Accelerated Lime-Soda Water Softening A. S. BEHRMAN AND W. H. GREEN International Filter Co., Chicago, Ill.
in softening the water supply of an entire community or for industrial processes in which cold water is required. Until a relatively few years ago, before modern softening equipment became available, a reduction in hardness to 70 or 90 p. p. m. (as calcium carbonate) was considered satisfactory performance in cold process softeners, even when a large excess of reagents was employed; and until quite recently an effluent hardness of 50 or 60 p. p. m. represented efficient softening, likewise with the use of an excess of reagents. The discrepancy between the theortically possible residual hardness of 20 to 25 p. p. m. and the considerably higher values obtained in actual practice has generally been ascribed in the past to the presence of unprecipitated “colloidal” calcium carbonate and magnesium hydroxide. The present writers consider it possibly more accurate and certainly more convenient to hold responsible for the discrepancy the formation of relatively stable supersaturated solutions of calcium carbonate and magnesium hydroxide, and will use this concept throughout the present paper.
If the reactions involved in the lime-soda softening process could be carried out to completion, the hardness of the treated water would be equivalent to the sum of the theoretical solubilities of calcium carbonate and magnesium hydroxide. In actual practice of cold-process lime-soda softening, however, this theoretical reduction of hardness is not achieved, owing to the formation of relatively stable supersaturated solutions of calcium carbonate and magnesium hydroxide. Partial destruction of this supersaturation has been most commonly effected heretofore by long periods of retention, aided by coagulants, such as the compounds of aluminum and iron. Previously precipitated sludge, providing a large amount of surface for the promotion of desupersaturation has been employed to a limited extent in the past. The present paper describes a recent development in lime-soda softening in which a novel method of utilizing, circulating, and conditioning the previously precipitated sludge and the freshly formed precipitate makes it possible to accomplish in a short holding time not only a close approach to theoretical completion of the chemical reactions involved, but also a high degree of clarification.
I
Heat and Coagulants as Aids to Desupersaturation Many expedients have been tried for destroying this supersaturation. Probably the most effective is heat. The residual hardness of water softened by the hot lime-soda process is frequently no higher than the theoretical solubility of calcium carbonate and magnesiun hydroxide in pure water; it may even be slightly less as a result of the common ion effect of the excess carbonate and hydro,xide alkalinity employed in the treatment as well as of the high pH environ-
N T H E lime-soda process of water soften-
ing, the calcium of the raw water is precipitated as calcium carbonate and the magnesium as magnesium hydroxide. This is true when lime alone is used as the precipitant, or when both lime and soda ash (sodium carbonate) are utilized. It follows that, if the reactions involved could be carried to completion, the hardness of the raw water would be reduced to a value equivalent to the combined theoretical solubilities of calcium carbonate and magnesium hydroxide. This combined value is approximately 20 to 25 parts per million (expressed as calcium carbonate) in pure water, and a little less at the high pH environment (typically 10.5 to 11.5) characteristic of limesods softening. I n actual practice, however, this theoretical efficiency of precipitation is not achieved, particularly when the softening process is carried out in the cold, as is necessarily the case
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129
ment already mentioned. The field of hot-process softening is obviously very limited, however. It is confined principally to the treatment of boiler feed water and to a few similar situations in which high temperature of the treated water is an asset, not a liability.
tening may be secured by the use of lime alone for the removal of the carbonate hardness, leaving the noncarbonate hardness untouched. In such cases the calcium (and to a lesser extent the magnesium) ions of the noncarbonate hardness may be expected to assist materially the precipitation of calcium
In cold-process softening, the principal aids to desupersaturation-i. e., those that have been employed most extensively and, within limits, quite successfully-are the coagulants. These comprise chiefly compounds of aluminum, such as aluminum sulfate and sodium aluminate, and compounds of iron, such as ferric and ferrous sulfates. Although the use of a coagulant obviously adds slightly to the cost of the watersoftening treatment, this cost is practically always more than compensated for by a reduction in hardness which is quite disproportionate to the amount of coagulant required. The use of as little as a half grain of coagulant per gallon (8.5 p. p. m.), for example, will frequently accomplish by desupersaturation a reduction in hardness of 1 to 2 grains per gallon (17 to 34 p. p. m.).
carbonate and magnesium hydroxide; it is thus possible to reduce the amounts of these compounds in the treated water to values more nearly approaching their theoretical solubilities.
