The filtration of public water-supplies
Hazen, Allen, 1869-1930
79.000 75.700 68 000 Cincinnati O Ohio Dordrecht Maas Rotterdam <( 68,000 68 000 Schiedam ft Altona Elbe 52,000 52,000 40,000 36,000 34,000 26 000 Hamburcr. « ( Stettin Oder Maci'deburcr .... Elbe Warsaw ..•- ••••. Weichsel Odessa Dneister Worms..... Rhine 25,000 22 000 Grand Forks. N. Dak Red River of the North Oder Frankfort on Oder 21,000 15,000 12,000 II 600 Bremen Weser Suburbs of Paris Seine Poufirhkeeosie. N. Y Hudson Pittsburc Penn..... Allechenv 11,400 9,400 9,200 8 200 Posen Wartha Hudson, N. Y H udson Albany N. Y 41 Breslau Oder 8 200 Brieg 7,500 4,634 I 660 Lawrence , Mass Merrimac Stuttgart Neckar Brunswick Ocker 650 171 Somersworth N. H Salmon PRELIMINARY PROCESSES TO REMOVE MUD. With both sand and mechanical filtration the difficulty and expense of treatment of a water increase nearly in direct propor- tion to the turbidity of the water as applied to the filter; and it is thus highly important to secure a water for filtration with as little turbidity as possible, and thus to develop to their economical limits the preliminary processes for the removal of mud. One of the most important of these processes is the use of reservoirs. Reservoirs serve two purposes in connection with waters drawn Digitized by VjOOQIC 134 FILTRATION OF PUBLIC WATER-SUPPLIES, from streams: they allow sedimentation, and they afford storage. If a water having a turbidity of i.oo is allowed to remain in a sedimentation-basin for 24 hours, its turbidity may be reduced by as much as 40 per cent, or to 0.60. If it is held a second day the additional reduction is much less. If samples are taken of the water in the reservoir before and after settling and sent to the chemist for analysis, he will probably report that from 70 to 80 per cent of the suspended matters have been removed by the process. The suspended matters are re- moved in much larger ratio than the turbidity. This arises from the fact that there is a certain proportion of comparatively coarse material in the water as it is taken from the river. This coarse material increases the weight of the suspended matters without increasing the turbidity in a corresponding degree. In 24 hours the coarser materials are removed completely, and at the end of that time only the clayey or finer particles remain in suspension. It is these clayey particles, however, that constitute the turbidity, which are most objectionable in appearance, and which are most difficult of removal by filtration or otherwise. Sedimentation thus removes the heavier matters from the water, but it does not remove the finer matters which principally affect the appearance of the water and are otherwise most trouble- some. A sedimentation of 24 hours removes practically all of the coarser matters, and the clayey material remaining at the end of that time can hardly be removed by further sedimentation. The economic limit of sedimentation is about 24 hours. Sedimentation has practically no effect upon the clearer waters between flood periods. Let us consider the effect of a sedimentation-basin, or reservoir liolding a 24-hours* supply of water, into which water is constantly pumped at one end, and from which an equal quantity is con- stantly withdrawn from the other, upon the water of a stream of such size that the time of passage of water from the feeders to the intake is less than 24 hours. During the period between storms Digitized by VjOOQIC TURBIDITY AND COLOR, 135 the water is comparatively clear and passes through the sedimen- tation basin without change. When a storm comes the water in the stream promptly becomes muddy, and muddy water is sup- plied to the reservoir; but owing to the time required for water to pass through it, the outflowing water remains clear for some hours. There is a gradual mixing, however, and long before the expiration of 24 hours somewhat muddy water appears at the outlet. The turbid-water period rarely lasts in streams of this size more than 24 hours, and at the expiration of that time the water in the sedimentation-basin is as muddy or muddier than the water flowing in the stream. After the height of the flood the stream clears itself by the flowing away of the turbid water much more rapidly than the water clears itself by sedimentation in the reservoir. That is to say, if at the time of maximum turbidity we take a certain quantity of water from the stream and put it aside to settle, at no time will the improvement by settling equal the improvement which has taken place in the stream from natural ■causes. Generally the improvement in the stream is several times as rapid as in the sedimentation-basin, and the water from it will at times have only a fraction of the turbidity of the water in the basin. Let us now consider what the sedimentation has done to im- prove the water. During the period of clear water, that is for most of the time, it has done nothing. For the first day of each flood period very much clearer water has been obtained from it than was flowing in the stream. For the first days following floods the water in the sedimentation-basin has been more muddy than the water in the stream. The only time when the sedimentation-basin