fl-i. 51 wsn > f f 4 :,- « ’ MILLER’S White Cloth Flap Binders, MANUFACTURED BY HAROLD E. MILLER 540-542-544-546 B’way, Albany, M. Y. SEND FOR PRICES. A COMPARATIVE STUDY THE METHODS USED FOR THE MEASUREMENT OF THE TURBIDITY OF WATER GEORGE C. WHIPPLE DANIEL D. JACKSON Reprinted from Technology Quarterly, Vol. XIII, No, 3 , September, 1900 UNIVERSITY OF ILLINOIS CHEMISTRY DEPARTMENT ARTHUR WILLIAM PALMER MEMORIAL LIBRARY 1904 W51 ytU \) L. V\ 274 George C. Whipple and Daniel D. Jackson. IT £ A COMPARATIVE STUDY OF THE METHODS USED FOR THE MEASUREMENT OF THE TURBIDITY OF WA TER. By GEORGE C. WHIPPLE and DANIEL D. JACKSON. Received July 25, 1900. At the present time, several different methods for the measure- ment of the amount of suspended matter in water are in common use. The most important of these are : i. The Gravimetric Method. 2. The Wire Method. 3. The Diaphanometer Method. 4. The use of Standards of Comparison. In addition to these are the Disc Method and the Photometer Method, the former applicable only to the comparatively clear water of lakes and reservoirs, and the latter too complicated for ordinary use. The weight of the suspended matter present in water is found by taking the difference between the total solids before and after filtra- tion through filter paper or through a Pasteur filter. The wire method, brought into use by Hazen, 1 consists in the observation of the visibility of a platinum wire lowered horizontally into the water. The turbidity scale is furnished by the reciprocal of the depth in inches at which the wire becomes invisible. The diaphanometer is an instrument devised by Hornung and improved by Parmelee and Ellms, 2 according to which the turbidity is measured by noting the limiting depth of liquid in a tube through which an image at the bottom can be discerned under standard con- ditions of illumination. The reciprocal scale is used as in the case of the wire* method. The use of kaolin standards of comparison was suggested by Mason, 3 but the authors have found that finely divided silica, obtained from diatomaceous earth, is more satisfactory. According to these 1 Hazen, Allen. The Filtration of Public Water Supplies. 3d edition. New York: Wiley. 2 Parmelee and Ellms. On Rapid Methods for the Estimation of the Weight of Sus pended Matter in Turbid Waters. Technology Quarterly , Vol. xii, No. 2, June, 1899. 3 Mason, Wm. P. Examination of Water. New York: Wiley. 4! * > o o Methods Used for Measurement of Turbidity of Water. 275 methods the turbidity of the water is estimated by comparing it with a series of standards of varying turbidity in glass tubes, the stand- ards being prepared by adding definite amounts of silica to distilled water, and the results expressed in parts per million of silica. 1 These four methods are all fairly satisfactory, but each one has its objections and its limitations. The wire method is the simplest and in many cases the most practical, but its results are only approxi- mately correct, and its use is limited to the hours of bright daylight. The gravimetric method is a long process, and requires careful manipulation and accurate weighing. It takes no account of the state of division of the suspended matter. The diaphanometer method is of more general applicability, but it demands a somewhat elaborate apparatus and a uniform source of light. The use of silica standards of comparison is satisfactory .for waters of low turbidity, but less so when the turbidity is very high. The comparisons cannot be well made with artificial light. Each of these methods, therefore, has its especial field of applica- bility. The wire method is best adapted to field work and to the study of the turbidity of streams where a single daily observation is sufficient ; the diaphanometer is of use in connection with the opera- tion of filters, where it is necessary to continue the observations through the night ; and the method of comparison with standards is suitable for general laboratory use. It is neither practical nor desir- able, therefore, to limit in any way the use of these various methods, but it is desirable that the relations that exist between the different methods be definitely known. The great importance of the knowl- edge of the turbidity of our American streams and the magnitude of the field of observation render it imperative that some standard of turbidity shall be universally recognized. Only thus can the results obtained by the different methods be made comparable. Such a standard must be one that is permanent and capable of exact duplication by different persons, and it must be of such a nature that comparisons may be readily made with any of the methods now in use. The scale of turbidity must be a uniform one, that is, the figures that express the turbidity must be directly proportional to the amount of suspended matter present in any given state of sub' division. 