Removal of Iron and Manganese from Water S. B. APPLEBAUM,The Permutit Company, New York, N. Y., AND AI. E. BRETSCHGER,Buffalo Electrochemical Company, Buffalo, N. Y. and then b r o w n i s h turbidity, A description of some of the dificulties caused which finally s e t t l e s out as a water supplies in amounts by iron and manganese both in industry and the precipitate. The first method g r e a t e r t h a n 0.3 to 0.5 home is given. The general methods of iron and of i r o n r e m o v a l thus closely p, p. m. cause difficulties (IS)in manganese removal consist of ( I ) oxidation and follows the reaction that occurs industry and the home. Chief in Nature. filtration and (2) base exchange. The oxidation among these are u n p l e a s a n t However, many waters do not t a s t e , u n s i g h t l y stains, and reactions are interfered with by a number of thus respond readily to this oxitroublesome deposits. factors, such as the presence of free carbon didation reaction. Discussion of Iron and manganese give a n oxide, low total solids, low p H value, organic some of the interfering factors a s t r i n g e n t , metallic taste t o matter, etc. follows. d r i n k i n g w a t e r ; this causes The various types of iron and manganese reF R E EC A R B O ND I O X I D E . trouble in cooking in the home, Since carbon dioxide is formed in the canning industry, and in moval plants consist of: ( 1 ) the simplest f o r m of on the right-hand side of the the production of beverages. aeration followed by Jiltration, (2) more thorough oxidation reaction, the presence Yellow to b l a c k p t a i n s a r e aeration for removal of free carbon dioxide of high amounts of carbon diformed by the precipitation of followed by Jiltration, (3) aeration followed by oxide or any acid. in the origiiron and manganese. This is basin settling and final filtration, (4) aeration, nal water interferes with this renoticeable i n t h e s t a i n i n g of action’s going t o c o m p l e t i o n porcelain b a s i n s , b a t h t u b s , lime feeding, basin settling, and Jiltration, bottles, glasses, and china, and (5) same as ( 4 ) using special coagulants, ( 6 ) (11). Many waters do contain interfering amounts of carbon also in the washing of linens, in oxidation and Jiltration by manganese zeolite, d i o x i d e . I n those cases i t is textile p l a n t s , in paper mills, and (7) removal of iron and manganese by necessary to remove the carbon and in the manufacture of raw sodium zeolite through base exchange without dioxide either by thorough aerawater ice where concentrations tion or addition of alkalies or in t h e core accentuate the ox idat ion. both. difficulty. Many reagents react The design of iron removal plant selected f o r The type of thorough aerawith iron and manganese to form a n y given water and set of engineering conditions tion necessary to remove carbon precipitates, such as tannin in requires experimental study, both in the laboradioxide differs f r o m the type leather which forms black iron tory and in the field. Four cases of acfual iron of aeration r e q u i r e d m e r e l y tannates as fast as ink. This for introduction of oxygen for occurs also in dyeing in textile removal installations, and results and experithe oxidation r e a c t i o n . The plants. ences obtained with them are given. latter involves the addition of Iron and manganese deposit o n l y 0.14 p. p. m. of o x y g e n on coils in cooling systems, requiring expensive- cleaning and reducing cooling efficiency. per 1 p. p. m. of iron. Such small amounts of oxygen can be I n municipal distribution systems such deposits increase the introduced by sniffling air into the suction of well pumps or by friction through the pipes and increase pumping costs. At a compressor, or by adding small amounts of oxidizing chemisudden high rates of flow the deposits are loosened and cause cals. But with high interfering amounts of carbon dioxide turbid water. The growth of iron bacteria, such as Cwn- present, a more thorough aeration than sniffling is required. othrix ( 7 ) , is promoted by iron and manganese. These orLow TOTAL SOLIDS. I n general, the well waters of high biganisms accumulate in stringy, slimy masses and produce carbonate hardness in the Middle R e s t respond readily to this disagreeable tastes and odors on decomposing. oxidation reaction. With waters low in total solids, however, I n general, iron and manganese may be removed from well even a thorough removal of the carbon dioxide by efficient waters by two methods. The first is by oxidation (8) to the aeration usually does not suffice, and additional steps are higher oxides and hydrates. These are insoluble and can required. therefore be removed by filtration. The iron oxidation reacLow PH VALUE. Iron and manganese in acid waters are tion is as follows: usually present as sulfates and require the addition of alkalies to raise the p H value above 8 or 9 before filtration. 4Fe(HCO& O2 2H20= 4Fe(OH)s 4- 8C02 ORGANICMATTER. Iron or manganese present in organic The manganese oxidation reaction is similar; the two factors combinations resist removal by aeration and filtration. In involved in this reaction are introduction of oxygen and such cases special coagulants, agitating devices for acceleraliberation of carbon dioxide. The second method for remov- tion of floc formation, and sedimentation basins are necessary ing iron and manganese is by base exchange through zeolite before filtration. without oxidation. EXCESSIVE AERATION. Excessive aeration ( 2 , 16) sometimes interferes with complete removal of iron and manganese, FACTORS INTERFERING WITH OXIDATION REACTIONS especially manganese. I n such cases better results have been Usually the oxidation reactions of the first method take obtained by restricted aeration, limiting the absorption of place quite readily. I n such cases, frequently, it suffices for oxygen to a fraction of saturation. MANGANESE.I n general, manganese (15) is more difficult the water to stand in contact with the air for a few hours, and its crystal clear condition (when drawn) changes to a milky to remove than iron and requires higher p H values. It
I
RON and m a n g a n e s e in
+ +
925
926
INDUSTRIAL AND ENGINEERING
CHEMISTRY
Vol. 26, No. 9
oxidizes more slowly and requires long detention periods in basins, together with addition of special reagents for complete formation of manganic precipitates; often its presence also causes the iron to resist oxidation and precipitation.