has been of use is during the first part of floods, that is, when the turbidity of the water in the stream is increasing. During this period it has been of service principally because of its storage capacity, yielding up water received from the stream previously, when it was less muddy. Such sedimentation as has been secured is merely incidental and generally not important in amount. Digitized by VjOOQIC 136 FILTRATION OF PUBLIC WATER-SUPPLIES. It will be obvious from the above that for these conditions storage is much more important than sedimentation. This brings us back to the old English idea of having storage-reservoirs large enough to carry water-works over flood periods without the use of flood-waters. Reservoirs of this kind were, and still are, consid- ered necessary for the successful utilization of waters of many English rivers, although these waters do not approach in turbidity the waters of some American streams. This idea of storage has been but little used in the United States. In the above case, if we use our reservoir for storage instead of as a sedimentation-basin, the average quality of the water can be greatly improved. The reservoir should ordinarily be kept full, and pumping to it should be stopped whenever the turbidity exceeds a certain limit, to be determined by experience ; and the reservoir is then to be drawn upon for the supply until the turbidity again falls to the normal. In the case assumed above, with a stream in which all of the water reaches the intake in 24 hours, a reservoir holding a 24-hours' supply, or in practice, to be safe, a somewhat larger one, would yield a water having a very much lower average turbidity than would be obtained with water pumped constantly from the stream without a reservoir. With a river having a watershed so long that 48 hours are required to bring the water down from the most remote feeders, a reservoir twice as large would be required, and would result in a still greater reduction in the average turbidity. As the stream becomes larger, and the turbid periods longer, the size of a reservoir necessary to utilize this action rapidly becomes larger, and the times during which it can be filled are shortened, and thus the engineering difficulties of the problem are increased. For moderately short streams, cost for cost, storage is far more effective than sedimentation, and we must come back to the old English practice of stopping our pumps during periods of maximum turbidity. Digitized by VjOOQIC TURBIDITY AND COLOR. 137 EFFECT OF MUD UPON SAND FILTERS. There are two aspects of the effect of mud upon the operation of sand filters which require particular consideration. The first relates to the rapidity of clogging, and consequently the frequency of scraping and the cost of operation ; while the second relates to the ability of the filters to yield well-clarified effluents. EFFECT OF TURBIDITY UPON THE LENGTH OF PERIOD. The amount of water which can be filtered between scrapings is directly dependent upon the turbidity of the raw water. The greater the turbidity, the more frequently will filters require to be scraped. In the experiments of the Pittsburg Filtration Commis- sion, with 4 feet of sand of an effective size of about 0.30 mil- limeter, and with rates of filtration of about three million gallons per acre daily, and with the loss of head limited to 4 feet, sand filters were operated as follows : For five periods the turbidi- ties of the raw water ranged from 0.035 to 0.062, and averaged 0.051, and the corresponding periods ranged from 102 to 136, and averaged 113 million gallons per acre filtered between scrapings. For ten periods the turbidities of the raw water ranged from 0.079 to 0.128, and averaged 0,102, and the periods averaged 78 million gallons per acre between scrapings. For fifteen other periods the turbidities of the raw water ranged from 0.134 to 0,269, ^^^ averaged 0.195, and the periods averaged 52 million gallons per acre between scrapings. In two other periods the turbidities of the raw water averaged 0.67, and the periods between scrapings averaged 16 million gallons. In all cases the turbidity is taken as that of the water applied to the filter. Usually this was the turbidity of the settled water, but in some cases raw water was applied, and in these case the turbidity of the raw water is taken. These results are approximately represented by the formula Period between scrapings, ) 12 }= million gallons per acre ) turbidity + 0.05* Digitized by VjOOQIC I3« FILTRATION OF PUBLIC WATER-SUPPLIES, Except for very clear waters the amount of water passed between scrapings is nearly inversely proportional to the turbidity. With twice as great an amount of turbidity, filters will have to be cleaned twice as often, the reserve area for cleaning will require to be twice as great, and the cost of scraping filters and of washing and replacing sand, which is the most important element in the cost of operation, will be doubled. With waters having turbidities of 0.20 upon this basis, the average period will be about 5 1 million gallons per acre between scrapings. This is about the average result obtained at the German works filtering river waters, and