1 Whipple, G. C., and Jackson, D. D. Silica Standards for the Determination of the Turbidity of Water. Technology Quarterly, Vol. xii, No. 4, December, 1S99. 276 George C. Whipple and Daniel D. Jackson. With this object in view, the authors have carefully studied the methods now in use and the relations that exist between them, and have reached the following conclusions : 1. No optical method based upon the reciprocal scale can serve as a standard, for such a scale is not a uniform one. 2. In order to obtain a uniform scale with an optical method, it is necessary to calibrate the apparatus used, to correspond with frac- tional dilutions of a water of definite turbidity. 3. The water of definite turbidity thus used should be considered as the standard of turbidity, and turbidity readings by all methods should be expressed in terms of this standard. 4. Such a definite standard of turbidity is found in the use of finely ground diatomaceous earth, as described by the authors. 1 The Gravimetric Method. The determination of the weight of suspended matter in water is not an exact process. For various reasons the results are liable to be in error by 5 parts per million, and at times by more than this amount. With very turbid waters the percentage of error, however, is small, but when the water contains less than 25 parts per million of suspended matter the error is considerable, and when below 5 parts per million the results are practically worthless. If the particles of suspended matter are large, they can be separated from the water by using a close filter-paper ; but when the water contains fine silt, clay, etc., it is necessary to use a Pasteur tube. The Wire Method. The accurate observation of turbidity by the wire method demands constant conditions of light and a wire of uniform size and brightness. Hazen has stated that the observation should be made in the open air during the middle part of the day, and with the wire shaded from direct sunlight. When the turbidity of the water is such that the wire can be seen at depths greater than about one foot, these condi- tions are necessary, but when the turbidity is greater than that men- tioned, the error from variations in light appears to be less. Thus the authors have found that when the turbidities were greater than .20 on the reciprocal scale, the same readings were obtained at all Loc. cit. Methods Used for Measurement of Turbidity of Water. 277 times during the day when the sun was more than about io° above the horizon ; that readings made under a porch agreed with those in the open air, and differed but little from those made indoors near a large window. Direct sunlight introduces an uncertainty in the read- ings, and is to be avoided. The nature of the receptacle containing the turbid water has practically no influence on the reading, provided that the diameter is at least twice the depth at which the wire becomes invisible. With receptacles of smaller diameter than this the opacity of the sides has a slight effect on the reading. Accord- ingly, a water pail 12 inches in diameter should not be used for reading turbidities below .17 on the reciprocal scale. It is prefer- able to have all the illumination from the top and not to use a glass jar so supported that light may enter from beneath. Differences in reading may be obtained by changing the distance of the eye from the water, as will be seen from the following table : Distance of eye above surface of water. Turbidity Readings. No. 1 . No. 2. No. 3 . No. 4. 4 inches. 