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Oxidizing Filters

Reverse Osmosis removal

For medium concentrations of iron, the use of an oxidizing filter can be a most effective means of treatment when the pH is 6.8 or above. When used, the oxidizing filter should be installed in the water line ahead of the softener. Oxidizing filters normally contain a base material that has been coated with manganese dioxide. This may be a manganese treated greensand, a manufactured manganese material, natural manganese bearing ores, and similar materials. These manganese oxides convert the soluble ferrous iron in the water into ferric iron. As the ferric hydroxide forms, it is filtered from the granular material in the tank. Periodic back-washing is, of course, necessary to remove the precipitated iron from the unit. Less frequently it is necessary to regenerate the filter bed.

When the iron accumulation in the filter becomes excessive, regeneration is necessary. First the unit is backwashed to remove precipitated iron. An appreciable volume of water is needed for even the smallest units to backwash the dense manganese material properly. This is a factor to consider carefully in any installation because of the importance of obtaining thorough removal of the filtered iron during this process.

The backwash rate of different filter materials varies considerably. In general, the rate is higher than for most softeners. Whenever these filters are selected, the water supply should be first checked to be certain that adequate volume is available for proper backwashing. This is essential for best results with any filter. The next step in the regeneration is to pass a solution of an oxidizing agent such as potassium permanganate through the filter bed. This deoxidizes and restores the manganese dioxide coating on the base filter materials. After rinsing, it is again ready for use.

OXIDATION AND FILTRATION

All concentrations of iron may be oxidized by feeding solutions of oxidizing agents such as chlorine (in the form of household hypochlorite bleach) or potassium permanganate into the water. This method is particularly valuable when the iron is combined with organic matter, when iron bacteria are present, or when the iron concentration is too high for other treatment methods.

Note 1:

Bleach and permanganate solutions should be prepared weekly. Neither is permanently stable in solution. As a result, a drift (the concentration of the solution gradually becomes weaker) of feed may occur.

Note 2:

To establish the strength of a bleach solution to be fed to a water system, the following factors must be known:

  • well pump capacity in gallons per hour;
  • chemical pump capacity in gallons per hour (it is wise to set adjustable pumps at the middle of their range, if possible. This will permit adjustments in both directions.);
  • desired chlorine feed in ppm; and
  • weekly water consumption in home.

Bleach containing 5.25% sodium hypochlorite has a strength of approximately 50,000 ppm chlorine. Thus:

Well pump capacity in gal/hr x desired chlorine feed x 128 oz/gal
Chemical pump setting in gal/hr x 50,000 ppm chlorine
= liquid oz of 5.25% hypochlorite bleach per gallon of solution

For example assume: A well pump capacity of 300 gal/hr; a chemical pump capacity of 30 gallons per day but set at 15 gallons per day (equal to 0.625 gal/hr); a desired chlorine feed of 4 ppm; 2,000 gallons of water used per week. Then:

300 gal/hr x 4 ppm x 128 oz/gal
0.625 gal/hr x 50,000 ppm              = 153,600/ 31,250     =  4.91 oz bleach/1 gallon of solution

Then:

Weekly water consumption x chemical pump setting in gal/hr
                             Well pump capacity in gal/hr

Thus:

            2,000 gallons per week x .625 gal/hr
                               300 gal/hr

= 6.67 x .625 = 4.16 gallons of solution.

As a reserve of solution is always advisable, in this case five gallons should be made up. Thus, dilute (5 oz. bleach per gallon x 5 gallons) 25 oz. of bleach to 5 gallons with soft water, where possible. Change the chemical pump setting if -. necessary to adjust the concentration.

Note 3: Permanganate

Well pump cap. gal/hr x 133 advoir oz/gal x desired KMnO,ppm
Chemical pump capacity in gal/hr x 1,000,000

= oz of permanganate per gallon of solution

Water consumption in gal/wk x chemical pump capacity in gal/hr
Well pump capacity in gal/hr

= gallons of solution for 1 week.


