BACKGROUND
Iron and manganese are only a concern in treated water supplies when they exceed concentrations above 0.3 mg/L for iron and 0.05 mg/L for manganese. At these elevated levels they are able to stain clothes, and plumbing fixtures such as sinks and toilets.

Iron and manganese are usually encountered as soluble forms in groundwater or deep reservoirs. When this water is exposed to oxygen, it picks up oxygen which slowly oxidizes the iron and manganese. This oxidation causes the iron and manganese to become insoluble precipitates. This also causes a color change in the water.

Generally speaking, groundwater supplies are more prone to have iron and manganese problems than are surface water supplies. If the water-bearing strata is principally an alluvial deposit (a deposit of materials, like mud sand and rocks, made by the action of water, such as in an eroding hillside into a valley), or of sandstone, or shale composition, it is more likely to have iron and manganese concentrations in it.

CHEMISTRY
They both may exist as:
a) soluble or
b) insoluble compounds in the water.

Groundwater, "the route":
As water percolates through the soil, the dissolved oxygen in it is consumed by the decomposing organic matter and microbes in the soil; and the decomposition also results in a reduction of the pH due to the microbial action. The iron and manganese atoms are "reduced" to the Fe+2 and Mn+2 state as the soil is a reducing environment that is created by the lower pH values and the absence of oxygen. When we pump the groundwater up to the surface, the oxygen in the air comes into contact with the water and enters the solution which starts the oxidation process; and the release of hydrogen sulfide (H2S) and carbon dioxide (CO2) from the groundwater into the atmosphere, raises the pH of the water. This allows for the soluble Fe+2 and Mn+2 states to change into the insoluble Fe+3 and Mn+3 states.

Surface water, "the route":
In deep raw water reservoirs, decomposing organic matter (such as algae and bacteria) consumes dissolved oxygen (especially during the winter months when ice and snow cover the impoundment,) further reducing the amount of dissolved oxygen in the water.

The iron and manganese atoms are "reduced" to the Fe+2 and Mn+2 state in the lower reaches of the reservoir due to the lower pH values and the absence of oxygen caused by the lack of wind action, algal action in photosynthesis, and the decomposition of the organic matter we spoke of.
As in the groundwater source, when the water comes to the surface, the oxygen in the air comes into contact with the water and enters the solution which starts the oxidation process; the release of hydrogen sulfide (H2S) and carbon dioxide (CO2) from the water into the atmosphere, raises the pH of the water. Once again, this allows for the soluble Fe+2 and Mn+2 states to change into the insoluble Fe+3 and Mn+3 states. In many deep reservoirs, high concentrations of iron and manganese may be found in the deep sections due to this phenomenon.

In pipes, "the route"
In piping, a filamentous, iron bacteria oxidize soluble iron into the insoluble iron state as an energy source, which causes the iron to "precipitate" in the pipe (the rough, encrustation’s that form on the interior walls of the pipe). Many times, the death and decay of these bacteria cause an additional taste and odor problem in the distribution system.

There are no long term solutions for the control of iron bacteria in distribution piping. Short term chlorination and subsequent flushing provides short term relief, but the only long term solution is the removal of iron and manganese from the water by treatment. Long term chlorination only increases the rate of oxidation , with a corresponding increase in colored water.

Treatment Processes and Notes:
In the class text, the "New York Manual" they stress: There are other conditions and constituents in the water that will affect the treatment process selected to remove iron and manganese from the water, but basically:
a) Soluble iron (Fe+2 ), in the absence of organic materials, (such conditions that exist with most groundwater sources), can be removed by
1) aeration or ozonation followed by
2) sedimentation and
3) filtration.
Note: If iron and manganese are both present and if they are combined with organic materials in the raw water, additional treatment will be required.

If the concentration to be removed is high, and/or the quantity of water to process makes it cost effective, the sedimentation step is provided to establish a detention time in the process, which allows the oxidation of the soluble Fe+2 and Mn+2 states to change into the insoluble Fe+3 and Mn+3 states. In some installations an appreciable quantity of the material precipitates out into the sedimentation tank as sludge. The remaining insoluble precipitates are then removed in the filtration process.

b) Manganese is not as easily oxidized as the iron. It is usually enhanced in its removal process by elevating the pH of the water to 8.5 or higher and then passing the water through an aeration basin which has coke beds coated with oxides. The manganese then goes through a catalytic conversion.

c) Iron oxidizes much faster than manganese. You may find at low levels of oxidation that manganese will not began its oxidation until almost all of the iron is oxidized. At higher dosage ratios of ozone to manganese (ie: 2.2 mg O3 / Mn mg), the manganese will be oxidized into permanganate. (Permanganate has a purple color in a very concentrated solution.) As such, permanganate may create a slight pink color to the water.

d) Well water is usually treated by aeration to release dissolved gases, such as carbon dioxide, and to start the oxidation process; then either potassium permanganate or chlorine is introduced to further oxidize the iron and manganese. Potassium permanganate is must faster in oxidization than is chlorine, and therefore requires less reaction time prior to sedimentation and subsequent filtration. For the removal of 1 mg/L of manganese it usually takes approximately 0.27 mg/L of dissolved oxygen, and for 1 mg/L of iron it will take approx. 0.14 mg/L dissolved oxygen. The filtration process following this step naturally develops a metal oxide coating which serves as a catalyst for the oxidation and removal of manganese.

e) Water softening with the high pH and the usual aeration (to reduce the C02 and lime requirement) will precipitate iron in a ferric (insoluble) form for removal by sedimentation and filtration. The lime treatment has also been shown to be an effective manner in which to remove organic-bound iron and manganese.

Manganese Zeolite Filters are generally pressure filters filled with oxide-coated natural greensand zeolite (glauconite). The water is passed through the filter, which acts like a water softener and removes the iron and manganese by having the oxidation and filtration steps carried out in the same vessel.

The zeolite, acts as a catalyst to promote oxidation of soluble Fe+2 and Mn+2 states to change into the insoluble Fe+3 and Mn+3 states ferrous. The insoluble compounds are subsequently filtered out in the zeolite media. Like a water softener, the media must be regenerated. It is regenerated by backwashing it with potassium permanganate. High backwash rates are required (eight gallons per minute per square foot) to remove the precipitated iron and manganese. The precipitates may also require backwashing aids, such as hydraulic, mechanical, and air scour to loosen the precipitates from the media, prior to the water backwash.

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