oxidation reaction. The basin also serves to catch precipitates that may slough off the coke a t times. No air trap is required on the filter because no excess air is present. Coke aerators have the advantage of catching some of the iron and manganese on the rough coke surfaces which act as TYPESOF IRON AND MANGANESE REMOVAL PLANTS catalysts in accelerating the oxidation reaction. However, The type of treatment plant required is determined by a the coke tends to clog in time, and periodic washing and renumber of factors such as: character of the n-ater, especially placement are required. Other types of aerators may be employed (e). A vertical the interfering factors present; engineering conditions; and pipe spouting upward is the simplest aerator, but the water is FILTER insufficiently broken up. To increase the dispersion as well as contact time between air and water, tables or trays may be added below the riser. In general, the action of any aerator consists in exposing the water to the air in thin films, jets, or drops so that each particle of water contacts air low in carbon dioxide content. Sometimes steps are used or a riffle boardi. e., an inclined surface with obstructions to flow. Tricklers similar to coke aerators may be employed, using brick or stone instead of coke, or using merely wooden boards arranged in staggered tiers. Khen wells are pumped by air lift, the air COMPRESSOR bubbles frequently suffice to provide efficient aeration. A popular form of aerator for larger plants consists of spray nozzles similar to spray pond cooling systems. FIGURE1. IRONAND MANGANESE REMOVAL FILTERW I T H AIR SNIFFLEROR COMPRESSOR Instead of sprinkling the water through the air, air may be blown through the water. Frequently the engineering condithe purpose for which water is used and the degree of iron or tions prevent lifting the water high enough to drop through an manganese removal required. Classification of the various aerator, whereas it is possible to install an air grid in a basin types of iron and manganese removal plants follows. below ground level and blow air up through the water. The WATERSLow IN CARBONDIOXIDE,RESPONDING READILY cost of blowing low-pressure air must be compared with the TO OXIDATION.Figure 1 shows the usual type of plant emcost of pumping the water against the additional head conployed for such waters, where the amounts of iron or man- sumed by the trickling type of aerator. ganese are not excessive. The small amount of oxygen FILTER needed for the oxidation reaction is introduced by sniffling air / into the well pump. An air trap releases any excess of air from the side of the filter above the filter bed. The use of a pressure filter as shown avoids repumping, but gravity filters may be used. If the amounts of iron and manganese present were so high that they would tend to clog the filter and require excessive PUMP filter washing, in time the iron and manganese would penetrate the filter bed and slip through into the effluent. Therefore, in such cases a contact filter is used ahead of the final filter. This contact filter uses coarse granules with greater storage capacity between the grains. Frequently the contact filter is FIGURE 2 . COKEAERATORAND FILTER arranged so that the bed is not submerged and is thus exposed to the air. Here also by using pressure shells reFigure 3 shows the theoretical amount of air that must be pumping is avoided, but gravity filters may be employed if blown through water to decrease the amount of carbon dioxide desired. to any desired amount. These curves were drawn for a temThe final filter should be run at rates of not over 2 to 3 perature of 59' F. For other temperatures corrections must gallons per square foot per minute. If sand is employed as be applied as shown in Table I. the filtering medium, a coarser sand may be used than in To use Figure 3, start at the base with the original amount of ordinary filtration practice because the iron and manganese dioxide in the water and pass vertically up to the curve hydroxide coatings on the grains act as catalytic agents, and carbon showing the final carbon dioxide desired. Then pass horizontally the entire bed is in action rather than the surface alone. It is to the amount of air at the left. Multiply the theoretical amount therefore necessary to season the bed directly after installa- of air thus obtained by a factor for temperature (Table I) and a tion until