THE AMOUNT OF COAGULANT WHICH VARIOUS WATERS WILL RECEIVE.
The amount of coagulant which can be safely used is dependent upon the alkalinity of the raw water. When sulphate of alumina is added to water it is decomposed, as explained above, with the formation of alumina, which is alone useful in the work of purification, and sulphuric acid, which combines with the calcium carbonate or lime present in the water. There should always be an excess of alkalinity or lime in the raw water. If for any reason there is not, there is nothing to combine with the liberated sulphuric acid, and the decomposition of the coagulant is not complete, and a portion of it goes undecomposed into the effluent. The effluent then has an acid reaction, and is unfit for domestic supply. When distributed through iron pipes, it attacks the iron, rusting the pipes, and giving rise to all the disagreeable consequences of an iron containing water.
The amount of lime in a water available to combine with the sulphuric acid can be determined by a very simple chemical operation, namely, by titration with standard acid with a suitable indicator. The amount of coagulant corresponding to a given quantity of lime can be readily and accurately calculated, but it is not regarded safe to use as much sulphate of alumina as corresponds to the lime. The quantity of coagulant used is not susceptible to exact control, but fluctuates somewhat, and if the exact theoretical quantity should be employed during 24 hours, there would surely be an excess during some portion of that time from which bad results would be experienced. It is therefore considered only prudent to use three quarters as much sulphate of alumina as corresponds to the lime in the water. With sulphate of alumina containing 17 per cent of soluble aluminum oxide and the corresponding amount of sulphuric acid, the amount which can be applied to a water in grains per gallon is slightly less than the alkalinity expressed in terms of parts in 100,000 of calcium carbonate.
Many waters contain sufficient lime to combine with the acid of all the coagulant which is necessary for their coagulation. Others will not, and it thus becomes an important matter to determine whether a given water is capable of decomposing sufficient coagulant for its treatment. It is usually the flood-flows of rivers which control in this respect. The water at such times requires much larger quantities of coagulant for its clarification, and it also usually contains much less lime than the low-water flows. The reason for this is obviously that the water of the flood-flows is largely rain-water which has come over the surface without coming into very intimate contact with the soil, and consequently without having taken from it much lime, while the low-water flows contain a considerable proportion of water which has percolated through the soil and has thus become charged with lime.
In some parts of the country, as, for instance, in New England, the soil and underlying rock are almost entirely free from lime, and rivers from such watersheds are capable of receiving only very small quantities of coagulant without injurious results.
The deficiency of alkalinity in raw water can be corrected by the addition to it of lime or of soda-ash. Lime has been used for this purpose in many cases. When used only in moderate amounts it hardens the water, and is thus seriously objectionable. The use of so large a quantity as would precipitate out, as in Clark’s process, has not been employed in practice. If it should be attempted, the amount of lime would require to be very accurately controlled, and the effluent would have to be treated with carbonic acid to make it suitable for supply.
Waters so hard as to require the use of the Clark process almost always have sufficient alkalinity, and do not require to be treated with lime in connection with the use of sulphate of alumina.
The use of soda-ash is free from the objections to the use of lime, but is more expensive, and would require to be used with caution. Its use has often been suggested, but I do not know that it has ever been employed in practice. In small works the use of a filtering material containing marble-dust, or other calcareous matter, would seem to have some advantages in case of deficiency of alkalinity, although it would harden the water so treated.
The alkalinities of a number of waters computed as parts in 100,000 of calcium carbonate (approximately equal to the safe doses of sulphate to alumina in grains per gallon) are as follows:
| Maximum. | Minimum. | Average. | |
|---|---|---|---|
| Boston water, 1898 | 2.87 | 0.33 | 1.08 |
| Conestoga Creek, Lancaster, Penn. | 12.20 | 3.70 | 6.80 |
| Allegheny River, Pittsburg | 8.00 | 1.02 | 2.90 |
| Mahoning River and tributaries, 1897 | 20.00 | 2.20 | 10.00 |
| Scioto River and tributaries, 1897 | 35.00 | 10.00 | 20.00 |
| Ohio River, Cincinnati, 1898 | 7.00 | 2.00 | 4.50 |
| Ohio River, Louisville | 10.87 | 2.12 | 6.70 |
| Lake Erie, Lorain, Ohio | 9.50 | ||
| Lake Michigan, Chicago | 11.50 |
CHAPTER X.
