Arnold, Donald S.

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 The Knowledge Bank at The Ohio State University Ohio State Engineer Title:  Water Purification Today Creators:  Arnold, Donald S.  Issue Date:  1943-12  Publisher:  Ohio State University, College of Engineering  Citation:  Ohio State Engineer, vol. 27, no. 2 (December, 1943), 7-8, 20, 28.  URI:  http://hdl.handle.net/1811/36018    WATER PURIFICATIONTODAYB y D O N A R N O L D , C H . E . , ' 4 2AQUA PUR A" the ancient Romans called it.l \ .To the chemist it is hydrogen hydroxideor H,0. To John Q. Citizen it is just plain water.He uses it to quench his thirst, wash, keep cleanin, and to water his garden, among other things.Nevertheless, without it, life as we know it wouldbe impossible. At first, man, living near a bodyof water as was his custom, would merely godown to the shore and secure the water as hefound need for it.The Industrial Revolution and its increase ofmetropolitan areas resulted in a considerableproblem of obtaining water supplies. Many ofthe epidemics of the past centuries resulted fromcontaminated water supplies. From this dangerhas gradually evolved the system of water treat-ment in use today.A satisfactory public water supply must con-tain a minimum of organisms which producedisease. It must be sparkling clear, colorless,good tasting, free from odors, reasonably soft,free from objectionable gas and minerals, neitherscale forming nor corrosive, plentiful, low in cost,and preferably cool. These conditions confrontthe engineer even before he has found a source ofwater. Some of the conditions may be remediedby treatment in the "water-works", but this im-poses an additional expense. Since nature veryrarely provides a supply which meets all thesespecifications, it is necessary that the best oneavailable be selected and treated to produce thedesired qualities that are found lacking.The earliest method was to permit the waterto stay for long periods in storage basins andreservoirs. The disadvantages of this system arethe high initial cost of the reservoirs and basinsand the tendency of the water to become stagnant.A second method is the slow sand filtration.This method requires filtration through one-acrebeds of sand 30 to 40 inches deep. To obtainsatisfactory results with this equipment it isnecessary that the water source have a low tur-bidity and a low color content. The defects ofthis system are the high expense of the filter con-struction, the large area required, and the needof a special water supply.The most widely used plan is the rapid sandfiltration which involves chemical treatment pre-vious to the filtration. The water is first areatedto free it from dissolved gases, to oxidize the ironinto the insoluble form (ferrous to ferric), andto oxidize the organic matter. This processingmay be accomplished by spraying the water fromnozzles, allowing it to flow over cascades, or per-mitting it to trickle through coke trays. The firstis the one most generally practiced.A coagulating chemical that dissociates in al-kaline water to form a gelatinous, sticky precipi-tate which absorbs color and entangles mud,bacteria and suspended matter is added to theareated water. Ferric sulphate, chlorinated fer-rous sulphate, ferric chloride, aluminum sulphate,sodium aluminate, lime, or a combination of fer-rous sulphate and lime may be used as the coagu-lating agent. The choice depends upon thealkalinity and organic content of the water to betreated. Aluminum chloride, chromic chloride,and titanium tetrachloride may be used for thispurpose but are of no practical use at presentbecause ferric chloride serves the purpose moreeconomically.The general principle of this coagulation isdemonstrated by the application of aluminum sul-phate which forms aluminum hydroxide, a reac-tion familiar to every general chemistry student.The lime is used where sufficient magnesium ispresent to precipitate as Mg(OH)., which actsas the coagulating agent.D e c e m b e r , 1 9 4 3 P a g e 7Settling Basins in Findlay, Ohio Plant—Courtesy Public Works Journal.These chemicals are given a violent and mo-mentary mixing with the water, thirty minutesto an hour of gentle agitation, and finally aboutsix hours of settling in the sedimentation basins.The agitation is performed by means of rotatingpaddles or propellers, diffused air supplied froma compressor, or a flocculator which is an im-proved type of paddle agitator.The finer particles which do not settle are re-moved by a sand filter. The gravity filter is com-posed of a concrete box with 18 to 24 inches ofgravel covering the outlet pipes which is in turncovered with 24 to 30 inches of filter sand. Awash trough is suspended above the sand whichcarries away the wash water after it is forcedback through the filter. This washing removesthe filtered matter and loosens the sand.The pressure filter is essentially of the sameconstruction as the gravity filter but is built ina metal shell. In this manner the pressure of thepump bringing the water to the filter is main-tained for pressure in the mains whereas a sec-ond pump is required for this purpose in the caseof the gravity filter. The disadvantage of thepressure filter lies in the impossibility of thor-oughly and conveniently inspecting the filteringmaterial and in the low wash-water efficiency.The danger of infection by the bacteria is re-moved