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.