Effect of Common-Ion Concentration Where a water is softened by conventional methods with both lime and soda to secure maximum possible precipitation of calcium and magnesium, it has long been the practice, especially in industrial water softening, to use an appreciable excess of both lime and soda in order to provide a residual concentration of hydroxide and carbonate ions beyond the amount theoretically required for combination and reaction. This empirical method of “forcing the precipitation” of calcium carbonate and magnesium hydroxide has a sound theoretical basis in the utilization of the effect of the increased common-ion concentration on the solubility product or, as more recently termed, the “activity constant.” What is not generally recognized, however, is the commonion effect of the calcium and magnesium (especially of calcium) ions in those cases where complete softening is not required, or even desired. The most important group of such cases is found in the softening of community water supplies, where softening to a residual hardness of 4 to 6 grains per gallon (68 to 102 p. p. m.) is usually the practical limit because of economic as well as technical considerations. I n softening many community water supplies, this desired degree of sof-
Excess Lime Treatment One application of the utilization of a large excess of hydroxide alkalinity was the so-called split treatment, now very little used. In this method a part of the water was dosed with the lime required for treating all the water, and the remainder of the raw water was added subsequently. The great overtreatment with lime of the first portion of the water accomplished enough extra reduction in the amount of residual calcium carbonate and magnesium hydroxide to make the amounts of these compounds in the final effluent less than if the whole flow of water had been treated with the average dose of lime in the conventional fashion. A later application of the use of excess hydroxide alkalinity was the method proposed by Hoover and Montgomery (4). The principle of this method is to employ enough lime to provide a hydroxide alkalinity of the order of 50 to 80 p. p. m. (in terms of calcium carbonate) to precipitate calcium carbonate and magnesium hydroxide with substantial completeness. After the conventional settling period the settled water is treated with carbon dioxide to precipitate the large excess of calcium hydroxide as calcium carbonate, which is subsequently removed by passing the water through a second sedimentation basin and finally through sand filters. Hoover and Montgomery report tests in several plants in which the effluent from this treatmerit had residual alkalinities of about 18 to 30 p. p. m. The principal difficulties in carrying out this type of treatment to produce an effluent containing substantially the theoretical amounts of calcium carbonate and magnesium hydroxide have been stated by Hoover to be the necessity for careful control of the carbonation, and especially the difficulty of removing the very finely divided calcium carbonate resulting from the carbonation before it reaches the filters and
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incrusts them. The use of returned sludge to accomplish the latter object did not work out satisfactorily ( 3 ) . Additional considerations which have perhaps tended to limit the use of the method are: (a) the materially increased cost
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sludge. Following this contact of the mixture of water and chemicals with the sludge, the sludge-carrying treated water passes upward to and through a suspension of the solid particles (kept suspended by the upward flow of water) in a tapered settling tank of increasing crosssectional area to provide decreasing upward velocity. The other method, developed by the writers’ organization, employs several novel principles, both of theory and of design and operation; the use of these principles has made possible the attainment of uniquely successful results.
Principles of Accelator Operation
ACCELATOR WATERSOFTENER, WILLIAMS BAY,WIS and space of installation for the additional sedimentation basins required; (b) the greater amount of carbon dioxide and larger carbonating apparatus necessary; and (c) the additional dosage of lime required to provide the excess hydroxide alkalinity.