by disinfection and sterilization. Chlorinehas generally been used for this purpose. In theearly days it was supplied by the application ofchloride of lime. Later, electrolytic cells weretried and abandoned. At the present time theliquid chlorine supplied in steel cylinders is inuse. Hypochlorites are found to be valuable inemergencies. Where a high residual concentra-tion of chlorine is required either a chloramineor an excess of chlorine is used. Ammonia andchlorine are added to the water separately tocombine into the chloramines in the water.The chloramines are relatively tasteless and ahigher concentration may be maintained withoutintroducing the chlorinous tastes. When an ex-cess of chlorine is used, dechlorination withsodium thiosulphate, sodium sulphite, sulphur di-oxide, or activated carbon is necessary.It has been found that lime-softening kills thebacteria sufficiently to eliminate the need for fur-ther disinfection.The alkalinity and the amount of calcium carbonate must be controlled to prevent the solutionof the iron of the pipes of the distribution systemand subsequent corrosion of these pipes. Waterwith a pH of 9 dissolves very little iron whilea lower pH results in enough dissolving to produce "red water." When the water is saturatecwith calcium carbonate, a protective coating o:the calcium carbonate is deposited on the pipesIf the saturation drops this coating is dissolvedTherefore a definite pH andi calcium carbonatecontent must be maintained in the water supplyas delivered to the consumer.This control of carbonates leads the researcheito the modern topic of water softening. Caven-dish noticed the reaction which is the basic pro-cess of our present softening system, namely thedeposition of carbonates from water supplies.The process was developed on the demand ofa number of the new steam-powered industrialplants. The first municipal softening plant in theUnited States was installed in Oberlin, Ohio, in1903. Many public and private plants using theClark process have been constructed since thattime.The hardness of water is considered in twoclasses: carbonate and non-carbonate, formerlycalled temporary and permanent respectively.The first class is composed of bicarbonates ofcalcium and magnesium. In the latter class is(Continued on page 20)P a g e 8 T h e O h i o S t a t e E n g i n e e rWATER PURIFICATION TODAY(Continued from page 8)calcium sulphate, commonly known as gypsum,and magnesium sulphate, the druggist's epsomsalts. In addition to these minerals, iron andaluminum compounds are found to be objec-tionable.The minerals which make water hard are unde-sirable due to the formation of soap curds bythe calcium and magnesium ions uniting with thesoap. This curding results in a precipitate inthe wash water and a wastage of soap in actuallysoftening the water before any washing may beaccomplished. Furthermore the carbonates pre-cipitate out in hot water pipes and boilers caus-ing a loss in heat efficiency. According to a re-port of the U. S. Bureau of Mines, the reductionin thermal conductivity of the pipes due to scale1/50 to 1/9 inch thick amounts to from 7 to 16%.One of the more important processes of soften-ing is the lime and soda ash method. The limeand soda ash are added to the raw water eithergravimetrically in the dry form or volumetricallyin milk of lime or in solution. Where there is astorage problem as in small plants, the hydratedlime is used. In larger plants the quicklime(CaO) is used as it is more economical. Theyare often mixed, agitated, and settled in a mannersimilar to the purification treatment. Alum(A12(SO4)3 is used as a coagulating agent.After settling and filtering, sodium hexameta-phosphate is added.This use of calgon has an ability to keep theminerals which produce hardness in solution, thusmaking the water soft although the minerals arepresent. It is especially important in preventingCourtesy Sewage Works Engr.66 Inch Pumps in Southwest Side Station in Chicago—Courtesy Public Works Journal.Shop Assembly of a Clarifierboiler scale but is harmless to even the finestfabrics and cloths.A development in precipitating machinery, thePermutit Spaulding Precipitator, reduces thetime of treatment from five hours to one hour orless. No settling takes place due to the substitu-tion of two fundamental chemical and hydraulicprinciples. The water and chemicals are intro-duced at the top center of the precipitator. Asthe mixture descends it is further mixed andbrought into contact with the precipitate fromthe water which has just passed. This presenceof the solid state with the precipitating solutionincreases the speed of coagulation. These par-ticles act as nuclei for the smaller particles.As the water continues to flow to the bottom,the particles, grow larger, settle, and leave thesmaller ones to act as new nuclei to continue theprocess. The mixture is continuously agitated.When the water reaches the bottom, it passesthrough a small outlet around the outer edge andflows upward through the outer, V-shaped tank.This tank has a bed of sludge supported by theupward flow of water. This acts as an upflowfilter and removes much of the remainder of theprecipitated matter. Clarification is obtained byreducing the upward velocity until it will nolonger maintain sludge suspension. The water istaken from the top of the settling tanks to thefilters. The V-shaped design