CHAPTER IX

REMOVAL OF IRON AND MANGANESE

Deferrization and demanganization

Common manifestations of the presence of iron salts in a water supply are rusty or "red" water, staining of linens in laundry operations, staining of utensils or food in cooking, brownish films formed on the inside of glasses and water bottles and brownish stains on plumbing fixtures. In its less disagreeable form iron may be exhibited only as a slight turbidity in the water. In aggravated form, the rusty deposits may unfit the water for any satisfactory use. The presence of iron is sometimes manifested by a slight "styptic" or "metallic" taste.

A not infrequent accompaniment of ferruginous waters from underground sources, is the appearance of "iron bacteria," e.g., crenothrix. These organisms established in the distribution system greatly aggravate the staining troubles named above, frequently cause clogging of service pipe and mains, and are responsible for disagreeable odors.

In considering the effects noted, a clear distinction should be made as to whether the troubles are due to soluble iron present in the source of supply or to metal dissolved from the distribution system or from house plumbing, particularly hot water systems.

Manganese in the water supply causes effects similar to those of iron, as to discoloration, staining, sediment, pipe clogging and assistance to organic growths. The manganese rust, however, has a darker appearance than the reddish iron rust and deposits of the oxide, if pure, are quite black. Manganese is frequently associated with iron in ground waters, but is usually present in smaller amounts than iron. There are notable exceptions, however, to this rule.

Amounts objectionable

Experience has shown that iron, if present in excess of 0.5 p.p.m. (expressed as Fe) will generally be objectionable to consumers. With pure ground waters 0.3 p.p.m. is the limiting amount beyond

which complaints of deposition and staining are to be expected. In waters containing considerable organic matter, a content of 1 p.p.m. or over may be tolerated without complaint. It may be noted here that the tentative standard of the United States Treasury Department applicable to water on interstate carriers suggests a limit of 0.3 p.p.m.

The amount of manganese required to produce complaint is not well established because this metal seldom is found unaccompanied by iron. The best information available is that the sum of these two metallic constituents may not exceed the limits stated above without complaint.

Methods of removal

No single method has been found applicable to all problems of iron and manganese removal because other impurities have a marked effect on behavior. These impurities in order of importance are organic matter, manganese, carbon dioxide and carbonates.

The processes found applicable are as follows: Aeration, sedimentation, coarse contact beds, sand filters and chemical treatment.

Aeration

Oxidation of the iron and manganese salts present in the ground water is a necessary preliminary to any subsequent removal process. The amount of oxygen required is very small, being only 0.14 part for each part of iron present, while ground water at ordinary temperature, when fully saturated, contains about 10 p.p.m. of dissolved oxygen.

It is often possible to supply the small amount of oxygen necessary for oxidation by adding chemicals such as permanganates, hypochlorites, chlorine, etc., but the addition of atmospheric oxygen is so simple and inexpensive as to leave no room for chemical treatment for this purpose. Oxidation therefore implies some form of aeration for bringing air in contact with water or water in contact with air. It is unnecessary to discuss here aerating devices in detail, but it is important to note, however, that some iron-bearing waters require a distinct limitation on the amount of aeration. At Lowell, Mass., for instance, it has been well established that, if aeration is carried beyond 50 per cent saturation of dissolved oxygen, the man

ganese is not removed on top of the slow sand filters, but the action is delayed and clogging of the lower part of the filter beds results. At Reading and Middleboro, Mass., complete aeration followed by filtration failed to remove iron satisfactorily from these supplies, but restricted aeration was successful. At Superior, Wis., the aerators were abandoned as distinctly harmful and the iron is now oxidized only by incidental handling of the water.

Except where organic matter or manganese is present in sufficient amount to make necessary a limit on aeration, it is generally advisable in connection with ground water to provide an efficient means of aeration in order to reduce to the lowest amount the carbon dioxide. and any hydrogen sulfide present as dissolved gases. Where ground water is pumped by air lift no other form of aeration may be needed, but carbon dioxide in excess of 50 to 60 p.p.m. generally requires additional aeration. In ground waters very low in alkalinity a secondary aeration may be necessary to secure satisfactory removal of carbon dioxide. In other words, the oxidized iron must be removed by contact action before a full release of carbon dioxide can be secured.