there is no serious diffi- culty in operating filters which require to be scraped with this frequency. With more turbid waters the period is decreased. With an average turbidity of 0.50 the average period is only 24 million gallons per acre between scrapings, a condition which means very difficult operation and a very high cost of cleaning. With much more turbid waters the difficulties are increased, and if the duration of turbid water should be long-continued, the operation of sand filters would clearly be impracticable, and the expense, also, would be prohibitive. In applying these figures to actual cases it must be borne in mind that the turbidity is only one of the several factors which control the length of period ; and that the turbidity of a water of a given stream is never constant, but fluctuates within wide limits; and that raw water can be applied to filters for a short time with- out injurious results, even though it is so turbid that its continued application would be fatal. It is very likely also that the suspended matters in different streams differ in their natures to such an extent that equal turbidi- ties would give quite different periods, although the Pittsburg results were so regular as to give confidence in their application to other conditions within reasonable limits, and when so applied they afford a most convenient method of computing the approximate cost of operation of filters for waters of known or estimated turbidities. Digitized by VjOOQIC EFFECT OF MUD UPON SAND FILTERS, 1 39 POWER OF SAND FILTERS TO PRODUCE CLEAR EFFLUENTS FROM MUDDY WATER. When the turbidity of the applied water is not too great it is entirely removed in the course of filtration. With extremely muddy raw waters, however, turbid effluents are often produced with sand filters. The conditions which control the passage of the finest suspended matters through filters have been studied by Mr. Fuller at Cincinnati at considerable length. They are similar in a general way to the conditions which control the removal of bacteria. That is to say, the removal is more complete with fine filter sand than with coarse sand; with a deep sand layer than with a shallow sand layer; and with low rates of filtration than with high rates. The practicable limits to the size of sand grain, depth of sand layer, and rate of filtration are established by other conditions, and the question remains whether within these limits a clear effluent can be produced. At Pittsburg the turbidity of the effluent from a sand filter operated as mentioned above, which received water which had passed through a sedimentation-basin holding about a 24-hours* supply, but without taking any advantage of storage to avoid the use of muddy water, was nearly always less than 0.02, which may be taken as the admissible limit of turbidity in a public water-supply. This limit was exceeded on less than 20 days out of 365, these days being during the winter and spring freshets, and on these days the excess was not such as would be likely to be particularly objectionable. For the water of the Allegheny River, then, sand filtration with one day's -sedimentation is capable of yielding a water not absolutely clear, but sufficiently clear to be quite satis- factory for the purpose of municipal water-supply. At Cincinnati, on the other hand, where the amount of sus- pended, matters was five times as great as at Pittsburg, the effluents which could be obtained by sand filtration without recourse to the use of alum, even under most favorable conditions, Digitized by VjOOQIC I40 FILTRATION OF PUBLIC WATER-SUPPLIES. were very much more turbid than those obtained at Pittsburg, and were, in fact, so turbid as to be seriously objectionable for the purpose of public water-supply. With rivers no more turbid than the Allegheny River at Pitts- burg, and rivers having floods of such short duration that the use of fJood-fJows can be avoided by the use of reservoirs, sand filters are adequate for clarification. For waters which are much muddier than the Allegheny, as, for instance, the Ohio at Cincinnati and at Louisville, sand filtration alone is inadequate. Mr. Fuller,* as a result of his Cincinnati experiments, has stated the case as follows: ** For the sake of explicitness it is desired to show, with the data of the fairly normal year of 1898, the proportion of the time when English filters (that is, sand filters) would be inapph'cable in the purification of the unsubsided Ohio River water at Cincinnati. This necessitates fixing an average limit of permissible suspended matter in this river water, and is a difficult matter from present evidence. ** In part this is due to variations in the character and in the relative amounts of the suspended silt, clay, and organic matter; and in part it is due to different amounts of clay stored in the sand layer, which affects materially the capacity of the filter to retain the clay of the applied water. During these investigations the unsubsided river- water was not regularly applied to filters; and, with the exception of the results of tests for a few days only, it is necessary to depend upon general information obtained with refer- ence to this point. So far