Formation of Floc by Aluminum Sulfate A. P. BLACK,OWENRICE,University of Florida, Gainesville, Fla.,
AND
EDWARD BARTOW
State University of Iowa, Iowa City, Iowa The indicator solutions formerly used gave a p~ in unbu#ered solu~ionsmore than tiE true p H as determined with properly adjusted (kohydrie) indicators. At l e d 2 liters of water should be used for all jar tests for plant control in order to eliminate effects due to the size of the cessel. Sulfate in amounts front 25 lo 250 p . p . ni. ezlend the zone of rapid floc formation for to the acid s&. Chloride ion ezerts relatiliely little effect below 6.5 p H but does eslend the zone toward the si&. ~ ~ results are.found to hold on a serniplant scale. Conlin~llous stirrirt!f is absolutely rwcessary the conduct of jar tests which will check euch other and give accurate data for plant operation.
atid (2) the size of the reaction vessels. Their values for pI1 important a d v a n c e s h a v e b e e n m a d e in were determined by the coloriwater purification practice in the lnetric past thirty years, not so much used were exceedingly dilute, b e c a u s e of the introduction of varying from 0.0002 .$Ito 0.002 M , and were in most cases very new m e t h o d s as h e c a u s e of a better understanding of the slightly buffered. Recent work various principles involved in by Xolthoff and Kameda (/I), the methods o r i g i n a l l y emand Fudge (8), Sclllegel and Steuher ( l o ) ,F~~~~~~ and ployed. The r e s e a r c h work o f t h e Acree ( 5 ) , and others has shown that serious errors may result in colloid chemist explains the process of coaguliation ( f o r m a t i o n the determination b ~ of the pH~ of unbuffered solutions by the colof floc) for the removal of color or turbidity. Colloid chemistry orimetric method unless certain has showii t h a t t h e o r i g i n a l precautions are taken. The pH ideas concerning the action of of tile indicator solution itself floc were largely incorrect, but must not he very different from that a more or less fortuitous that of the unbuffered soluti~n choice of coagulants was a happy treated o r i t will p r o d u c e a one. Even with full knowledge of tlie phenomena, no better correspnding change in the pH of the solution. A coloricoagulants have been found. Saville (9) shows that the color mctric method for adjusting indicator solutions described by present in natural waters is colloidal. Acree and Fswcett (1) permits adjustment to any desired Miller (7) seems to have been the first to p i n t out the ef- pH value or series of values within the useful range of the ficient role of the trivalent aluminum and ferric ions in neu- particular indicator. It is thus possihio to employ an inditralioiiig and precipitating the negatively charged color col- cator solution termed “isohydric” of approximately the same loids, and to conclude that the absorptive property of the pII value as that of the solution being tested. Isohydric indigehtinous floc plays, as a rule, a relatively unimportant part cator solut.ions are recommended wherever accurate results in the process. on unbuffered solutions are desired. This p i n t becomes of Miller summarizes his own work together with the thorough importance when it is remcinbered that indicator solutions study of Theriault and Clark (21) of the relationship between prepared by the directions of Clark (S) are, almost without hydrogen-ion concentration and floc formation. The follow- exception, more acid than the acid limit of the virage-i. e., ing conditions must be fulfilled for a successful clarification useful range of the particular indicator. The indicator soluand coagulation: (I) There must be present a certain mini- tions now furnished by most leading firms are adjusted to the mum quantity of aluminum or m i d - p o i n t s of their respecferric cation; (2) there should tive ranges, thus differing by be present an anion of strong riot m o r e t h a n 0.8 pI1 unit c o a g u l a t i n g power; (3) the from tlie pK of the s o l u t i o n pH must he carefully adjusted. tested. The indicator soluDart.ow and Peterson (B) tions employed in this nork showed that even moderate w e r e a d j u s t e d in s t e p s of a m o u n t s of c e r t a i n anions 0.3 pH unit, thus making i t present in natural waters propossihle to select one practiducealargeeffeotonthe rateof eally isohydric with the solucoagulation and the optimum tion heing tested. The writers p r e c i p i t a t i o n of alum floc. hare now in progress a careThey point out that the work fulstudyof the a d j u s t m e n t of Theriault and Clark (11) on FIGURE1. LABOR*TORY Srmarsc Macrr~se of indicator solutions by both theeffectofpHmaynot hecorthe c o l o r i m e t r i c method of rectly interpreted unless they take into account. the relatively Faweett and Acree (5) and by the quinhydrone electrode, and strong concentrations of buffer salts. They call attention of the buffer capacity or, better, the buffer index of natural also to the now familiar fact that uniform stirring of Iabora- and treated waters. tory samples when any type of floc is heinl: studied is ahsoThe volumes of solution employed by Fawcett and Acree lutely essential for duplicat.ion of results. Their studies indi- were very small (200 ml.), and the method of stirring did not cate that thc presence of dissolved sa1t.s may play a role fully permit continuous observation. Theriault and Clark (11) as important as pH in the location of the point of opt.imum floc pointed out the decided effect which the size of the containing formation. vessel exerts on the time of floe formation, and Willcomb ( H ) , It is probable that the results of the work of Bartow and in a paper which appeared after this work was completed, Peterson might be changed by two modificat.ions of their has presented an admirable discussion of mixing, both in tlie niekhod of procedure: (1) the method of determining pII laboratory and in the plant.