00 .50 .26 .145 8 inches. .71 .45 .25 .140 12 inches. .73 .47 .26 .140 20 inches. .78 .48 .28 .145 50 inches. .82 .49 .29 .150 As might be expected, these differences are greatest in the case of very turbid waters. The readings are most accurate when the distance of the wire from the eye is about equal to that of normal vision. Throughout the present investigations this distance has been pre- served as nearly as possible. In field work, however, it is a com- mon practice to read the rod in a standing position, the wire thus being from four to six feet from the eye. The size of the platinum wire used has a slight effect on the results. At Pittsburg it was found that a wire 0.4 mm. in diameter gave readings 25 per cent, higher than those obtained with the stand- ard wire, 1 mm. in diameter, and that a wire larger than the standard gave somewhat lower results. The authors have compared turbidities 2yS George C. Whipple and Daniel D. Jackson . with wires i mm. and 1.5 mm. in diameters, and have obtained results that were in substantial agreement with each other. Comparisons between the wire method and the disc method 1 have shown that the latter gives somewhat lower results, — that is, the disc can be seen at greater depths than the wire. This is shown by the following table : Turbidity by wire method. Turbidity by disc method. Per cent, which the turbidity by disc method was of the turbidity by wire method. .098 .087 90 .120 .105 87 .135 .125 91 .183 .165 92 .450 .363 80 .609 .444 73 .917 .625 68 With high turbidities the differences are considerable, but with com- paratively clear water the results of the two methods approach each other. Turbidity reading by the wire method may be therefore extended into the clear water of lakes by the substitution of the disc, without the introduction of serious error. It has been generally assumed that the turbidity of water is in- versely proportional to the depth at which the wire becomes invisible, and the reciprocal scale is based upon this assumption. Hazen states “that if a water having a turbidity of 1.00 is mixed with an equal volume of clear water, the mixture will have a turbidity of 0.50, and advantage is taken of this fact for the measurement of high turbidities by dilution.” Upon this assumption also depends the accuracy of the elaborate apparatus for turbidity measurement used in the filtration experiments at Washington, 2 D. C., where high turbidities were read by dilution with clear water and where low turbidities were read by admixture with a water of known turbidity. 1 Whipple, G. C. The Microscopy of Drinking Water, p. 73. New York : Wiley. 2 Report of Colonel A. M. Miller, on the Feasibility and Propriety of Filtering the Water Supply of Washington, D. C. Senate Document , No. 259, 56th Congress, 1st session. Methods Used for Measurement of Turbidity of Water. 279 According to the experience of the authors, the assumption that the turbidity is inversely proportional to the depth of the disappearing wire, is only approximately correct when the distance of the wire from the eye is substantially that of normal vision. 1 If a water in which the disappearing wire has a depth of one inch is diluted with an equal volume of clear water, the wire will not disappear at a depth of exactly two inches, but at a depth somewhat less than two inches ; or, in other words, a turbid water, when diluted with an equal vol- ume of clear water, will have a turbidity, according to the reciprocal scale, of somewhat more than one-half the original turbidity. This has been shown by scores of observations, but a single example will serve as an illustration. Turbidity calculated according to the reciprocal scale. Observed turbidity. Original water 1.60 1.60 Diluted ^ with clear water . .80 .83 Diluted ^ with clear water . .40 .47 Diluted ^ with clear water . .20 .26 Diluted j-q with clear water . .10 .145 This fact has been noticed by other observers, but its important bearing upon the value of turbidity readings, expressed in terms of the reciprocal scale, has not been appreciated. Parmelee and Ellms, as well as Hazen, found that the ratio between the weight of sus- pended matter and turbidity increased with the turbidity, but the reason for this was attributed to the differences in the size of the sus- pended particles, — larger particles as a rule being present in greater amounts in the water of any stream during periods when the turbidi- ties are high. If, however, it is true that turbid waters diluted one- half do not have half the turbidity according to the reciprocal scale, it follows that the present method of stating results is not the best one, as the figures that express turbidity do not give true comparisons of the amount of suspended matter present. The size of the particles does have, indeed, an important effect upon the turbidity, and a given weight of fine clay particles does produce a greater opacity in the 1 The assumption is more nearly correct, however, for observations made with the eye at a distance of five or six feet from the wire, as in Hazen’s practice, and this fact should be remembered in the present discussion. 