as the information goes, it appears that an average of 125 parts per million is a conservative estimate of the amount of suspended matters in the unsubsided river- water, which could be regularly and satisfactorily handled by English filters. But at times this estimated average would be too low,, and at other times too high. . .. ** While English filters are able to remove satis^'actorily on an * Report on Water Purification at Cincinnati, page 378. Digitized by VjOOQIC EFFECT OF MUD UPON SAND FILTERS, l^l average about 125 parts of silt and clay of the unsubsided water, actual experience shows that they can regularly handle suspended clay in subsided water in amounts ranging only as high as from 30 to 70 parts (depending upon the amount of the clay stored in the sand layer), and averaging about 50 parts per million. But it is true that for two or three days on short rises in the river, or at the beginning of long freshets, the retentive capacity of the sand layer allows of satisfactory results with the clay in the applied water considerably in excess of 70 parts. If this capacity is greatly overtaxed, however, the advantage is merely temporary, as the stored clay is washed out later, producing markedly turbid effluents." Translating Mr. Fuller's results into terms of turbidity, the 125 parts per million of suspended matters in the raw water repre- sent a turbidity of about 0.40, and the 30 to 70 parts of suspended matters in the settled water represent turbidities from 0.20 to 0.40, the average of 50 parts of suspended matters corresponding to a turbidity of about 0.30. Upon this basis, then, sand filters are capable of treating raw waters with average turbidities up to 0.40, or settled waters with average turbidities up to 0.30, but waters more turbid than this are incapable of being successfully treated without the use of coagulants or other aids to the process. These results are in general accordance with the results of the experiments at Pitts- burg, and demonstrate that while sand filters as generally used in Europe are adequate for the clarification of many, if not most, river waters in the United States, there are other waters carrying mud in such quantities as to make the process inapplicable to them. EFFECT OF MUD UPON BACTERIAL EFFICIENCY OF FILTERS. The question is naturally raised as to whether or not the pres «nce of large quantities of mud in the raw water will not seriously Digitized by VjOOQIC 142 FILTRATION OF PUBLIC WATER-SUPPLIES. interfere with the bacterial efficiency of filters. Experiments at Cincinnati and Pittsburg have given most conclusive and satisfac- tory information upon this point. Up to the point where the effluents become* quite turbid, the mud in the raw water has no influence upon the bacterial efficiency; and even somewhat beyond this point, with effluents so turbid that they would hardly be suit- able for the purpose of a public water-supply, the bacterial efficiency remains substantially equal to that obtained with the clearest waters. Only in the case of excessive quantities of mud, where, for other reasons, sand filters can hardly be considered applicable, is there a moderate reduction in bacterial efficiency. As men- tioned above, particles constituting turbidity are often much smaller than the bacteria, and in addition, the bacteria probably have an adhesive power far in excess of that of the clay particles. For these reasons clay particles are able to pass filters under con- ditions which almost entirely prevent the passage of bacteria. On the other hand, it does not necessarily follow that the removal of turbidity is accompanied by high bacterial efficiency. Although this is often the case, there are marked exceptions, particularly in connection with the use of coagulants, where very good clarification is obtained, and notwithstanding this, effluents are produced containing comparatively large numbers of bacteria. LIMITS TO THE USE OF SUBSIDENCE FOR THE PRELIMINARY TREATMENT OF MUDDY WATERS. When water is too muddy to be applied directly to filters, the most obvious treatment is to remove as much of the sediment as possible by sedimentation. Sedimentation-basins are considered as essential parts of filtration plants for the treatment of muddy waters. The effect of sedimentation, as noted above, is to remove principally the larger particles in the raw water. By doing this the deposit upon the surface of the filters and the cost of operation are greatly reduced. These larger particles are mainly removed by a comparatively Digitized by VjOOQIC EFFECT OF MUD UPON SAND FILTERS, I43 short period of sedimentation, and the improvement effected after the first 24 hours is comparatively slight. The particles remain- ing in suspension at the end of this time consist almost entirely of very fine clay, and the rate of their settlement through the water is extremely slow; and currents in the basin, due to temperature changes, winds, etc., almost entirely offset the natural tendency of the sediment to fall to the bottom. There is thus a practical limit to the effect of sedimentation which is soon reached, and it has not been found feasible to extend the process so as to allow much more turbid waters