Iron in a soluble form is commonly found in groundwater, the result of the water’s contact with rocks and other aquifer material that contain iron. The soluble iron, called ferrous iron, is soluble in water and does not cause emitter clogging in microirrigation systems. Under certain conditions, however, the soluble iron can be oxidized, or converted to insoluble iron, (ferric iron). This iron precipitates out of the water as an iron oxide, a reddish brown deposit. Concentrations as low as 0.2 ppm can clog a drip irrigation system.
- Pumping of groundwater lowers the water pressure in the well, releasing dissolved carbon dioxide gas and raising the water's pH.
- Aeration of the groundwater can occur if water cascades in the well or if there are leaks on the suction side of a centrifugal pump used for pumping shallow groundwater. Aeration causes ferrous iron to oxidize into ferric iron. At a pH of less than about 5, the oxidation rate is much slower than at a higher pH, so little ferric iron is formed. Ferric iron is insoluble in water that is alkaline or weakly acidic. Aerated water with a pH between 7 and 8.5 contains mostly insoluble ferric iron.
- Frequently, bacteria are found when iron is present in the water. The source of the bacteria may be unclear, but it may be the result of well contamination during drilling. These bacteria get their energy by converting soluble ferrous iron to insoluble ferric iron. This process results in iron precipitation along with the formation of slime inside pipelines, drip lines, and emitters.
Sampling for Iron
If the water is to be analyzed for iron, the sample must be acidified to a pH of 4 or less or the ferrous iron in solution will precipitate as ferric iron before it can reach the lab. If this happens, the water analysis will show little iron in solution. Thus, if iron and other dissolved constituents are to be determined, you must collect two samples: an acidified sample to test for iron and an unacidified sample to test for the remaining constituents in solution. The laboratory can provide a sample bottle with the correct amount of acid in it that you can use when gathering the iron sample. Do not rinse the bottle before you collect the sample.
Manganese resembles iron in its chemical behavior, but it is less abundant than iron. It can occur as a soluble manganous manganese or manganese bicarbonate, which changes to an insoluble manganese hydroxide when it reacts with atmospheric oxygen. Manganese hydroxide precipitates as a black deposit. The oxidation rate of manganese is very slow at a pH less than 9.5. Since most irrigation water has a pH much lower than 9.5, manganese clogging is less of a problem than iron clogging. However, manganese precipitation can cause clogging even at concentrations as low as 0.1 ppm.
Water Treatment to Prevent Clogging Problems
Once iron or manganese has precipitated, it is very difficult to dissolve the precipitate. The focus should be on treating the irrigation water to precipitate the chemicals before you use the water in a microirrigation system or on preventing their precipitation. Recommended treatment methods are as follows.
Aerate the water to oxidize the iron or manganese, then allow the precipitate to settle out before you use the water for irrigation. This method requires using a reservoir or settling basin with a flow-through water velocity less than 1 foot per second. Chlorine or copper compounds may be needed to control algae growth in the reservoir or basin. Aeration may be the only practical method for dealing with high iron concentrations. It causes oxygen to dissolve in the water, which changes the iron or manganese to the insoluble forms; it releases dissolved carbon dioxide gas, increasing the pH of the water; and it releases or partially releases other dissolved gases such as hydrogen sulfide.
The oxidation rate under aeration depends on the pH of the water and the length of contact time. At a pH of 5, the oxidation rate of iron is slow, and little of the ferrous iron is oxidized. At a pH of 7, almost all of the ferrous iron can be oxidized by air after only 15 minutes of contact time.
For manganese, the pH must be greater than 9.5 for any substantial oxidation to occur.
You can inject chlorine into the water to oxidize the iron or manganese and cause precipitation. Injection must occur upstream of the filter to remove the precipitate. The rate at which the chlorine is mixed in the water is also a factor in the process. Centrifugal sand separators installed immediately downstream from the injection point can increase the mixing of chlorine in the water. Some rules of thumb are as follows:
- Inject chlorine at a concentration of 0.64 times the ferrous or soluble iron concentration in order to maintain 1 ppm of free residual chlorine at the end of the drip line. Chlorine oxidizes the iron at a lower pH than does aeration. For practical purposes, however, a pH of about 6.5 to 7.5 is recommended for chlorine oxidation.
- Inject chlorine at a concentration of 1.3 times the soluble manganese concentration. The pH must be at least 9.5 for any appreciable oxidation to occur.
After you inject chlorine to cause iron precipitation, discharge the water into a reservoir where the iron can settle out or use a sand media filter to remove the precipitated iron.
Continuously inject acid to decrease the pH to 4 or less in order to prevent precipitation. At a low pH, the oxidation rate of ferrous iron to ferric iron is very slow. Acid must be injected into the water before the iron or manganese changes to the insoluble form. Few problems with clogging have been reported for iron concentrations of 2 to 4 ppm at a pH of 5 to 5.5. This practice, however, can be expensive and may damage the irrigation system.
Inject an inhibitor that prevents iron or manganese precipitation. Compounds such as polymeric acid, polyphosphate, and phosphonic acid have been used as inhibitors. These compounds must be injected prior to any oxidation of ferrous iron to ferric iron, and they do not work if ferric iron has already formed. The injection rate for the inhibitors is usually less than 5 ppm.
In municipal water treatment, a polyphosphate such as sodium hexametaphosphate is added to the water before the iron is oxidized, thus preventing flocculation of the small individual particles. Recommended injection rates are 2 mg/l sodium hexametaphosphate for each 1 mg/l iron or manganese. Since chelation is expensive it should be used in agricultural systems only after careful evaluation.
Chlorine is recommended for controlling iron bacteria. Inject chlorine continuously to maintain a free chlorine concentration of 1 ppm or inject periodically for 1 to 2 hours at a concentration of 10 to 20 ppm. Also, inject chlorine into the well to control any bacteria that may be growing there.
Iron Sulfide and Hydrogen Sulfide
Iron sulfide, which is very insoluble even in acid solutions, can form a black precipitate when the hydrogen sulfide concentration in the water is greater than 0.2 ppm. The methods for precipitating iron sulfide before the water reaches an irrigation system are the same as for iron oxide precipitation. For hydrogen sulfide, inject chlorine at a concentration of 4 to 8 times the hydrogen sulfide concentration.
Barnes, D., and F. Wilson. 1983. Chemistry and unit operation in water treatment. London and New York: Applied Science Publishers.
Keller, J., and R. D. Bliesner. 1990. Sprinkle and trickle irrigation. New York: Van Nostrand Reinhold.
Nordell, E. 1961. Water treatment for industrial and other uses. New York: Reinhold.