Previously Precipitated Sludge For many years it has been known that if the mixture of raw water and softening chemicals is stirred in contact with previously precipitated sludge, the chemical and physical reactions involved are materially accelerated, both as to completeness and as to the time required. The patents of Koyl ( 5 ) ,Sutro and Booth (8), and Green and Behrman (8) show both appreciation of the beneficial effect of previously precipitated sludge and a successively closer approach to a practical method of securing this effect. It is not necessary that the solid matter stirred in contact with the raw water and chemicals be previously precipitated sludge. It has been the writers’ experience that almost any type of suspended matter, such as finely divided sand, will have a similar effect. Obviously, however, previously precipitated sludge is the most conveniently available type of suspended matter to employ; and it is only to be expected that suspended particles having the same chemical composition as the substances to be precipitated would have an additional advantage for that reason. Until the past few years there has been only a limited utilization of the accelerating effect of previously precipitated sludge. Within that time two different processes have been developed independently which have attracted the attention of the water purification chemist and engineer. I n the method and apparatus of Spaulding (7) the water and chemicals are mixed together as in previous practice, and this mixture is subsequently brought into contact with the sludge. The mixture of raw water and chemicals descends into the lower portion of the apparatus, where a slowly moving horizontal paddle keeps in suspension a large mass of retained
Figure 1 is a diagrammatic cross-sectional view of one form of the ‘LAccelator” (by which name this method and equipment are now designated) as applied to water softening. Reference to this figure will make clear the following novel principles of accelator operation, described in their proper sequence : The raw water and treating chemicals are not first mixed together, as in previous practice, and then brought in contact with suspended particles of previously precipitated sludge. Instead, the chemicals alone are first mixed with a slurry of the precipitate; the raw water is then rapidly mixed into and with the chemical-slurry mixture, with the result that the chemical reactions take place in the presence of the enormous surface area presented by the old particles. As a result there is not the usual tendency to form a highly supersaturated solution of calcium carbonate and magnesium hydrate that must later be desupersaturated; instead, a large proportion of the newly formed substance separates out as a deposit on the surface of the solid particles present. This has advantageous results in several directions. One consequence is that there is no formation of an enormous number of neand extremely fine particles to be agglomerated and then settled out. Instead, the old particles on which the new precipitate deposits, grow comparatively large and heavy, forming a suspension from which the treated water readily separates in a clarified condition. These slurry particles are hard and crystalline. They are present in the slurry as single crystals and as clusters cemented together by the accretion that takes place; as a consequence, the suspension or slurry may be circulated and agitated very vigorously without the breakdown of particles characteristic of the usual precipitate. Instead of the common limiting agitator speed of about 2 feet per second, speeds many times greater may be employed. As a matter of fact, speeds of 10 feet and more per second are commonly utilized. This is a practical advantage in carrying out the treatment, particularly in reducing the size of apparatus required. As the combined result of these factors, when the throughput water finally escapes from the mixing zone, it separates so readily from the slurry particles that clarification is immediate; and the water is a t or so close to the point of stable equilibrium that there would be no advantage in providing further contact or longer retention. A new method is employed for ensuring intimate and adequate contact of solid particles and liquid. I n the conventional type of stirring employed in previous practice, using a simple horizontal. paddle, the mixture of lime and other chemicals is added to a relatively enormous volume of liquid containing the suspended particles. With the slow hori-
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zontal motion to which these paddles are necessarily limited, the opportunities for collision are uncertain and left largely to chance. Furthermore, there is usually ample opportunity for short-circuiting, so that some of the water may escape from the stirring zone with little or no contact with the solid particles. I n the Accelator, however (Figure l), the mixture of chemical-impregnated slurry and raw water is subjected to a positive and rapid circulation and contact, first in the lower primary mixing and reaction zone, and then in the upper secondary mixing and reaction zone. Usually one volume of new or throughput water and four or five volumes of slurry are circulated a t relatively high velocity through this portion of the system. By means of these highly localized zones of reaction and the positive contact of solid particles and liquid thus provided, adequate opportunity is assured for the deposition of calcium carbonate and magnesium hydroxide. Short-circuiting of the water without contact with the slurry is prevented. The combined time in both the lower and upper mixing and reaction zones is about 5 minutes. The throughput of treated water then escapes upwardly through the slurry pool into the clarified water zone of uniform cross section and finally over an effluent launder or weir to the filters. It will be observed that with a mixture of four or five volumes of slurry to one of throughput the water travels the circuit into and through the mixing zones four or fives times before it finally emerges as throughput; it is in close contact with the solid particles the entire time. As a result of carrying out the treatment and handling the slurry in this way, the slurry is brought into a condition such that water readily separates from it. The circulation