A N Y fundamentally
811
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812
I N D U S T R I A L A N D E N G I N E E R I N G C H E RI I S T R Y
Vol. 25, No. 7
EFFECT O F CHLORIDE AND SULFaTE I O N S ON TIMEO F FORVA-x-ere diluted to 1000 ml. to yield a working solution, 50 ml. of which when added to 2 liters of water represented a dosage of TION OF BLUJ~ISUM FLOC 2 grains per gallon.
Six battery jars of 3.5 liters capacity TTere used for the coagulation experiments (Figure 1). Two liters of water were used for each test. The water in the battery jars was stirred by glass stirrers, each of which had two 3 X 15 cm. blades.
A 0.077 N sodium hydroxide solution was prepared, 8 ml. of which were exactly equivalent to a 2-grain alum dosage.
Solutions of 33.54 grams sodium sulfate (?u’azSO~~lOHzO) ,per liter, 16.38 grams sodium chloride per liter, 13.77 grams sodium bicarbonate per liter, and 0.014 N sulfuric acid were of such strength that convenient small volumes could be used to furnish the dosages of the ingredients desired. As soon R S the floc had formed in a jar, a sample was removed from the machine without interrupting the stirring and was placed in a clean Erlenmeyer flask, tightly stoppered, and allowed to settle until clear and bright. This usually required about 2 hours. Ten-ml. samples of the water were then carefully pipetted into color-comparison tubes filled with air free of carbon dioxide and containing ten drops of indicator solutions of varying pH values. Thorough mixing was secured without shaking by allowing the water to flow from the pipet tip into the indicator solution in the tubes. The tubes were stoppered to exclude carbon dioxide and then compared with buffer color standards in a LaMotte Roulette comparator. Ordinarily determinations were made on each sample with indicator of three different pH values, from which data a value for the hydrogen-ion concentration, reliable to 0.05 pH, could be obtained. The indicator solutions mere adjusted to the desired pH values by the method of Fawcett and Acree ( 5 ) .
EFFECT OF INDICATOR PH. The p H of the indicator solution exerted, as expected, a large effect upon the p H of the waters tested (Table I, columns 7 and 8). Where the p H of the indicator solution is not exactly the same as that of the solution tested, the actual pH is obtained by interpolation (Table I, column 9). It should not be inferred that the
6.5
1.0
.
7.3
8.1
pl!
