt or clay particles, chemical precipitates, or other contaminants) for which they can provide a “glue” to bind particles together.

Levels of Concern

Certainly any waters that appear “green” prior to use are capable of causing biological clogging but even surface waters which appear clean may be a clogging hazard. Since surface water quality can change drastically across the season, often caused by rising temperatures and falling water levels, it is often not worthwhile to attempt to quantify the biological clogging hazard. It is better to monitor the microirrigation system for any sign of biological clogging and if it appears, treat the water. Often there is a history of biological clogging problems and the manager knows that treatment is required.

Treatment

Biological treatment methods involve using a biocide that kills the biological contaminant. The two most common biocides used with microirrigation systems are chlorine and copper. Historically, chlorine products have been most frequently injected into microirrigation systems while copper products have been used to control biological growth in ponds and reservoirs. This has changed somewhat with the availability of new copper-based formulations developed for injection into microirrigation systems.

The following are recommended levels of chlorine for biological contaminant control:

Injection Method and chlorine concentration at the end of the last lateral

Continuous injection 1-2 ppm

Periodic injection 10-20 ppm

 

Contact time between the water with chlorine and biological contaminant is important. Periodic chlorine injections should be at least 4 hours and longer is better. Chlorine injections can continue up to system shutdown, with the chlorinated water left sitting in the lines. This may have limited effect on above-ground lines since they tend to drain out at the lowest point(s), but it may help clean up other parts of the system.

Copper levels to provide effective biocide protection are also quite low, often copper levels less than 5 pm are effective. Follow manufacturer’s recommendation for formulations containing copper.

Chemical Precipitate Clogging

Most chemical precipitate clogging problems are associated with groundwater sources. Elements in solution in the groundwater may precipitate above ground and the precipitates may clog the microirrigation system’s small emitter passageways.

There are many potential chemical precipitates which can cause clogging problems, but calcium carbonate (lime) and iron are two of the more common problems. Lime precipitation is the most common and can occurwhen calcium and bicarbonate levels in the water are 2 meq/l or higher and the water pH is 7.5 or higher. 

The most common treatment for lime precipitation clogging is to lower water pH to 7.0 or below. A pH in the range of 6 to 6.5 is effective in removing the calcium carbonate precipitate while not being of risk to system components. 

Iron precipitate clogging is not as common as lime precipitation but it is more difficult to deal with. Iron precipitate clogging can occur when the iron levels are 0.2 ppm or higher, although most problems occur when iron levels are 1 ppm or higher. Water pH only needs to be 4.0 or higher for iron precipitation so this pH level includes nearly all waters.

Most people deal with iron precipitation problems by pumping the groundwater into a pond or reservoir where the iron precipitates and settles out. Adequate time is needed for the small precipitates to settle. This dictates an adequately sized pond. 

A relatively new way of dealing with iron and calcium carbonate precipitation problems is to continually inject a product containing phosphonate or phosphonic acid. The phosphonate (or phosphonic acid), injected at rates of 5 ppm or less, interferes with the precipitation. There are a number of anti-clogging formulations on the market which contain phosphonate or phosphonic acid as their active ingredient. Phosphonate or phosphonic acid products may be very beneficial for iron clogging problems, for which only aeration/precipitation and settling are currently the only solutions.

S-AV-CULT-IR[1]

Microirrigation, Fertilizers and Clogging

Microirrigation (drip, microsprinkler, fan jet) applies water through small openings and can easily be prone to clogging. For this reason, filtration is used to avoid introducing sediments into the system and all fertilizers are injected before the filters to avoid their clogging the system. Before introducing a mixture of fertilizers into an untried system a jar test should be performed to make sure there are no chemical interactions between the irrigation water and anything that is introduced that might cause precipitation and eventual clogging. Depending on the size of emitters openings, some types are more or less prone to clogging. Anytime you use a new brand of fertilizer, make sure you do a jar test, because there have many problems in the past of just reading the label, seeing that it is soluble and then finding out to one’s horror that every emitters is clogged. For prevention of chemical and biological clogging see the article by Schwankl in this newsletter.

Microirrigation systems work best with pre-solubilized, liquid fertilizer solutions. In season application of dry fertilizers over the top of micro systems is very inefficient since only a fraction of the soil surface receives water necessary to solubilize the dry material. Very finally ground materials, such as gypsum or potassium sulfate can be suspended in solution by injectors if the materials meet the specifications of the irrigation system.  The injectors continuously agitate the materials in the irrigation water to prevent settling out. The irrigation systems should be flushed after every use to prevent the materials from settling the ends of irrigation lines after the system is turned off. 

Microirrigation can supply small, frequent does of nutrients throughout the growing season. Plant roots proliferate in the emitter wetted area which makes for a more active zone for taking up nutrients. Many growers have found that nitrogen fertilizer rates can be reduced due to the increased efficiency of uptake. Nutrients that require a larger root system than the microirrigation wetted pattern might need more frequent application than under sprinkler or furrow, such as potassium or micronutrients. Leaf tissue testing is a helpful too to adjust fertilizer applications, especially with a new system.

All pressurized irrigation systems require a certain amount of time to fill all laterals with water and achieve operating pressure. Injected fertilizer also requires a certain amount of time to distribute throughout the irrigation system. The ideal time to inject fertilizer is in the middle of the irrigation set. If injection takes place before the system is fully pressurized, there is a lack of uniformity if fertilizer placement (see http://cesantaclara.ucdavis.edu/files/19603.pdf). If the irrigations system is shut down before the fertilizer is fully distributed, fertilizer remains in the laterals, encouraging microbial growth that can lead to plugging. During long irrigation sets, soil mobile nutrients, such as nitrate-nitrogen should be applied near the end of the, while still allowing adequate time for system flushing.

And the mantra with all microsystems to avoid clogging is flush, flush, flush.

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