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in the Accelator is divided into two parts. I n the mixing zones the flow is rapid to the point of being violent and turbulent; in the outer spaces between the mixing chamber and the basin wall the circulation is relatively quiescent through an outward-downward-inward path. The agitation during this quiescent flow is not enough to retain the water in the slurry. The water may be said to be “squeezed out” of the upper surface of the slurry pool, emerging clarified to such a degree that the turbidity is characteristically less than 10 p. p. m. There is no gradual clarification from bottom to top as in ordinary sedimentation. The change is abrupt from thick slurry to clear water. The solids in the slurry, which have been thickened by the escape of water, are carried downward and inward by the return circulation, and so back to the mixing chamber. The slurry is diluted in the mixing chamber by the raw water and thickened in the separating zone by the escape of water. It should be noted here, however, that the solids do not settle out. With this type of treatment there is no real sedimentation. This method of water treatment departs fundamentally from what have hitherto been regarded as standard principles of design and operation. The old conceptions of rate of sedimentation and time of retention have no application here. Instead, capacity must be visualized in terms of rates of flow in gallons per square foot per minute, just as is done regularly with filters. The limiting rate a t which clear water may be separated from the slurry has not yet been definitely established. In the earlier Accelator installations, upward rates of flow were usually of the order of 1 gallon per minute per square foot of slurry pool surface (Figure 1). As experience demon-
n
SEDIMENTATION
B A S I N
U CONV€NTlONAL
SOFTENING
PLANT
n AREA PRETREATMENT PLANT AREA RECARBONAT1ON BASIN AREA PIL reus EXCAVATION PRETREATMENT PLANT CONCRETE PRETREATMENT PLANT
ACCELATOR SOFTENING PLANT
CONVCNTlQNAL
268
S Q . F7:
80
SQ. F7:
/810 80
SG!.FX
360
SP.FT
300 I78
c u . YDS.
360 195
C U YDS.
59
cu. YDS.
S O f T N I N G PLANT Sd(.PT SQ. FT.
CU. YDS.
FIGURE 2. COMPARISON OF AREAAXD VOLUMEREQUIREMENTS OF CONVENTIONAL AND ACCELATOR WATER SOFTENERS
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INDUSTRIAL AND ENGINEERING CHEMISTRY
strated that this rate was far below the permissible maximum, the rate was gradually stepped up until the standard in plants now being built is 3 gallons per minute per square foot. Considerably higher rates have been shown to be practical, and entirely satisfactory results have been obtained at rates in excess of 4 gallons per minute per square foot by operating equipment designed for a 3-gallon rate at these higher rates
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excess solids is to settle them and then discharge them from a low point in the apparatus. In the Accelator, on the other hand, excess solids may be said to be floated or skimmed off. AS Figure 1 shows, a slurry concentrator compartment is built into one side of the apparatus, the top of this compartment determining the slurry level. This compartment is not a part of the circulating: zone. When, therefore. as a result of the Lydraulic leveling out of the slurry zone solid particles are carried to any point above the concentrator compartment, they no longer have the upward rise of water to keep them in suspension. They fall, therefore, into the concentrator compartment and are discharged continuously to waste through a controlled opening. In this way is accomplished automatically the twofold purpose of removal of excess solids and the maintenance in the apparatus of sufficient slurry for proper operation. The concentration of solids in the circulating slurry ranges from about 1 to 2 per cent by weight, while in the slurry discharged from the concentrator the solids are usually 10 per cent or more. The volume of water lost with the discharge of solids is therefore greatly reduced, and is appreciably less than required with conventional methods of sludge blowdown.
Results of Treatment -4CCELATOR WATER SOFTENER, GEORGETOWN, TEXAS
for a sufficient period, In such test runs the limiting factor has not been the separation of the water from the slurry, but either insufficient facilities for supplying additional raw water or the fact that the mixing chamber was too small to provide the necessary contact conditions. The significance of these high rates of flow becomes evident when it is remembered that the standard rates of flow in sand filters are 2 gallons per minute per square foot of filter area for municipal filter plants and 3 gallons for industrial plants. In other words, the area of the Accelator is usually no greater than the area of the sand filters following it and is sometimes even less. The conventional lime-soda softening plant typically employs a 4-hour period for reaction and sedimentation. With the Accelator there is no such arbitrary period of retention. The size of the apparatus is determined by the upward rate of rise considered safely permissible for a given water under the particular set of conditions obtaining. As already stated, this rate is now normally 3 gallons per minute per square foot of slurry pool area. With a water depth of 12 feet and a rise of 3 gallons per minute per square foot, the holding or retention time figures out a t 30 minutes. From this, however, are deductions for filleted corners and the like. The resultant economy in construction costs is too obvious to require detailed discussion a t this time. A graphic comparison, however, is shown in Figure 2, of the principal items of construction required for a conventional type of water-softening plant and for the Accelator; each plant has a treating capacity of a million gallons per day.