FIGURE2 The blades were attached to vertical glass rods, 1 x 25 cm.;
these in turn were attached t o vertical steel drive shafts by heavy-walled rubber tubing so that each might be disconnected and a jar removed during a run. The vertical shafts were supported on an iron framework and on the upper end had pulleys, all driven by one continuous belt from a small electric motor. Identical conditions of illumination and the formation of Tyndall cones were produced by rays from 100watt Mazda bulbs, controlled by a single snitch, the light from which passed through 3/~-inch (0.95-cm.) holes in a long wooden box placed behind the jars. The Tyndall cones produced were so sharp that it was usually possible to duplicate the shorter floc times t o 15 seconds in successive experiments, T o exclude heating effect from the bulbs, the side of the box nearest the jars was lined with asbestos board which, together with a 1-inch (2.5-cm.) air gap between the box and the jars caused a maximum rise of only 1 C. during the longest runs, The apparatus with the exception of the motor and gear reducer was built in the University of Florida machine shops at little cost. Willcomb (12) describes the construction of laboratory stirring machines of this general type. GENERALMETHOD. Exactly 2 liters of distilled water were placed in each of the six jars. Enough of a standard solution of ftluminum sulfate was added to give 2 grains of alum to each jar. The mixture was stirred until thoroughly mixed. Varying amounts of standard alkali were added, the amounts being governed by experience. The time of addition for each jar was noted by means of a stop watch. The time a t which a welldefined floc formed in each jar was carefully noted, and the total elapsed time for each jar was corrected for the time required to add the alkali. STANDARD SOLUTIONS.-4 171-gram sample of very pure aluminum sulfate, Alz(S01)3~18Hz0, was dissolved in distilled water and made up to 2 liters. Sixteen ml. of the stork solution
PE
FIGURE3 variations will be as great in the case of natural or treated waters. To determine the extent of such variations, a further investigation has been undertaken. EFFECT OF SPEED OF STIRRING ON TIMEOF FLOC FORMATION. Too rapid stirring breaks u p the floc, whereas with too s l o stirring ~ its formation is slow, Trials were made at 26, 52, and 78 r. p. m., corresponding t o peripheral speeds of 0.5, 1, and 1.5 feet per second. There was little difference between the floc times a t 52 and 78 r. p. m., but about twice as much time was required for a good floc a t 26 r. p. m. The
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1933
813
difference was not due to the difference in time required for mixing, because a t 78 r. p. m. a dye was thoroughly mixed in 10 seconds, a t 52 r. p.m. in 15 seconds, and a t 26 r. p. m. in 60 seconds, whereas a floc that formed with a given dosage in 5 minutes a t 52 r. p. m. required 11 minutes a t 26 r. p. m., and one that formed in 22 minutes a t 52 r. p. m. required 45 minutes a t 26 r. p. m. Therefore, 52 r. p. m. was adopted as the rate for all experiments. TABLEI. EFFECTOF INDICATOR PH ON PH SOLUTIONS
OF
flocculation was plotted against volume of collecting vessel, the curve became flat near a volume of 2000 cc. Hence it would seem desirable that a t least this volume of water should be used in jar tests for plant control. EFFECTO F SULFATE I O N OK FLOCFORMATION. Sulfate ion as pure sodium sulfate (NazSOd.lOHzO) was added to the water with the alum in dosages of 25, 50, 125, and 250 p. p. m., and the jars were thoroughly stirred before addition of alkali (Table 111, Figure 3). UNBUFFERED
(Temperature, 26' C . ; dose, 2 grains per gallon; alum volume solution, 2000 ml.) P H -------TIME------Indica- Ohsvd. Actual JIR NaOH Initial Final Total INDICATOR tor soh. soln. Ml. Min. M i n . Min. Bromo6.30 6.70) 6.85 6.90,- 6.90 :3 9 thymol 0 39 7.29 7 . 0 5 \ blue 6.30 6.65 Bromothymol 6 85 6 70 P 6 . 7 0 $5 44'/2 '/l 7129 6 : 9 0 ! blue Phenol 1 12 11 red 7 -
i i
l'/a
16
14?/a
lO'/a
5
11
Is/#
12
6
11
2'/k
12
ga/4
Phenol red Phenol red Phenol red
' 7.00 j7.60 8.20 7.00 7.60 8.20
.)
7.55 7.601 7.70 \ 7.50( 7.60 r 7.75\
7.60 7.60
FLOC FORMATION WITH ALUJI AND ALKALI,Experiments were made in the forination of floc using a dosage equivalent to 2 grains of alum per gallon. Alkalinity was furnished first by sodium hydroxide and secondly by sodium bicarbonate (Table 11, Figure 2). TABLE 11. TIMEOF FORMATION OF ALUMFLOC WITH SODIUM AND BICARBONATE AT VARIOUSPH VALUES HYDROXIDE pH 6.60 6.70 6.80 7.15 7.30
NaOH-Time pH
Time
Min.