to be brought within the range which can be economically treated by sand filtration. Digitized by VjOOQIC 144 FILTRATION OF PUBLIC WATER-SUPPLIES. CHAPTER IX. THE COAGULATION OF WATERS. The coagulation of water consists in the addition to it of some substance which forms an inorganic precipitate in the water, the presence of which has a physical action upon the suspended matters, and allows them to be more readily removed by subsi- dence or filtration. The most common coagulant is sulphate of alumina. When this substance is added to water it is decomposed into its com- ponent parts, sulphuric acid and alumina, the former of which combines with the lime or other base present in the water, or in case enough of this is lacking, it remains partly as free acid and partly undecomposed in its original condition ; while the alumina forms a gelatinous precipitate which draws together and surrounds the suspended matters present in the water, including the bacteria, and allows them to be much more easily removed by filtration than would otherwise be the case. In addition, the alumina has a chemical attraction for dissolved organic matters, and the chemical purification may be more complete at very high rates than would be possible with sand filtration without coagulant at any rate, however low. Coagulants have been employed in connection with filtration from very early times. As early as 183 1 D'Arcet published in the ** Annales d'hygi^ne publique," * an account of the purification of Nile water in Egypt by adding alum to the water, and afterwards filtering it through small household filters. More recently alum has been repeatedly used in connection with sand filters, particu- * Translation in German in Dingler*s Polytechnical Journal, 1832, 386. Digitized by VjOOQIC THE COAGULATION OF WATERS. 1 45 larly at Leeuwarden, Groningen, and Schiedam in Holland, where the river waters used for public supplies are colored by peaty matter which cannot be removed by simple filtration. SUBSTANCES USED FOR COAGULATION. Mr. Fuller* has given a very full account of the substances which can be used for the clarification of waters. Without taking tip all of the unusual substances which have been suggested, the most important of the coagulants will be briefly described below. Lime. — Lime has been extensively used in connection with the purification of sewage, and also for softening water. Lime is first -slaked and converted into calcium hydrate, which is afterwards dissolved in water, and applied to the water under treatment. The amount of lime to be used is fixed by the amount of carbonic acid in the water. So much lime is always used as will exactly convert the whole of the carbonic acid of the water into normal carbonate of lime. This substance is but slightly soluble in water and it precipitates. The precipitate is crystalline rather than flocculent, and is not as well adapted to aid in the removal of clayey matters as some other substances, although its action in this respect is considerable. The precipitate is quite heavy, and is largely removed by sedimentation, although filtration must be used to complete the process. Water which has been treated with lime is slightly caustic; that is to say, there is a deficiency of carbonic acid in it, and it deposits lime in the pipes, in pumps, etc. ; and although the precipitated calcium carbonate is much softer than steel, it rapidly destroys pumps used for lifting it. Principally for these reasons it is necessary to supply carbonic acid to water which has been treated in this way, and this is done by bringing it in contact with flue-gases, or by the direct addition of carbonic acid. The use of lime for softening waters is known as Clark's process. It was patented in England many years ago, and the * Water Purification at Louisville, page 378. Digitized by VjOOQIC 146 FILTRATION OF PUBLIC WATER-SUPPLIES. patent has now expired. Various ingenious devices have beeit constructed for facilitating various parts of the operation. The process has hardly been used in the United States, but there is a large field for it in connection with the softening of very hard waters, and where such waters also contain iron or clay, these substances will be incidentally removed by the process. Larger quantities of lime have an action upon the suspetnded matters which is entirely different from that secured in Clark's process, and the action upon bacteria is particularly noteworthy. This action was noted in experiments at Lawrence,* where it was found that sewage was almost completely sterilized by the applica- tion of considerable quantities of lime. An extremely interesting series of experiments upon the application of large quantities of lime to water was made by Mr. Fuller in iSgg.