Removal of Sludge A unique feature of the Accelator is the method of removing the precipitated solids in excess of the amount required for proper operation. The conventional method of removing
The novel features of design and operation that have been described would obviously be of little practical importance if the quality of the treated water was not satisfactory. Therefore, of special interest is the fact that the physical and chemical quality of water treated in the Accelator is not only as good as that of water treated in conventional types of plants but is decidedly better in several important respects.
Turbidity of Effluent As a result of the conditioning of the slurry previously described, the turbidity of the effluent from the Accelator is characteristically less than 10 p, p. m. When a coagulant is employed (usually about a half grain per gallon of aluminum sulfate or its equivalent in sodium aluminate or the iron coagulants) in conjunction with the softening chemicals, which is considered good practice in any type of lime-soda softening process, the turbidity of the effluent is characteristically less than 5 p. p. m. Not only the effluent but the entire body of water above the slurry has these low turbidities. The top of the slurry pool, as already pointed out, is a sharply divided line; and the turbidity of the water after it leaves the slurry pool is not changed. When, therefore, we look down into the tank of an Accelator in operation, we see through a depth of clear water to the top of the slurry, which appears as a flat or gently undulating white blanket. Filters following the Accelator naturally have little work to do; the filter runs are consequently long, and the wash water consumption correspondingly low.
Chemical Characteristics of Effluent When the hardness of a raw water is due so predominantly to calcium compounds that the desired degree of softening may be secured without the necessity for removing magnesium, the residual calcium carbonate in the Accelator effluent may frequently be reduced to a value closely approaching,
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and sometimes reaching, the theoretical solubility. At the same time, the effluent water is so stable that recarbonation may frequently be dispensed with. After all, when it is remembered that the familiar calcium carbonate saturation test of a water consists essentially in correcting either supersaturation or undersaturation by subjecting the water to contact with an excess of finely divided calcium carbonate, it is not surprising that the adequate contact provided in the Accelator should likewise produce a treated water exactly saturated with calcium carbonate and therefore stable. It follows logically that stabilization with respect to calcium carbonate may be accomplished a t practically any desired point on the solubility curve of calcium carbonate, in which calcium carbonate alkalinity is plotted against pH [compare Baylis (1) and Langelier (S)]. I n previous practice it has been necessary to use an excess of lime to “force” the precipitation of calcium carbonate and then (at least in municipal softening practice) to recarbonate to remove the undesirable hydroxide alkalinity. With the Accelator, on the other hand, stabilization with respect to calcium carbonate may be accomplished a t almost any desired pH, so that a stable water may be produced even if the water is undertreated with lime. In many such cases the need for recarbonation is eliminated. For example, a t a typical Accelator softening plant a t Georgetown, Texas, the treated water had a phenolphthalein alkalinity of 10 p. p. m., a methyl orange alkalinity of 45 p. p. m. (a bicarbonate alkalinity, therefore, of 25 p. p. m.), and a pH of only 8.9. The calcium carbonate saturation test showed this to be a stable water, which eliminated the necessity for recarbonation. When appreciable amounts of magnesium have to be removed, it will usually not be found possible to avoid recarbonation, if the treated water is to be used as a potable supply. To precipitate magnesium hydroxide requires a pH of at least about 10.2; and this is higher than standard practice permits for community supplies.
Economy of Treating Chemicals For the same reason that the conventional excess of lime is not necessary in the Accelator, the use of an excess of soda is likewise avoided. Accordingly, it is possible to remove just as much or little of the noncarbonate hardness as may be desired, supplying for that purpose the stoichiometric amount of soda indicated. Another appreciable saving in lime accrues, as would be expected, from the more complete utilization of this reagent brought about by the repeated circulation of the slurry.* Contrasted with the appreciable losses of unused lime typical of conventional types of softening plants, experience with the Accelator has indicated in several cases a utilization very closely approaching the theoretical maximum, based on the available calcium hydroxide (or oxide) in the particular lime employed.
