Silica Sol as Auxiliary Coagulant with Copperas GRAPHICAL DETERMINATION OF OPTIMUM PROPORTIONS OF DUAL COAGULANTS IN WATER TREATMENT A. A. HIRSCH State Department of Education, Baton Rouge, La.
AYLIS discovered the importance of colloidal silica in turbidity removal by coagulants (1-4, 6); and techniques were developed by Baylis (1,6) and by Graf and Schworm (8, 11) for the preparation and application of activated silica solutions as an adjunct to alum and some of the other coagulants. Following their work, a number of investigators have reported successful experiences in the use of silica sol with chemical treatments, usually with alum. Only one investigation (10) failed to note generally beneficial effects. Activated silica has also given favorable results in the chemical. treatment of sewage (7, 9). Nevertheless when first announced, universal benefits were disclaimed for this reagent, and its limitations in practical use have been indicated. Applicability of activated sodium silicate to the treatment of a given water is a matter of individuality as regards character of water, process, and plant. This paper relates some laboratory studies with activated silica sol on the Lower Mississippi River water, a t the Carrollton plant of the New Orleans Sewerage and Water Board, to determine its advantages as an auxiliary coagulant in treating a slightly alkaline, fairly hard, colorless, moderately turbid, but readily flocculated raw water. Plant treatment consists in preliminary grit removal, lime softening t o bicarbonate neutralization, intervening primary sedimentation, coagulation by copperas, and final sedimentation. Using the pair copperas-activated silica sol, it is evident that a given clarity in the settled water might be attained with an infinite number of combinations simply by varying the amounts and ratios of the chemicals. The problem therefore resolved itself into the selection of the most economical proportions of coagulants based on cost. A direct graphical method was employed that involved a minimum of calculation and gave immediately a program for the optimum coagulant adjustment t o yield any desired turbidity level in the settled water.
the bottom careful12 bent at the ex erimentally determined best angle-namely, 42 ; over this dezection was slipped sufficient rubber tubing to reach almost to the side of the vessel. After a flash mix and about 10 minutes of stirring at 175 r. p. m., the next chemical was added without removing the sample from the machine; the same rate continued for 10 minutes more. The speed was then lowered progressively to about 10 r. p. m., allowing 0.6-2 hour total slow speed stirring according to the needs of each particular series for good flocculation. Quiescent settling ranged from 16 to 24 hours before pipetting off a 400-ml. portion of the supernatant for reading on a Hellige turbidimeter. Identical procedure was followed throughout a given run. Silica sol was prepared from 41.1 B6. sodium silicate (containing 8.85 per cent Na2O and 28.7 per cent SiOz; ratio of Na2O t o Si02, 1to 3.22) after dilution t o 1.5 per cent Si02, almost complete neutralization down to 1200 p. p. m. calcium carbonate alkalinity using 12.6 per cent of the sodium silicate weight of 66" BB. sulfuric acid, 2-hour aging, and final dilution to 0.56 per cent Si02 (or 1.95 per cent sodium silicate content). This same batch of Baylis sol was used throughout all experiments; its age at the time of dosing varied from 6 t o 12 days. By applying coagulant doses in geometric series, a wide range of experimental results was obtained. RESULTS OF JAR TESTS
RAWWATERWITH SILICA SOL. Silica sol alone does not coagulate turbid natural waters (1). Silica sol failed utterly to flocculate 60 p. p. m. turbidity in raw Mississippi River water
EXPERIMENTAL CONDITIONS
During the period of these experiments the raw water characteristics and plant feed data were as follows: Color, p. p. m. Turbidity, p. P. m. Si02 Free Cor, p. p. m. Alkalinity, p. p. m. CaCOa
10 62 0 106 8.2 41 147 145
E r u s t a n t s , p. p. m. CaCOa P. m. CaCOa Total hardness Total hrrrdnesd t)falod.) Total dissolved solids (105' C . ) , p. p. m. 249
Ca, p. p. m. Mg,p.-p. m.