28 o George C. Whipple and Daniel D. Jackson. water than the same weight of coarse silt particles ; but the effect of the water itself also has an important influence. Th^ amount of absorption of light by water is greater than is sometimes supposed. Wild has given the following coefficients of absorption for distilled water : Temperature. Intensity of light after passing through io cm. of distilled water. 24.4° C 0.9179 17.0 09397 6.2 0.9477 Not only does the absorption vary with the temperature, it varies greatly with the character of the dissolved substances. In colored waters the absorption is much greater than in clear waters, but the coefficient is at present unknown. In clear waters the intensity of the light at various depths may be stated approximately as follows : Depth in inches. Intensity of light. 0 1.00 1 .98 5 .92 10 .85 15 .79 20 .72 It is apparent, therefore, that the absorption of light by the water exerts an important influence on the visibility of the wire. The absorption of light by the particles of suspended matter at different depths is an even more important factor, but its amount is unknown. Furthermore, the opaque particles act as a screen to shut out the object in view, and the distance of these particles from the eye must be taken into account. The optical phenomena involved are compli- cated, and it is unnecessary to analyze their relative importance. It is sufficient to determine at what depths the wire becomes invisible in waters of definite turbidity with different degrees of dilution, and from these to formulate a law for practical application. Methods Used for Measurement of Turbidity of Water. 281 Distilled water rendered turbid by the addition of 1,000 parts per million of finely divided silica was put into a jar, and the depth of the disappearing wire observed. It was then diluted with distilled water to various degrees, and a series of similar observations made. The results of these observations are shown in Figure 1. o / OO 2 1 1 1 | 1 F>FF?TS Psrej M/LUO/V OF S/L/CF 00 300 400 500 600 700 8< OO 9 00 /c )00 1 — tr=r=*-~ *--«* — \ 'X A y wS- £ Jr 7 r /o' d . ft I / X 1 7 / / 1 •i DIAGRAM SHOWING THE DEPTH AT WHICH A PL A T/NU/Vt WIRE WILL BECOME / HV/S/BLE IN WA TERS MADE TURBID BY DIFFEREN T AMOUNTS OF SILICA. s - \ >o \ $ - ^ //? / 1 / l / l I l / 1 1 1 fri — 1 l i | 1 L 1 f f 1 /o « to on CU Fig. 1. It was found that when the water contained 666 parts of silica per million, the wire disappeared at a depth of one inch. This turbidity therefore corresponds to unity of the present method of stating results by the use of the wire. Starting from this point, the curve of the reciprocal scale is shown by the dotted line. According to this curve the wire should have disappeared at two inches when the water con- 282 George C. Whipple and Daniel D. Jackson. tained 333 parts of silica, at three inches when it contained 222 parts, and so on. The diagram shows, however, that the depths of the disappearing wire did not follow the reciprocal curve, but did follow a somewhat similar curve. Within the limits of practical observa- tion this curve was found to have the following formula : 5 _ 400 d — 0.4 where 5 represents the number of parts per million of the silica standard, and d represents the depth of the wire in inches, or ^ 1. 