Min.
38.0 23.5 10.5 6.0 5.0
7.40 7.45 7.60 7.65 7.70
3.75 4.0 6.5 9.0 21.0
r
pH
6.55 6.65 6.70 6.80 6.90
NaHCOl Time pH
Time
Min.
Mzn.
19.0 15.0 10.0 6.0 d.0
6.95 7.0 7 10 7 30 7 70
5.5 5.25 4.0 3.75 3.75
DE
FIGURE4
While the floc times were slightly less than those of Bartow and Peterson (2) and the pH values shifted to the acid side, the coagulation zones established (Figure 3) do not differ IONON TIMEOF FORMATIONessentially from theirs. With increasing amounts of sulfate TABLE111. EFFECTOF SULFATE OF ALUMFLOC ion, the zone of rapid floc formation is broadened on the acid (pH varied b y addition of sodium hydroxide) side, extending to pH 4.8 where the highest dose was used. 25 P. P . 36. ,so4 50 P. P. M. so4 125 P.P. Y. 804 250 P.P. M . so1 pH Time pH Time pH Time pH Time EFFECTOF CHLORIDE IONON FLOC FORMATION. Chloride Mzn. Min. Min. Min. ion was added as pure sodium chloride in doses of 25, 50, 4.65 27.0 14.25 4.50 18.0 21.0 125, and 250 p. p. m. (Table IV, Figure 4). By comparison 14.25 4.80 8.0 4.60 4.25 7.0 5.10 12.0 4.70 4.0 4.0 6.5 with the effect of the test when no chloride was added (E'igure 5.85 10.0 4.90 3.75 3.75 6.5 6.10 5.75 8.25 5.75 3.50 3.5 1) it is seen that the lower doses have no effect and the higher 6.25 5.0 7.50 6.10 6.70 3.25 3.25 doses relatively little effect in extending the zone of rapid 6.25 6.70 3.25 4.0 6.50 7.10 3.50 4.25 5.25 6.85 7.30 3.50 5.75 7.05 coagulation toward the acid side. It is extended slightly 7.0 4.75 6.0 7.15 7.70 4.0 11.5 7.10 4.75 7.0 7.30 4.5 . . toward the alkaline side by the higher doses. 7.30 5.25 5.75 8.25 7.50 .. .. 7.50 13.0 ... .. 5 5.... 0 77 .. 67 00 293 .. 00 .. ... TABLEIV. EFFECTOF CHLORIDE 103ON TIMEOF FORMATION OF ALUM FLOC The experiments of Bartow and Peterson ( 2 ) showed the (pH varied with sodium hydroxide) optimum time to be a t p H 7.25, whereas the optimum time 25 P. P. M. c1 50 P. P. M. c1 125 P. P. M. c 1 250 P. P. 36. cl pH Time pH Time pH Time pH Time found in the present experiments was a t pH 7.40. The difMin. Min. Xin. Min. ference is undoubtedly due to the use here of adjusted indi6.60 26.0 6.40 23.0 5.85 20.0 5.30 37.0 6 . 5 0 6 . 7 0 1 0 . 5 6 . 1 5 1 5 . 0 5 . i 5 19.0 1 2 . 0 cator solutions. Their value was more acid as would be ex7.0 6.75 6.20 6.0 6.45 6.25 8.25 7.5 pected. Minimum floc time of the present writers was 4 6.90 6.50 7.30 5.25 6.70 4.75 5.25 6.0 7.70 20.0 6.90 5.0 4.25 2.20 7.10 4.5 minutes, compared with 12 minutes for Bartow and Peterson. 7.10 7.80 34.0 , .60 7.0 4.50 7.35 4.75 .. ... 7.50 7.50 7.80 7.5 4.,5 6.5 The difference may be due to different methods of stirring, 7.80 .... ... 15.0 .... ... 87 .. 09 0 162..205 z.90 but, as their stirring was rapid and thorough, it is probably 19.0 ... ... .. ... , . 9 5 3 6.0 .. ... * . . . . due to the volume of water and the size of the container used-2 liters and 3.5-liter jars here and 200 ml. and 300-ml. bottles for Bartow and Peterson. The effect of the chloride and sulfate ions on the positively Theriault and Clark (11) reported that, when the time of charged colloidal floc conforms, qualitatively a t least, to
a14
I N D U S ' f I( I A 1.