40.3 11.0
Na (oalod.), p. p. m. 22.3 9 1 0 2 (gravimetric), p. p. m. 5.4 804, p. p. m. 42.3 32 C1, p. m. Plant &sage, grains/gal. Lime 4.3 Copperas 0.34 Temperature, F. 49
The stirring machine used for this work was shop-made with belt-driven arrangement for treating twelve 1-gallon samples simultaneously. The stirring arm waa a, a/l&nch brass rod with
Figure 1. Effect of Order of Addition of Silica Sol and 4 Grains per Gallon of Quicklime Slurry on Final Turbidity of Mississippi River Water
a
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turbiditv. Reference to the dashed lines will be made later. Addition of activated silicate to a given copperas dose, except for the uncertain dip zone with low dosages of both materials, will improve final clarities down to a residual value where the curves flatten out parallel to the silicate axis. A limit in benefits possible from silicate is thus indicated. Increase in copperas lowers final turbidities, without exception. Iron alone can produce fairly low turbidities, particularly if the dosage is high enough; silicate alone is valueless. However, their combination is most effective, as the decided vertex in the curves shows. Floc break with sufficient iron present was aided by a very small silica addition. LIME-SOFTENED WATERWITH SILICA SOL AND COPPERAS.This combination is of particular interest, as the silicate meshes into the lime-iron process used locally. Starting with lime-softened primary settled water from the plant, various combined doses of silica sol and copperas were added t o explore their composite flocculating value. Results of the trial coagulations (Figure 3) show maximum curvature in the turbidity lines somewhat near the silicate axis. The curves run out at lower silicate than iron doses, an indication of the superiority of, silicate if either is to be used alone. Curves for
SILICATE
Figure 2. Copperas-Silica Sol Coagulation of Raw Mississippi River Water Figures
underlined represent turbidities.
20-hour
settled
and even, with ascending dosage, caused a small increase in final turbidity of the settled water by its action as a dispersion agent. L I h l E D iJrATER WITH SILICA SOL. Activated silica effectively coagulates Lower Mississippi River water that has first been lime-softened to balance bicarbonates. Floc growth was faster when silicate followed liming, but final clarity of the supernatant was improved if the lime mas added last. Comparative results as affected by the order of chemical addition are shown in Figure 1. I n subsequent experiments silica sol was added in advance of the main coagulant. As noted by previous observers, silica sol in practical amounts did not affect the pH of the water. RAW WATER WITH SILICASOL ANI COPPERAS.This combination is of interest for coagulating turbidity without the advantage of lime softening. Silicate materially enhances the action of copperas as Figure 2 shows. The two coagulant doses are plotted along the coordinate axes with final turbidities smoothed in parametrically. The figures underlined are the experimentally observed turbidity readings used to interpolate the contours. I n this way the plot shows all combinations of coagulants possible to produce a desired settled water
Figure 3.
Copperas-Silica Sol Coagulation of Limed, Settled Water
Plant quicklime dose, 4.39 grains er gallon. Figures underlined give 18-hour settred turbidities.