MECHANICAL FILTERS.
The term mechanical filters is used to designate a general class of filters differing in many respects quite radically from the sand filters previously described. They had their origin in the United States, and consisted originally of iron or wooden cylinders filled with sand through which the water was forced at rates of one to two hundred million gallons per acre daily, or from fifty to one hundred times the rates usually employed with sand filters. These filters were first used in paper-mills to remove from the large volumes of water required the comparatively large particles, which would otherwise affect the appearance and texture of the paper; and in their earlier forms they were entirely inadequate to remove the finer particles, such as the bacteria, and the clay particles which constitute the turbidity of river waters. Various improvements in construction have since been made, and, in connection with the use of coagulants, much more satisfactory results can now be obtained with filters of this class; and their use has been extended from manufacturing operations to municipal supplies, in many cases with most satisfactory results.
The information gathered in regard to the conditions essential to the successful design and operation of these filters in the last few years is very great, and may be briefly reviewed.
PROVIDENCE EXPERIMENTS.[39]
The first data of importance were secured from a series of experiments conducted by Mr. Edmund B. Weston of Providence, R. I., in 1893 and 1894, upon the Pawtuxet river water used by that city. The experimental filter was 30 inches in diameter, and had a layer of sand 2 feet 10 inches deep. The sand was washed by the use of a reverse current, the sand being stirred by a revolving rake at the same time. The amount of coagulant employed was about 0.7 of a grain per gallon. The raw water was practically free from turbidity, and the filter was operated to remove color and bacteria.
The removal of color, as stated in Mr. Weston’s report, amounted to from 70 to 90 per cent. The experiments extended over a period of ten months. The rate of filtration employed was about 128 million gallons per acre daily. The bacterial results of the first six months’ operations were rejected by Mr. Weston on account of defective methods of manipulation.
During the period from November 17, 1893, to January 30, 1894, the average bacterial efficiency of filtration was about 95 per cent, and the manipulation was considered to be in every respect satisfactory. The efficiency was occasionally below 90 per cent, but for four selected weeks was as high as 98.6 per cent. The average amount of sulphate of alumina used, as calculated from Mr. Weston’s tables, was two thirds of a grain per gallon. The highest efficiency followed the application of a solution of caustic soda to the filtering material. The first day following this treatment the bacterial efficiency was above 99 per cent. Afterwards it decreased until January 30, when the experiments were stopped. The high bacterial efficiency following the use of caustic soda was of such short duration as to suggest very grave doubts as to its practical value. It is extremely unfortunate that the experiments stopped only a week after this experiment, and the results were never repeated. I consider that the average bacterial efficiency of about 95 per cent obtained for the period of October 17 to January 30, when the manipulation was considered to be in every way satisfactory, more nearly represents what can be obtained under these conditions than the results for certain periods, particularly after the use of the caustic soda.
LOUISVILLE EXPERIMENTS.[40]
These experiments were inaugurated by the Louisville Water Company in connection with the manufacturers of certain patented filters. Mr. Charles Hermany, Chief Engineer of the Company, had general charge of the experiments. Mr. George W. Fuller was Chief Chemist and Bacteriologist and had direct charge of the work and has made a most elaborate report upon the same. In these examinations many devices were investigated; but the two which particularly deserve our attention are the filters known as the Warren Filter and the Jewell Filter.