016 == d r — 1.016 where d' represents the depth of the wire in centimetres, or £ 400 t 1 — 0.4 t where t represents the turbidity expressed in terms of the reciprocal scale. This curve is shown on the diagram by the full line. Observations were then made with other substances than silica, to see if this formula could be generally applied. Natural turbid waters from various sources were diluted with distilled water and the depth of the disappearing wire ascertained. Some of these results are shown in Figure 2. As in Figure 1, the dotted line represents the curve of reciprocals, and the full line the curve obtained by plotting the above formula. In Figure 2, however, the abscissae represent not weights of silica, but a uniform scale of turbidity, where unity is the turbidity that causes the wire to disappear at the depth of one inch. The black spots represent the observations obtained by the authors using material that was obtained from the experimental filters at Pittsburg, 1 Pa., to pro- duce the turbidity. It will be seen that they follow the curve very closely. The circles represent observations obtained by Parmelee and Ellms on the water of the Ohio River. These also follow the general curve. Observations with natural waters and with waters rendered artificially turbid with kaolin gave similar results, while waters ren- 1 This material was kindly furnished by Mr. Wm. R. Copeland, Bacteriologist-in-charge. Methods Used for Measurement cf Turbidity of Water . 283, dered turbid with colored substances, such as coal-dust, lamp-black, iron oxide, etc., gave but slightly different results. In no case did the observations follow the curve of reciprocals, and in every case the curve was similar to that obtained for silica. It is apparent, therefore, that the above formula may be given general application within the limits of the ordinary observations of 0 .JO 2 ru^e/ry Accot 0 so 4 101*1$ TO A UA/jrOt}M SCALE. 0 so &o 70 8 0 D r /£ 0 __o A > 6 $ 8 * ■N * " /o £ fc- * n 14 L fa b *>A m./ 0 /V 4* 5 / A j U / 1 1 DIAGRAM SHOWING THE DEPTH AT WHICH /) PL/ T/ HU At W/PE W/LL BECOME / //VISIBLE /A/ W/TEPS OF D/FFEPENT TC/PB/D/TY • OBsepva r/ofjs bv wh/pple jwo jacksoa/ O OBSCP VA T/O/VS 0Y PA/fAVBlEE A/VO PL LAPS ' / 1 1 1 1 • 1 1 1 1 1 1 1 1 1 f 1 f 1 1 1 1 /6 1 . 1 ’ 1 1 1 1 1 8 1 1 1 1 1 i 1 1 1 1 1 Fig. 2. turbidity with the wire method. By its use turbidities obtained by the wire method may be expressed in terms of a uniform scale, a decided advantage over the reciprocal scale. This position is further strengthened by a comparison of the tur- bidity readings with the amounts of suspended solids. It has been found that the ratio between suspended solids and turbidity expressed by reciprocals of the depth of the disappearing wire is far from constant, but that the ratio between suspended solids and turbidity 284 George C. Whipple and Daniel D. Jackson. calculated to silica from the wire readings is nearly a constant for samples of water from any particular locality. The following obser- vations of suspended solids and turbidity by the wire method, obtained, by Parmelee and Ellms for the Ohio River, illustrate this fact. Weight of sus- pended solids. (Parts per million.) Turbidity by wire method. (Reciprocal scale.) Turbidity reduced to silica standard. Weight divided by turbidity, accord- ing to recipro- cal scale. Weight divided by tur- bidity in terms of silica standard. 33 0.17 75 195 .44 62 0.30 136 198 .45 126 0.49 244 252 .51 248 0.86 527 275 .47 342 1.02 690 335 .51 453 1.14 833 344 .54 528 1.43 1,333 370 .40 Average, .47 Hazen, in his Filtration of Public Water Supplies, gives the fol- lowing equivalents of turbidity readings in weight of suspended matter for river waters with particles of average size. If these turbidities are reduced to silica by the formula, the ratio between turbidity and sus- pended matter becomes practically a constant when the turbidity by the wire method is less than 1.00. Turbidity by wire method. (Reciprocal scale.) Turbidity reduced to silica standard. Weight of sus- pended solids. (Parts per million.) Weight divided by turbidity accord- ing to recipro- cal scale. Weight divided by turbid- ity according to silica standard. 0.05 • 21 13 260 .62 0.10 42 26 260 .62 0.20 88 55 275 .62 0.30 136 85 283 .62 0.40 189 116 290 .61 0.50 249 150 300 .60 1.00 666 360 360 .54 Average, .61 Methods Used for Measurement of Turbidity of Water. 28 5 The following table will be found convenient for transforming tur- bidities obtained by the use of the wire at the distance of normal vision, and expressed according to the reciprocal scale into parts per million of silica. TABLE FOR TRANSFORMING TURBIDITIES OBTAINED BY USING THE WIRE METHOD WITH THE RECIPROCAL SCALE INTO TURBIDITIES EXPRESSED IN PARTS PER MILLION OF THE SILICA STANDARD. Wire .OO .OI .02 •03 .04 •°5 .06 .07 .08 .09 reading. Parts of ■ Silica Per Million. .00 00 4 8 12 16 21 25 29 33 37 .10 42 46 50 54 58 63 68 73 78 83 .20 88 92 96 101 106 111 116 121 126 131 .30 136 141 146 153 158 ! 