July, 1943
IN D U S T R I A L A N D E N G IN E E R IN G C H E M I S T R Y
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system. I n Figure 5 the price in dollars per ton of coagulant X is laid off along the coagulant Y in any arbitrary units. Inversely, the cost per ton of coagulant Y is marked off along the coagulant X axis in the same units, after this figure is adjusted for the difference in p. p. m. scale factors, as indicated in the diagram. Along the “equal cost slope” connecting these two points the total coagulant cost is a constant. By drawing lines parallel to the equal cost slope, tangent to all of the residual turbidity curves, the locus of the most economical coagulant adjustment is obtained through the points of tangency. Any other equal cost slope would cut a given turbidity curve as a secant and, from the direction of curvature, involve a higher cost. Locating an economy line by this method obviates the necessity for replotting the data directly on a cost basis. Correctness in establishing the slope of a constant cost line may be checked by multiplying its intercept dosage on any axis by the corresponding cost for the same coagulant; the products so found should be equal along either axis. ECONOMICS OF SILICATE-COPPERAS PAIR
Equal cost slopes and the loci of most economic pairing are indicated in Figures 2, 3, and 4 by the dashed lines. For purposes of calculation, prices per ton f. 0. b. plant are taken as follows: sodium silicate Figure 4. Filtrate Turbidities (through Whatman No. 41 Paper) Following the Copperas-Silica Sol Coagulations in Figure 3 $20, 66’ B6. sulfuric acid $15, and copperas Figures underlined are turbidimeter readings. $15. The cost of sulfuric acid for partially neutralizing the silicate amounts to $1.89 per ton of sodium silicate, or slightly . less than 10 per cent of the sodium silicate cost. very clear water are asymptotic t o the iron axis and demFor silicate-copperas clarification of raw river water (Figure onstrate the inapplicability of copperas alone in this region. 2) greatest economy is gained, particularly for the clearer From the pronounced convex inward type of curvature, it is settled samples, when the sodium silicate dose is roughly apparent that mixed coagulation surpasses a single coagulant three fourths of the copperas dose. in flocculating value and enables lower residual turbidities. Floc containing silicate was larger, especially when first formed, than when it contained copperas alone; mechanical envelopment of soil particles was thus promoted. But on aging, the silicated floc shrank to a smaller volume and showed greater resistance to breaking on agitation. Tough compact floc is not only ideal for filtering, but may also retard scouring of reservoirs and lengthen cycles between cleaning. Iron up to 4 p. p. m. also aided bottom floc consolidation a t a given silicate level. With either iron or silicate alone as coagulants, the silicate showed the greater degree of final floc compaction. Supernatant samples were also filtered through Whatman No. 41 paper in a 4-inch funnel and filtrate turbidities were determined. Results are plotted in Figure 4. Again the vertex in the turbidity lines is very distinct, a n indication of the advantages of bicoagulants. As before, some curves which follow along the copperas axis bend downward and outward a t the silicate axis; therefore, if a single coagulant is to be selected, activated silica sol is superior to copperas on a paper filter performance basis. The reversed curvature, near the silicate axis, is probably due to experimental uncertainties in measuring very low turbidities. I I I I I 2 3 4 5 LOCATION O F OPTIMA
Obviously, the higher the cost of a coagulant per ton, the greater will be the relative amount of its associate in the dual
COAGULANT
X
PRM.
Figure 5. Method of Establishing Minimum Cost in a Bicoagulant System
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INDUSTRIAL AND ENGINEERING CHEMISTRY
When coagulating lime-softened water (Figure 3) the economy line indicates the value of using approximately as much silicate as copperas beyond a very small iron lead until a dosage of 3 p. p. m. copperas is reached; treatment for lower turbidities can be more economically selected by increasing only the silica sol and still retaining slightly over 3 p. p. m. copperas as a constant feed. The high indicated relative amount of silica here reverses the roles of main and auxiliary coagulants and gives more value to activated sodium silicate than do previous studies. The economy of high silica proportions can be appreciated from its power to produce very clear water. However, if high settled-water turbidities are tolerable, the limiting condition points to the economy of copperas alone. For inferior filtrate quality (Figure 4 ) copperas unaided is the economical choice up to 2 p. p. m., but for improved clarity activated sodium silicate is best added in the proportion of 2 p. p. m. for every 1 p. p. m. copperas above a 2 p. p. m. copperas level. Figure 4 indicates that economical silicate dosage approximately equals the copperas dose at and above the 4 p. p. m. level. From these observations it is evident that considerably more emphasis is due to activated silica sol in its joint performance with copperas than has been accorded heretofore. APPLICATION TO PLANT PRACTICE