sufficient coating forms on the grains. Owing to factor for aerator efficiency. the greater penetration of iron and manganese into the bed, TABLE 1. CORRECTION FACTOFS~ it is advisable to use air wash to assist the water wash in cleanCORRECTION CORRECTION ing the bed. TEMP. FACTOR TEMP. FACTOR F. F. FFr.4TERS HIGHIN CARBOX DIOXIDEREQUIRING CARBON 95 0.56 32 1.70 DIOXIDE REMOVAL BY ALKALIFEED OR EFFICIENT AERATOR 104 0.49 41 1.40 113 0.44 50 1.18 BEFORE FINAL FILTRATION. In this case Figure 1 may 12% 0.39 59 1.00 be employed, provided an alkali feed is installed to neu140 0.29 68 0.86 212 0 77 0.74 tralize the carbon dioxide. Otherwise some type of efficient 86 0.63 aerator should precede the filter. Aeration is more economia The, theoretical volume of air determined from Figure 3 for 59' F. must cal than alkali addition because no chemical costs are in- be multiplied by these factors for other temperatures. volved. If the water supply is high in carbon dioxide and in iron and Figure 2 shows a coke tray aerator. The receiving basin below the aerator provides additional time to complete the manganese, it is frequently advisable to install a contact
il
n
I
fi
filter between the aerator and final filter. With very high iron and manganese present, a settling basin may even be desirable instead of the contact filter. FINAL I
927
IN DUSTR IAL AN D EN GI N EER IN G CH EM ISTRY
September, 1934
cnz C O N T E N T 2
N P P M
3
SO0
400
300
derivatives do not respond alike to the same coagulant. In some cases activated carbon has been found very effective, in others, prechlorination. The best treatment must be determined by experiment. The treatment plant should therefore be designed so that the pH values may be altered a t will a t various stages in the process and also so that different coagulants may be added. MANGANESE ZEOLITE. This zeolite (4) ha3 been found very effective in the removal of iron and manganese from water, especially where complete removal is desired for industrial purposes. Manganese zeolite consists of sodium zeolite that has been treated by manganous chloride to convert it to manganous zeolite by base exchange and then oxidized by sodium or potassium permanganate. The reactions involved in this production of manganese zeolite may be written as follows (Z stands for zeolite): Na20.Z MnO Z
200
+ MnClz hlnO Z + 2NaCl + 2T\’aMn04= S a 2 0Z + hlnO.Mn,O, =
(1) (2)
The higher oxides of manganese formed in reaction 2 are really carried by the zeolite on the surface of the grains. The zeolite proper acts as an active carrier and catalytic reagent. The oxides themselves do the work of iron and manganese removal. These oxides give up oxygen to the water for the necessary oxidation reaction., thus :
100
NalO.Z
+ MnO.Mn,O; + 2Mn(HCO3)2 = NazO.Z + 5MnO2 + 4C02 + 2 H 2 0
(3)
After the higher oxides have given up most of their available oxygen in this manner, they are reoxidized by passing perF I G U R E 3. ‘THEORETICAL AMOUNTOF I I I R R E manganate solution through the bed. The amount of perC 4 R B O N DIOXIDE FROM W 4 T E R QUIRED TO REXOVE manganate required is about 0.01 pound per 1000 gallons AT 59’ F. for the average water. One or two regenerations per week WATEROF LOW PH VALUE THATDOES XOT READILY are usually sufficient, although daily backwashings are desirable. RESPOND TO OXIDATIONEVENAFTER THOROI-GH AERATION. Figure 5 shows a typical manganese zeolite layout. The Figure 4 shows the type of plant required. Lime is usually employed to raise the p H value for larger plants, soda ash for contact filter ahead of the manganese zeolite units is used smaller plants. Lime, however, usually gives better results whenever the amounts of iron and manganese are high. The because of the coagulating effect of the calcium carbonate pre- manganese zeolite removes completely the residual iron and cipitate formed. Hut lime increases the hardness, and where manganese left in the effluent of the contact filter. this is not desired soda ash is preferred. An LlME T A N K E L E C T R O CWfM C A L agitating c h a m b e r is CONTUOLUNIT MElER FEEDPIPE shown for floc accelera\ P“ FEED tion. T h e s e t t l i n g PUMP OR A N O C E A R S basin has a sludge reX NG C H A U B L R mover. Contact filters are usually not 0 SERVICE SEl7LING BASIN required because of the settling basin. ,0CON,90L
YY
/
WATERCONTAINING I R O N AND ;\fANGANESE I N ORGANIC COMBINATIONS. In this case co-
A RCR D
T UA R POw A