These filters were operated for two periods, namely, from October 18, 1895, to July 30, 1896, and from April 5 to July 24, 1897. The investigations were directed toward the clarification of the river water from the mud, and to the removal of bacteria. The water was substantially free from color. The character of the water at this point was such that in its best condition at least three fourths of a grain of sulphate of alumina were necessary for its coagulation, and with this and with larger quantities of coagulant fair bacterial purification was nearly always obtained. The problem studied therefore was principally that of clarification from mud. The average efficiencies, as shown by the total averages, (page 248,) were as follows: Warren filter, bacterial efficiency, 96.7 per cent; Jewell filter, 96.0 per cent.
LORAIN TESTS.[41]
These tests were made by the author of a set of Jewell filters at Lorain, Ohio. The filters were six in number, each 17 feet in diameter, having an effective filtering area of 226 square feet each, or 1356 square feet in all. The construction of the filters was in all respects similar to the Jewell filter used at Louisville. The raw water was from Lake Erie, and during the examination was always comparatively clear, but contained considerable numbers of bacteria. The problem was thus entirely one of bacterial efficiency. The question of clarification hardly presented itself. Although the water became turbid at times it did not approach in muddiness the condition of the Ohio River water, and an amount of coagulant sufficient for a tolerable bacterial efficiency in all cases was more than sufficient for clarification.
A summary of the results obtained is as follows:
| Week Ending 6:00 P.M. | Average Rate of Filtration, Gallons per Sq. Ft. Min. | Sulphate of Alumina, Grains per Gallon. | Bacteria in Lake Water. | Bacteria in Effluent. | Bacterial Efficiency per cent. |
|---|---|---|---|---|---|
| June 19 | 1.06 | 2.58 | 1441 | 16 | 98.9 |
| 26 | 1.10 | 2.50 | 385 | 6 | 98.4 |
| July 3 | 1.11 | 2.27 | 367 | 9 | 97.5 |
| 10 | 1.28 | 1.07 | 154 | 14 | 90.9 |
| 17 | 1.14 | 0.94 | 189 | 26 | 86.3 |
| Average | 1.14 | 1.83 | 507 | 14 | 96.4 |
| The average bacterial efficiency was 96.4 per cent with 1.83 grains ofsulphate of alumina per gallon. | |||||
PITTSBURG EXPERIMENTS.[42]
The Pittsburg experiments were inaugurated by the Pittsburg Filtration Commission. The operation of the filters extended from January to August, 1898. A Jewell and a Warren filter were used similar in design to those used at Louisville. The raw water contained large numbers of bacteria, and was also often very turbid, although less turbid than at Louisville. At times more coagulant was necessary for clarification than was required for bacterial efficiency; while as a rule more was required for satisfactory bacterial purification than was necessary for clarification. The opportunities were therefore favorable for the study of both of these conditions. The amount of coagulant necessary for clarification has been mentioned in connection with coagulation.
The results secured upon the relation of the quantity of coagulant to the number of bacteria in the effluent were more complete than any other experiments available, and are therefore here reproduced from the Pittsburg report nearly in full.
It was found that the amount of sulphate of alumina employed was more important than any other factor in determining the bacterial efficiency, and special experiments were made to establish the effect of more and of less coagulant than used in the ordinary work. These experiments were made upon the Warren filter during May, and with the Jewell filter during June. The monthly averages for these months are thus abnormal and are not to be considered. The remaining six months for each filter may be taken as normal and as representing approximately the work of these filters under ordinary careful working conditions.
During the six months when the Warren filter was in normal order the raw water contained 11,531 bacteria and the effluent 201, the average bacterial efficiency being 98.26 per cent. The bacterial efficiency was very constant, ranging only, by months, from 97.48 to 98.96 per cent. During the same period a sand filter receiving the same water yielded an effluent having an average of 105 bacteria per cubic centimeter.
The Jewell filter, for the six months in which it was in normal order, received raw water containing an average of 11,481 bacteria and yielded an effluent containing an average of 293, the bacterial efficiency being 97.45 per cent, and ranging, in different months, from 93.23 to 98.61 per cent.