of the outer tankprovides this reduction of pressure and also main-tains a uniform composition of the filter bed.The general design has been found to be flexibleenough to be applicable to existing settling tanksand basins.The advantages of this precipitator are many,(Continued on page 28)P a g e 2 0 T h e O h i o S t a t e E n g i n e e rCIRCUITS OF VICTORY!THIS Signal Corps lineman and his comrades are1 building and keeping open the telephone lines thathelp to coordinate attack and defense in every battlezone. Not only on land, but also at sea and in the air,telephone and radio equipment made by WesternElectric is helping to bring Victory closer.This Company —for 61 years the manufacturer forthe Bell Telephone System —is today a vast arsenalof military communications equipment. This field ofEngineering for manufacture offers a wide range ofopportunity to men who are interested in the technicalproblems of production.• • •Buy War Bonds regularly—from now till Victory!Western ElectricIN PEACE...SOURCE DF SUPPLY FDR THE BELL SYSTEMIN WAR.. .ARSENAL DF COMMUNICATIONS EQUIPMENT.D e c e m b e r , 1 9 4 3 P a g e 21WATER PURIFICATION TODAY(Continued from page 20)It cuts the time of treatment. Due to the actionof the sludge in increasing the precipitation, lesschemicals are required which in turn reducesthe need for recarbonation. The sludge suspen-sion system provides fresh sludge; the older par-ticles settle to the bottom and are blown out adrain pipe. It automatically adjusts itself tovarying rates of flow by raising or lowering theheight of the filter bed. (This is actually done bythe velocity of the water itself.) Longer filterruns are permitted due to a better job of particleremoval done by the filter bed of sludge. Thepresence of particles in the sludge filter auto-matically provides the balance required for theprotection of the distribution system.In the lime-soda process the lime precipitatesthe hardness minerals while the soda ash and CO2of the recarbonation counteract the excess limeused.A variation of this is the overtreatment of aportion of the water and mixing this with an-other part of the raw water. This is known asthe split treatment.Aluminum sulphate is added to decrease thetime required for coagulation of the precipitateas well as to remove the soluble magnesium saltsas insoluble magnesium aluminates.The second method consists of replacing thecalcium and magnesium with sodium which willnot produce the reactions of the hard water but—Courtesy Sewage Works Engr.Engine Room in Pumping Stationrather remains in solution. This is accomplishedby the use of zeolites, which are complex insolublemineral compounds of double silicates of alum-inum or iron or both. They are found naturallyas the greensand zeolites and are manufacturedsynthetically.The zeolites are used as filter sand in bothgravity and pressure type filters, the latter typebeing commonly used in the home and small shop.As the water flows through the sand, the Na re-places the Ca and Mg. The sand also filters outthe suspended particles. After a period of time,the Na is all replaced and the sand is said to beexhausted. To regenerate it, it is merely neces-sary to pass a salt (NaCl) solution through thesand. This reverses the process and the sodiumreplaces the calcium and magnesium which arewashed out as soluble chlorides. This filter withthe necessary regenerations may be used indef-initely. An example of municipal softening isthe plant at Orrville, Ohio. It was built in thesummer of 1940 at a cost of $37,000. It is alime-soda ash type treating water from two wellswhich has a hardness of 300 parts per million,The lime treatment reduces this to 92 in the set-tling basins. (It first passes through the mixingtank and the clarifier where it is agitated afterthe chemicals are added and mixed.) The recar-bonation reduces it further to 82 and the filter to60. After the addition of the sodium hexameta-phosphate and passage through the distributionsystem, it emerges from the tap with a hardnessof 54 parts per million. After several months itwas found that recarbonation was unnecessarywith the elimination of overtreatment with limeand was consequently abandoned. The use of limesoftening also eliminated the use of chlorine inthe purification of the water. The sludge is usedto flocculate the sewage diminishing the amountof ferric chloride and ferrous sulphate used there.The average of one month showed the averagedaily treatment of 420,000 gallons of watertreated with 26.3 pounds of alum, 926 pounds oflime and 2 pounds of calgon. All of the reagentsare first dissolved in water then added by auto-matic machinery.In this plant 250,000 gallons were treated thefirst month at a cost of $20.20 per million gal-lons for chemicals. One year later the consump-tion had been boosted to 400,000 gallons per dayand the cost of chemicals reduced steadily to$12.71 per million gallons, each successive monthbeing lower than the one preceding it.By a careful control of the reagents, the tastehas been maintained and there has been a def-inite improvement in the performance of thehome water-heating systems and a reduction inthe soap consumption.P a g e 2 8 T h e O h i o S t a t e E n g i n e e r

Water Purification Today

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Water Purification Today

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