Example: Well pump capacity is 300 gal/hr; chemical pump set at 15 gal/day or .625 gal/hr; desired feed of permanganate is 20 ppm; and weekly water consumption is 4,000 gallons. Thus:

300 gal/hr x 133 x 20 ppm =    798,000=      1.28 oz KMnO,
6.25 gal/hr x 1,000,000             625,000       per 1 gallon of water

4,000 gal per week x .625     =  8.33 gallons needed
       300 gal per hour

Thus, dissolve 11.5 oz (1.28 oz x 9) of Potassium Permanganate in 9 gallons of water.

Note 4:

Solutions of sodium hypochlorite are stable only when very strongly alkaline. In fact, in the process of manufacture, free chlorine combines with hydroxide ions: Cl2 + 2OH- H20 + Cl- + C10-. When the hypochlorite solution is added to water and the alkalinity neutralized, the chlorine is released. This action is speeded as the acidity is increased. Thus, the oxidizing power of the chlorine is released more rapidly at low pH values.

Potassium permanganate is a strong oxidizing agent across a broad range of pH values. In neutral or alkaline solutions the permanganate is reduced to the insoluble manganese dioxide, MnO2 However, in very acid water with pH below 4 it may be reduced all the way to the soluble manganous ion, Mn++. Thus, consideration should be given to the pH of the water when the oxidizing agent is selected.

Note 5:

If organic or chelated iron is present, contact times for both chlorine and permanganate may have to be increased significantly. The precise amount of time cannot, unfortunately, be calculated and may have to be adjusted on the basis of experience.

A variety of chemical solution feeders are used for this purpose, including positive displacement pumps, eductors, and several types of suction devices.

Like the manganese dioxide in an iron filter, the chemical oxidizing agents convert ferrous iron to the ferric state. The precipitated iron is then removed from the water by filtration.

When chlorine is used as the oxidizing agent, and sufficient contact time is made available, not only will the iron be removed, but also the water will be disinfected. Activated carbon filters are commonly used to remove the precipitated iron as well as any excess chlorine in the water. When potassium permanganate is fed, an iron filter is usually used to remove the precipitated iron.

With this method of treatment, the oxidizing agent must be introduced into the water ahead of the pressure tank. Under normal conditions, the pressure tank or a similar vessel will serve to mix the oxidizing agent thoroughly with the water.

When only simple ferrous iron is present in the water, it can be filtered immediately after it leaves the pressure tank.

Where iron bacteria are present in the water, two approaches have been used. In one method, short contact times have been used, usually with high concentrations of chlorine, to kill the bacteria. This approach depends upon the filter to remove the dead bacteria with any precipitated iron present. It is probable that oxidation of the bacteria continues on the filter bed.

Top 5 Water Contaminants

In the other approach, longer contact time is used, usually with relatively low concentrations of either chlorine or permanganate, to obtain more complete oxidation of the bacteria ahead of the filter. Reports indicate that both methods have been widely and successfully used.

Water treatment authorities have found that one of the most aggravating forms of iron is organic (chelated) iron. Iron in this form does not respond to the more simple iron treatment methods, for it is bound into organic materials which both tie up the iron in a non-ionic form, and are unusually resistant to oxidation. Thus, neither softeners nor iron filters will effectively remove this iron.

Strong oxidizing agents and long contact times are frequently the only answer to the presence of organic iron. High chlorine concentrations have been effective in some cases, but some authorities point out that it has been their experience that in most waters potassium permanganate will far surpass chlorine in oxidizing the organic iron. The key to the process is undoubtedly in assuring sufficient contact time between the oxidizing agent and the organic matter to insure complete reaction. Unfortunately, this cannot be estimated without tests on the water, but in many cases 20 to 30 minutes of contact are necessary. Where the proper conditions are provided, this type of treatment produces excellent results regardless of the quantity of iron in the water.

Depending upon the amount of sludge produced, however, care must be exercised to insure frequent backwashes to keep the filter bed clean. In some cases retention tanks or settling basins are provided to reduce the sludge load on the filters. More details on the operation of a chemical feed pump will be given later in this lesson. (-->Next)

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