Iron & Manganese Removal

The following processes can remove iron & manganese:

Filter media can include sand, filter charcoal, and manganese greensand. Selection of the correct filter aide (nonionic, anionic, and cationic) and optimum dosage rates depend on extensive knowledge of the raw water chemistry.

aBsorption or aDsorption?

Absorption means to take into, such as a sponge takes up water within itself, but does not alter its own shape.
Adsorption means to retain on the surface of something, such as picking up sugar with a wetted finger. The only change to the finger is a coating of sugar. Only a certain amount of sugar can be picked up this way before the finger’s surface reaches its capacity.

Manganese dioxide has the ability to adsorb iron and manganese. How much depends on the manganese dioxide’s state of regeneration.

Adsorption of iron and manganese leaves both in an un-oxidized state. So, there is almost no particulate build-up within the filter bed. Manganese dioxide is the coating on manganese greensand. In some studies the reactive rating of the manganese dioxide coating on manganese greensand was determined to be about 75-80%.

Manganese dioxide ore called pyrolusite, mined from ground deposits, can be over 90% manganese dioxide with a reactive rating of 100%.

Manganese dioxide adsorption media (pyrolusite) has been the filter media of choice for iron and manganese removal in England for about 70 years. Biological Activity will very likely lead to the conclusion that a portion of the manganese is also affected by biological organisms.

BioFe and BioMn are the terms applied to iron or manganese that forms part of a bacteria colony (biofilm, slimes, etc.). Fe & Mn may be held captive in solution, be in the process of oxidation, in an oxidized form through intra-cellular or extra-cellular activity, or undergoing reduction from a particulate form back to a soluble form.  The presence of both ferric and ferrous iron, and iron bacteria will usually indicate the presence of colloidal iron as well. High iron residuals in the filtered water may be mostly colloidal material. Colloidal iron and manganese is usually defined as that material which is oxidized but which will pass a 0.1 micron filter membrane.

As biological colonies are stressed by strong oxidants (chlorine, potassium permanganate, ozone, etc.) colloidal iron and manganese are often released, showing up as elevated residuals in the filtered water.

If oxidant dosages are increased and residuals of iron and/or manganese also increase, it is likely BioFe and BioMn are undergoing oxidative changes and are responsible for the rising residuals.

Empirical data indicates colloidal manganese levels are usually considerably higher than colloidal iron levels in the same water. Unrestricted colloidal manganese and the colloidal manganese released by stressed bacteria colonies can account for most of the manganese residual in filtered water.

If all treatment factors are constant and iron and manganese residuals seem to rise and fall without explanation, the cyclic nature of bacteria colonies in which iron and manganese is captured, processed and released, may account for the variances.

Typically iron is filterable within a minute or so of coming into contact with an oxidant. If this quick reaction is slowed, it may be an indication the iron is forming part of an organic compound, usually certain organic acids. If oxidized water is sampled for ferrous iron over time and the oxidation to a filterable state requires extended time, the slowness to oxidize may be caused by organic compounds into which iron has been taken by ion exchange. The same slowness to oxidize or go to a filterable state applies to manganese that has been taken into an organic compound by ion exchange.  Send a sample of the well water to an independent lab for total organic carbon analysis. Empirical data indicates levels over 2 mg/L total organic carbons can result in elevated iron and/or manganese residuals.

If, after a filter has been at rest for some time (say, over an hour) and the odour of hydrogen sulphide (a.k.a. rotten egg gas) is present immediately after the filter is put back into service or put into a backwash cycle, organic compounds are present. This is just a warning flag that organic compounds are present and may signal an interference with oxidation and/or filtration of oxidized iron and/or manganese.

How can I tell if organic carbon compounds are having an adverse impact on my Fe & Mn removal process?

Tips:

The following conditions may indicate the presence of œiron bacteria and/or organic carbon compounds:

Iron Bacteria is present when:

1. Brown to black slimes are found in a toilet tank.

2. There is stringy material in a water sample.

3. There are tubercles present on the inside of steel filter vessels.

4. Well pumps begin to lose production capacity when still mechanically sound.

5. Chemical analysis shows both ferric and ferrous iron are present.

6. A positive reaction is shown using a BART

Reaction Test) for IRB (Iron Reducing Bacteria). This test was developed in Western Canada and is sold by Hach through PrairieChem. If the presence of iron bacteria is suspected, this test should be undertaken. It is inexpensive and a reliable indicator.

7. Many of the same species of bacteria commonly referred to as œiron bacteria also interact with manganese. If manganese exists in the same water being tested for iron bacteria, a positive BART

Oxidation followed by filtration. The oxidant used can be oxygen (from air), and/or chlorine, and/or potassium permanganate. Filter media is sand, or sand and coal. Achieving low residuals assumes almost all of the Fe & Mn is in solut ion, not part of a biological colony and not forming part of organic compounds.Oxidation followed by filtration and adsorption.

Often potassium permanganate is dosed, followed by filtration through manganese greensand, sometimes with a coal cap. This process works well if the Fe & Mn can be readily oxidized and/or adsorbed.Oxidation followed by applying a filter aide, then filtration.Oxidation of the iron only, filtration, then removal of the manganese by adsorption.

In-situ species determination often indicates this is a good process choice. It is all done in the same filter vessel. Adsorption of iron and manganese at low rates.

Typical rates ranges from 1.5 to 3.5 USgpm/sq.ft. (3.67 to 8.57 m/hr).Adsorption of iron and manganese at high rates.
Up to 16 USgpm/sq.ft. (39.16 m/hr) 3 Just about every well in Western Canada has some level of iron bacteria present.

This means ferrous iron, ferric iron (oxidized), BioFe and colloidal iron likely all coexist in the water being treated.
Further, it is likely manganese exists in solution, in oxidized form, as BioMn and as colloids, all in the same well water.

Exposing bacterial colonies to strong oxidants such as chlorine, potassium permanganate or ozone kills the bacteria resulting in the release of colloidal iron and manganese so held. Granular media filters without using a filter aide cannot filter colloids. A significant percentage of prairie well waters have some level of organic compounds present. Where organic compounds exist, some of the iron and manganese often behaves as though bound by certain of the compounds.

It is often difficult to filter out oxidized iron and manganese that is a part of an organic compound. Filter aides may be required to assist in filtration. High Rate Adsorption Iron & Manganese Removal Process (pp) is a recent development. The process, which is patent pending, has performed well in waters containing both high levels of iron bacteria and organic carbons, plus hydrogen sulphide and ammonia. Arsenic can also be reduced to very low levels using this process.

No process changes should be made to existing ones without the benefit of a pilot study.