The following are symptoms of iron and manganese fouling:
- Discoloration of membranes.
- Poor salt rejection on individual membrane test data.
- Low flow on individual membrane test data.
- Possible high pressure differential on individual membrane test data.
- High iron or manganese values reported in feedwater.
- High pressure differential reported on first array.
The recommended cleaning approach is a low pH clean (RoClean P703) followed by a high pH clean (RoClean P111 or L211). Results can be further improved by heating the cleaning solution to the maximum allowed by the membrane manufacturer.
Steps to preventing iron and manganese fouling include:
- Conduct a complete and accurate water analysis.
- If the water supply contains a high amount of ferrous iron, it is important to prevent it from oxidizing. Oxidation can be caused by exposure to chlorine or aeration of the water (which occurs in the multimedia filter or other similar equipment).
- Iron and manganese can be removed using a greensand filter regenerated with potassium permanganate. However, if potassium permanganate is overdose or is not properly rinsed from the greensand filter, it will oxidize the membrane surface.
- Iron can be removed by chlorinating the water then removing the oxidized iron or manganese with a multimedia filter.
Additional Notes:
Iron in water can be found as ferrous or ferric. Ferrous iron is dissolved iron that has not precipitated. Exposure to air turns ferrous iron into ferric (oxidized) iron, which essentially, has become rust.
Ferrous iron can exist in relatively high levels and not precipitate so long as it is not oxidized. It becomes a filtration issue if it becomes oxidized. Ferric iron fouling can be removed using RoClean P703.
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