163 168 173 178 183 .40 189 195 201 207 213 219 225 231 237 243 .50 249 256 262 268 275 281 287 295 302 309 .60 316 323 329 336 345 ! 351 358 366 373 381 .70 389 397 404 412 421 429 437 445 453 460 .80 470 478 487 497 506 515 524 534 543 552 .90 562 572 582 592 602 612 623 635 645 656 1.00 666 ... ... Wire .OO .IO .20 •30 .40 •50 1 .60 .70 | .80 .90 reading. Parts of Silica Per Million. 1.00 666 786 923 1,084 1,273 1,407 1,777 2,127 2,564 3,174 2.00 4,000 ... ... .... It is preferable, however, to have the turbidity rod graduated in terms of silica as well as in reciprocals. Such a graduated rod is shown in Figure 3. For the purpose of graduating the rod, or for transforming turbidities expressed in depths of the wire, the following table will be found useful. 286 George C. Whipple and Daniel D. Jackson . Fig. 3. — Turbidity Rod. Methods Used for Measurement of Turbidity of Water . 287 TABLE SHOWING THE DEPTH OF WIRE IN INCHES AND CENTIMETERS CORRE- SPONDING TO TURBIDITIES EXPRESSED IN TERMS OF THE SILICA STANDARD. Silica. Depth of wire. Silica. Depth of wire. Silica. Depth of wire. Inches. Centime- ters. Inches. Centime- ters. Inches. Centime- ters. 0 .... 65 6.54 16.5 240 2 06 5.2 1 400.40 1017.0 70 6.09 15.5 250 2 00 5 1 2 200.40 509.0 75 5.73 14.5 260 1.93 4.9 3 133.73 339.5 80 5.40 13.7 270 1.88 4.8 4 100.40 264.2 85 5.08 13.0 280 1.83 4.7 5 80.40 223.4 90 4.84 12.2 290 1.78 4.6 ■6 67.06 170.3 95 4.60 11 7 300 1.73 4.5 7 57 58 146 3 100 4.40 11.2 400 1.40 3.5 8 50.40 138 2 no 4.04 10.2 450 1.29 3.3 9 44.84 113 8 120 3.73 94 500 1.20 3.1 10 40.40 1118 130 3.48 9.0 600 1.06 2 7 15 27.00 68.6 140 3.25 85 666 1.00 2.5 20 20 40 61.0 150 3.06 7.9 700 96 2.4 25 16 40 417 160 2.90 7.3 800 90 23 30 13 73 348 170 2.75 7.0 900 84 2.1 35 11 83 30 0 180 2.62 66 1,000 80 2.0 40 10.40 26.4 190 2.50 63 — 45 9 28 23 6 200 2 40 6.1 — .50 8.40 213 210 2 30 5.8 — — 55 7.67 19.5 220 2.23 5.6 — — 60 7 07 17.9 230 2.13 5.3 ... The Diaphanometer Method. Parmelee and Ellms, in their use of the diaphanometer, found that the ratio between the suspended solids and the turbidity expressed in terms of the reciprocal scale was not a constant, but increased with i.the turbidity, as shown in the following table : 288 George C. Whipple and Daniel D. Jackson. Weight of suspended solids. (Parts per million.) Turbidity by diaphanometer, using reciprocal scale. W eight divided by turbidity. 33 .07 465 62 .13 484 126 .21 583 248 .38 657 342 .46 745 453 .55 820 528 .62 855 Accordingly, they adopted the following formula for transforming turbidities into weights of suspended matter : Weight in parts per million = 600 (Turbidity — 0.0 1), where the turbidity equals the reciprocal of the depth in inches of the water in the diaphanometer tube. On account of the nature of sus- pended matter in the water used in their work this formula is neces- sarily only of local application. Through the courtesy of Mr. George W. Fuller the authors have had the privilege of experimenting with the diaphanometer used by Parmelee and Ellms at Cincinnati. Turbidity observations have been made upon waters rendered artificially turbid by the addition of silica standard in various amounts. The results are shown in Figure 4. It was found that, with a turbidity produced by 1,000 parts of silica per million, the cross of light in the diaphanometer tube disappeared from view at a depth of 1.96 inches. With this as a starting point, the reciprocal scale would follow the curve shown by the dotted line. The observations, however, did not follow this curve. The curve of observations was similar to that obtained with the wire method, but the variations from the reciprocal curve were somewhat greater than was the case with the wire method. The curve of observations was found to have the following formula : d — 1 where 5 represents the number of parts of silica per million and d equals the depth of the liquid in the tube in inches, •or Methods Used for Measurement of Turbidity of Water. 289 c = 2 >433 d' — 2.54 where d! equals the depth in centimetres, or 5 = 958 t 1 — t where t represents the turbidity according to the reciprocal scale. 0 /OO 2 PAR 70 3 rs P£R AO/LL/OA/ OP S/L/tA 00 +OO sroo 600 7 00 8 00 £ 1 OO !C IOO 2 4 O Q < D T / < AS S' b O O 0 A Of,/ /