A
Pi D E S G I N E E R I N G C H E M I S T I t Y
Schulz's rule, the monovalent chloride ion exerting far less effect than the bivalent sulfate ion. ELPERlflESTAL t$',4TER
TREATMEKT I'LArT
It would he of interest to check certain narts of the work done in jars in a semiplant or a plant scale, ;-here large qnantities of water can be treated. Water treatment plants cannot carry on experimental work which might interfere with a m i formly satisfactory quality of water. Data on experimental work of plant-.scale size is therefore almost, entirely lacking in
Val. 25, No. 7
volumes of solution a t a practically uniform rate over a period of several hours was satisfactorily accomplished by the use of a siphon (Fignre 5 ) operating from a reservoir of solution held at constant level by means of a large inverted flask of the solution. Each siphon was fitted with a glass stopcock, and the rate of flow adjusted. wine a ston aatch and eraduated cylinder. Each siphon 'was cxecked hourly during the course of a run, and, if a variation of more than 2 per cent was noted, the run in question was repeated. The sodium sulfate solution or mixed acid-su1fat.esolution was added a t the upper end of the first mixing basin. The rapid and thorough mixing provided in that basin for a period of almost 2 minutes insnrcd a uniform water a,t the point of addition of the coagulant. The alum s o l u t i o n was alloved to flow directly into the 2-inch pipe which discharged the water from the first to the second mixing basin, thus insuring thorough mixing as the water entered the basin. The water treated was the regular Gainesville mnnicipal supply, having R mineral content as follon~s:
PILO~ED~ InRorder ~ : . thnt the msolbs obtained
the literature. The work of Bat.es mentioncd by Dartorv and Peterson ( 2 ) showed that an equivalent amount of sodium sulfate replaced sulfuric acid as a n accessory agcnt in alum coagulation and furnished identical results without measurably affecting the pR. Experiments on a semiplant or plant scale were carried on a t the University of Florida, which has an expcrimental water treatment plant designed for teaching and experimental purposes. The plant (Figure 5 ) consists of an aeration hasin, a 600-gallon constant-level storage tank, four mixing basins, tn-o settling basins of approximately 400 @dlons earh, a, rapid sand filter, and a wash water reservoir. Water i s circulated through the plant partly by gravity and parily hg R small Fairbanks-Morse motor-driven pump a t a rate of 310 gallons per hour. It is fitted with a system of piping and valves which permits great flexibility in ibs operation. .4 small control laboratory is provided. The first mixing basin is divided into eleven small compartments by round-tlie-end baffles and is nsed for mixing the added chemicals. The next two mixing basins are divided by over-and-under baffles into fifteen small compartments in such a way that, u-hen format,ion of floc is observed in a compartment,, the time of floc formation after the addition of alum in tlie first compartmcnt can he calculated. Thc rate of flow was measured by a carefully calibrated water meter and cheeked by weighing the water delivered during measured time intervals. A iiniform rate of flow through the plant was insured by an overflow pipe at the top of the storage tank which maintained a constant level in the tank. R-ater was delivered from the bottom of the tank to t h e pump a t constant head. The calculated floc times were checked both by use of dye and a suspension of alum floc. Solutions of alum, sodium sulfatc, hydrochloric acid, and sulfuric a rid m r e prepared of such strength that, xhen fed a t known 1-ulumes and at known rates, they gave the desired dosages. ADDITIONOF SOLUTIOXS. The addition of relatively mall
....".....I.~~~.~ .. oiilv on clear dam since it wurs found that the Doint of first definite floc formation 'could best hk determined -by employing the ~~~
Tyndall cone produced by a beam of sunlight reflected down into the mrnnartment, bv .___ -.... * ." R, mirror. Bv nartlv " shadine the comgartment and proper manipulation of the mirror, it
