6. Additional Information About Iron, Manganese, And Sulfide
6.1. Iron and manganese
Emitter plugging from iron precipitates and iron-reducing bacteria is especially difficult to control. In some geographic areas, iron causes very serious scale formation and emitter plugging problems.
- Dissolved iron in irrigation water is usually caused by microbial activity.
- Iron and manganese concentrations as low as 0.2 ppm can cause a bacterial growth problem.
- Iron bacterial growth appears reddish, while manganese bacterial growth is blacker in color. These bacteria oxidize iron or manganese in the irrigation water.
- Iron precipitation and rapid bacterial growth create enough material to plug a micro-irrigation system in a few weeks.
- Iron bacteria are notoriously difficult to kill, partly because they may live in the irrigation well. Periodic acid or chlorine treatments of the well are sometimes effective.
- It is not clear if iron bacteria exist in groundwater before well construction and multiply as water is pumped, or if they get into the aquifer from the soil during well construction.
- Iron oxide can form without bacteria after an irrigation system has shut down. Contact between air and water left in the line causes iron to precipitate.
- Polyphosphates and polymaleic acid can effectively sequester iron and manganese so they remain suspended and move through the irrigation system.
- If the iron concentration is less than 3.0 ppm, water conditioning can be an effective, economical treatment.
- At higher iron concentrations, water conditioning will be costly and may be ineffective.
- Since chlorine is an oxidizer, it can precipitate iron and manganese, removing them from the irrigation water.
- Injecting chlorine gas to precipitate iron followed by filtering through fine media or discs has worked to remove iron (Bar, 1995). It is important to thoroughly mix the chlorine and water.
- Chlorine will also kill bacteria that oxidize iron and manganese, eliminating growth.
- Iron can be removed from well water by pumping it into a reservoir and aerating (oxidizing) it.
- Iron (and manganese) precipitate in the reservoir before the water is pumped for irrigation.
- Chlorination is still required since the reservoir water takes on characteristics of surface water.
- Dissolved iron and manganese in the presence of sulfides can form a black, insoluble precipitate. Iron greater than 0.6 ppm combined with sulfide greater than 2.0 ppm in the water creates iron sulfide sludge.
- Sulfide problems are associated with well water, and the well casing can become rapidly clogged.
- Wells that draw water from two formations, one high in sulfides and the other high in iron, are candidates to form iron sulfide sludge. Combine aeration, acidification, and chlorination to treat this problem.