Although these curves definitely demonstrate the economy of sodium silicate in conjunction with lime softening and final coagulation with copperas, application of the Baylis sol
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to plant operation has been restricted at Carrollton to use as a palliative during periods of basin outage or obscure intervals of poorly coagulable turbidity, particularly in cold weather. When it has been employed, the progress of enhanced floc formation produced by preceding 0.3 grain per gallon of copperas with 0.1 grain per gallon of silica could be followed as a front advancing through the course of an aroundthe-end mixing basin. However, a t the Algiers plant (capacity of 6 million gallons per day with limited mixing and settling facilities), serving a municipal division on the west bank of the river, adoption of silica sol as routine in connection with the lime-iron process has resulted in improved performance. LITERATURE CITED
(1) Baylis, J. R., J . Am. Water W o r k s Assoc., 29, 1355-96 (1937). ( 2 ) Baylis, J. R., Water W o r k s Eng., 90, 971-4, 1035-9, 1101-5 (1937). (3) Baylis, J. R., Water W o r k s & Sezmrage, 83,470-3 (1936). (4) Ibid., 84, 61-3 (1937). (5) Ibid., 84, 221-5 (1937). (6) Ibid., 85, 855-8 (1938). (7) Christenson, C. W., a n d Lavine, Irwin, Trans. Am. I n s t . Chem. E ~ Q ~ 36, s . ,71-90 (1940). (8) Graf, A. V., and Schworm, W. B., Water Works Eng., 90, 1514 (1937). (9) Hurwitr, E., and Williamson, F. M., Sswaaa W o r k s J . . 12. 562-70 (1940). (10) Lordley, H. E., and Smith, M. C., J . Am. Water W o r k s Assoc., 31, 2149 (1939). (11) Schworm, W. B., Rept. 24th Ann Missouri Water Sewage Conf., 10, 37-41 (1938).
FUNGAL AMYLASES AS SACCHARIFYING AGENTS IN THE
Alcoholic Fermentation of Corn LU CHENG HAO, ELLIS I. FULMER,
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N TWO previous papers (3, 6 ) data were presented on
the use of mold amylase preparations in the saccharification of corn mash for alcoholic fermentation. The molds were cultured on wheat bran in rotating drums. In a general review of the use of microbial amylases in the alcoholic fermentation, one of the authors ( 5 ) discussed briefly the relative merits of these materials and malt. The present paper describes a new and more efficient laboratory method for growing the molds and compares the efficiencies of the mold-bran preparations from twentyseven strains of molds, representing four genera, as saccharifying agents in the alcoholic fermentation of corn. FERMENTATION PROCEDURES
From the results of preliminary investigations, the folloiving standard procedures were adopted. The stock cultures of the molds were kept on wort-agar slants. For cultivating the molds in flasks, transfers were made from well sporulated cultures to wheat bran mashes. The latter were prepared by mixing equal weights of wheat bran and 0.3 N hydrochloric acid in Erlenmeyer flasks and sterilizing for 30 minutes a t 15 pounds per square inch (1 kg. per sq. cm.) steam pressure. The bran mashes were heavily inoculated from well sporulated stock cultures and the flasks, lying on their sides, were
AND L. A. UNDERKOFLER Iowa State College, Ames, Iowa
incubated a t 30" C. For most of the work here reported, 500-ml. Erlenmeyer flasks containing 25 grams of bran were employed, but it was found later that more rapid growth and sporulation are secured if 10 grams of bran are used in 250-ml. flasks. After abundant sporulation had taken place, the cultures were used as inoculum for larger batches of bran mash. The well sporulated mold cultures on the bran may be allowed to dry undisturbed in the flasks and kept a t incubator or room temperature for many months without loss of potency as inoculum. The mold amylase preparations were produced by growing the molds on the wheat bran mash in special 3-quart aluminum pots equipped for aeration. The apparatus is simply constructed (Figure 1); it is a modification of that employed by Beresford and Christensen ( d ) , and has several advantages over the drum method previously employed in these laboratories (3, 6 ) . It takes less space and requires no special mechanical devices. There is no disturbance of the mold mycelium during growth and more uniform aeration is obtained. The growth of the molds is more rapid, and the pot