Sedimentation

The usefulness of plain sedimentation is limited to water containing considerable alkalinity, little organic matter and relatively high amounts of iron and manganese. Usually precipitation is too slow to make sedimentation economical, for satisfactory removal is seldom obtainable unless more than 24 hours retention is provided. Plain sedimentation is therefore less useful than other processes to be described.

In connection with chemical precipitation referred to later, sedimentation has obvious usefulness.

Sedimentation following coarse contact beds is useful in taking care of occasional "unloading" caused by rapid change in rate.

Coarse contact beds

Except in the presence of high organic matter, dissolved iron in ground water after adequate aeration will precipitate as red ferric oxide on any surface in contact with the water. This behavior is utilized in connection with artificial beds of various materials arranged to provide large contact surface. The kind of material is not important and may consist of broken stone, round boulders,

coal, coke, gravel, shavings or wooden slats. Deposition takes place whether the contact material is supported with free access of air or is submerged.

Downward versus upward flow. Downward flow contact beds have an advantage when used to control the degree of aeration by varying submergence as is done at Lowell, Mass. The downward flow method, however, retains less iron and is less easily cleaned. Upward flow contact beds, on the contrary, are easily cleaned, less apt to clog and have high efficiency.

Cleaning. The usual way of cleaning contact beds is by quick opening of a flush valve which allows the bed to drain quickly, dragging the accumulated iron deposits out. The flushing method suffices for beds of very coarse material, but, in the case of gravel, clogging is apt to occur unless some auxiliary means of cleaning is provided. In the plant at Long Beach, N. Y., the contact beds of gravel are underlaid with perforated pipe underdrains through which wash water may be applied. Occasional use of this method of washing at intervals of 10 days supplement the daily flushings and serve to keep the beds in good condition.

Limitations of contact devices. It has been the usual experience with contact beds that, while they are able to deal with water of very high iron content, yet the residual iron in the effluent is apt to exceed the desirable limit, making the addition of sand filters necessary for complete removal. This was found true at Memphis, Tenn.

The above limitations as to incomplete removal by the contact bed do not always hold, as the Long Beach, N. Y., plant removes iron virtually completely.

Sand filters

After aeration sand filters may be used as the sole iron removal device or merely as a finishing treatment. Except where used to follow chemical treatment, sand filters are simply contact devices, but the contact action is usually to the upper portion of the sand bed. Sand filters are generally capable of more complete iron removal than can be gotten by coarse contact beds and sedimentation. In addition, sand filters constitute an important safeguard where the well water is subject to bacterial pollution. Sand filters are more consistent in behavior than contact beds, which sometimes unload, especially where there is an increased demand,

When manganese is present sand filters are useful in retaining the manganese not precipitated with iron in prior processes. Disadvantages of the sand filter are the greater investment cost compared with other processes and the additional head required for filtration. Successful results have been gotten from iron removal from both slow sand and rapid sand types of filter. The slow sand filter requires less attention in its operation and, in some cases, the additional contact period due to the slower rate of filtration may be advantageous in effecting complete removal. The rapid sand filter occupies less space, represents a smaller investment and will take care of water with a higher iron content.

Chemical treatment

Lime and alum are sometimes used in connection with iron removal. Their application involves sedimentation facilities to take care of precipitated material. With moderately alkaline ground water containing large amounts of iron (in excess of 5 p.p.m.) and little organic matter, iron precipitates rapidly after aeration. The action is hastened by the application of lime. Unless preceded by efficient aeration, treatment with lime adds considerably to the hardness of the water by combining with the remaining carbon dioxide. There is little advantage in this method except as a temporary expedient. It is well recognized that organic matter retards or prevents coagulation and precipitation of inorganic salts. Where organic matter is present in sufficient amount as to interfere with removal of iron by contact action, coagulation with aluminum sulfate, alone or assisted by lime, and followed by settling has been found successful as a preliminary step to contact action.

Sometimes where aeration and contact fail to coagulate a small amount of iron in the presence of a relatively large amount of peaty organic matter, potassium permanganate may be added, as at Merrimack, N. H.

1