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TABLEI. CONDENSED TYPICAL OPERATING DATA Accelator water softenera A B C D Treating capacity, gal./min.: Rated 500 26 600 600 As operated 350 650 200-500 32-26 Flow rate, gal./min./sq. ft.: As designed 1.12 1.85 2.2 2.00 As operated 0.65 2.00 0.86-2.2 2.46-2.0 Retention time, min.: As designed 66 61 4s 39 As operated 114 56 48 33-39 Raw water Characteristics, p. p. m.: Turbidity 2 10 2 640 1250 Total hardness (as CaCOz) 300 350 370 106 102 ’ Calcium hardness (as CaCOs) 222 160 280 84 78 Magnesium hardness (as CaCOa) 78 190 90 22 24 Total alkalinity (as CaCOd 266 370 314 61 ~. 74 Free carbon dibxide 15 16 27 Efauent from accelator, before reoarbonation or filtration, p. p. m.: Turbidity 3 5 10 10 10 Total hardness (as CaCOa) 85 112 97 68 50 Alkalinity (as CaCOa): Phenolphthalein 31 83 10 15 23 Methyl orange 46 124 45 30 37 Chemicals used for treatment, Ib./ 1 0 0 0 gal.: Hyhrated lime [approx. 90% available Ca(OHh1 2 . 2 4 3.66 1.78b 0.84 0.92 Soda ash (98% NazCOa) 0.053 0 0 0 0.44 Aluminum sulfate (filter alum) 0.106 0.145 0.07 0 0 Ferric sulfate (Ferrisul) 0 0 0 0.25 0.25
.. ..
Z . A , Anna I11 * B Williams Bay Wis.; C Georgetown, Texas (no reoarbonation); b,,aern&+tratorAoceiator watkr softener, Baton Rouge, La. (no recarbonation or filtration). b Equivalent of the 1.36 pounds quicklime actually used when these tests were made.
The chemical characteristics of the treated water shown in the table do not, of course, represent the maximum reduction of hardness that would be possible by properly modifying the chemical dosage but, rather, the result of the specific chemical treatment decided on for reasons of over-all economy or desirability under the local conditions obtaining.
Other Uses of t h e Accelator The utility of the Accelator type of treatment and apparatus is not limited to water softening, although of the approximately thirty-five plants which have been installed since the first in 1935, the majority are used for that purpose; the treating capacities range from a few thousand gallons per day for small industries to several million gallons per day for large industries and communities. .4n appreciable number of plants, however, are employed successfully for clarification of turbid or colored waters where softening is not required. The water is usually treated with aluminum sulfate or other coagulant and any auxiliary reagents necessary. The slurry therefore consists essentially of aluminum hydroxide and coagulated colloidal matter. The same general principles of design and operation apply in Accelator clarification practice as in softening, and the same order of low turbidity in the effluent is secured.
Typical Operating Data
Literature Cited
Table I gives condensed operating data for four Accelator softening plants. Three of these installations ( A , B, C) soften the community supplies of the cities indicated. All three supplies are derived from wells. Data for the fourth plant (D), a small demonstration installation operating on Mississippi River water, are inserted primarily to indicate the possibilities of simultaneous softening and clarification by this method of a highly turbid surface water. All three of the city installations ( A , B, C) were made before the recently adopted normal rate of upward flow (3 gallons per minute per square foot of slurry pool surface) was established.
(1) Baylis, J. R., IND. ENQ.C H ~ M19, . , 777 (1927). (2) Green, W. H., a n d Behrman, A. S., U. S. P a t e n t 1,653,272 (Dec. 20, 1927). (3) Hoover, C. P., J. Am. Water Works Assoc., 29, 1694-5 (1937). (4) Hoover, C. P., and Montgomery, J. M., Ibid.,21, 1218-24 (1929). (5) Koyl, C. H., U. S. P a t e n t s 653,011 (July 3, 1900) and 677,669 (July 2, 1901). (6) Langelier, W. F., J. Am. Water Works Assoc., 28, 1500-21 (1936). (7) Spaulding, C. H., U. S. P a t e n t 2,021,672 (Nov. 19, 1935). (8) Sutro, H. H., a n d Booth, L. M., Ibid., 797,759 (Aug. 22, 1905). RECEIVED October 4, 1938. Presented before the Division of Water, Sewage, and Sanitation Chemistry at the 96th Meeting of the American Chemical Society, Milwaukee, Wis., September 5 to 9, 1938.