-f The bacterial results were extremely favorable, although the necessity for remov- ing the excess of lime afterward is a somewhat serious matter, and in these experiments it was not entirely accomp'lished. Aluminum Compounds, — Sulphate of alumina is most commonly employed. It can be obtained in a state of considerable purity at a very moderate price, and important improvements in the methods used for its manufacture have been recently introduced. Potash and soda alums have no advantage over sulphate of alumina, and, in fact, are less efficient per pound, while their costs are greater. Chloride of alumina is practically equivalent to the sul- phate in purifying power, but is more expensive. Sodium Aluminate has been examined by Mr. Fuller, who states that experience has shown that its use is impracticable in the case of the Ohio River water. Compounds of Iron, — Iron forms two classes of compounds, namely, ferrous and ferric salts. When the ferrous salts are applied to water, under certain conditions, ferrous hydrate is pre- * Special Report Mass. State Board of Health 1890, Purification of Sewage and Water, page 747' f Water Purification at Cincinnati, p. 485. Digitized by VjOOQIC THE COAGULATION OF WATERS. 147 cipitated, but this substance is not entirely insoluble in water containing carbonic acid. Under some conditions the precipitated ferrous hydrate is oxidized by oxygen present in the water to ferric hydrate, and so far as this is the case, good resylts can be obtained. Ferrous sulphate is not as readily oxidized when applied to water as is the ferric carbonate present in many natural waters, and for this reason ferrous sulphate has not been success- fully used in water purification. In the treatment of sewage, where the requirements are somewhat different, it has been one of the most satisfactory coagulants. Ferric sulphate acts in much the same way as sulphate of alumina, and is entirely suitable for use where sulphate of alumina could be employed, but it has not been used in practice, due prob- ably to its increased cost as compared with its effect, and to the practical difficulties of applying it in the desired quantities due to its physical condition. Metallic Iron : The Anderson Process. — The use* of metallic iron for water purification in connection with a moderately slow filtration through filters of the usual form is known as Anderson's process (patented), and has been used at Antwerp and elsewhere on a large scale, and has been experimentally examined at a number of other places. The process consists in agitating the water in contact with metallic iron, a portion of which is taken into solution as ferrous carbonate. Upon subsequent aeration this is supposed to become oxidized and precipitate out as ferric hydrate, -with all the good and none of the bad effects which follow the use of alum. The precipitate is partially removed by sedimentation, while filtration completes the process. The process is admirable theoretically,, and in an experimental way upon a very small scale often gives most satisfactory results, muddy waters very difficult of filtration, and colored peaty waters yielding promptly clear and colorless effluents. In applying the process on a larger scale, however, with peaty Digitized by VjOOQIC 148 FILTRATION OF PUBLIC WATER-SUPPLIES. waters at least, it seems impossible to get enough iron to go into solution in the time which can be allowed, and the small quantity which is taken up either remains in solution or else slowly and incompletely precipitates out, without the good effects which follow the sudden and complete precipitation of a larger quantity, and in this case the color is seldom reduced, and may even be increased above the color of the raw water by the iron remaining in solution. The ingenuity of those who have studied the process has not yet found any adequate means of avoiding these important prac- tical objections; and even at Antwerp a great extension of the filtering area, as well as the use of alum at times of unusual pollu- tion, is good evidence that simple filtration, in distinction from the effect of the iron, is relied upon much more than formerly. At Dordrecht also, where the process has been long in use, the rate of filtration does not exceed the ordinary limits; nor is the result, so far as I could' ascertain, in any way superior to that obtained a few miles away at Rotterdam, by ordinary filtration, with substantially the same raw water. The results obtained at Boulogne-sur-Seine, near Paris, have been closely watched by the public chemist and bacteriologist of Paris, and have been very favorable, and a number of new plants of very considerable capacity have been built, to supply some of the suburbs of Paris, but even in these cases only moderate rates of filtration are employed which would yield excellent eJHuents without the iron. Compounds of Manganese, — Manganese forms compounds simi- lar to those of iron, that is to say manganous and manganic salts, but their use in connection with water filtration has not been ifound possible. In addition, manganese forms a series of com- pounds, known as manganates and permanganates, quite different in their structure and action from the others. These compounds contain an excess of oxygen which they give up verj" readily to organic matters capable of absorbing oxygen, and because of this Digitized by VjOOQIC THE COAGULATION OF WATERS. 149 power, they have been extensively used in the treatment of sewage. Applied to the treatment of waters their action is very slight, and the compounds are so expensive that they have not been employed for this purpose. Theoretically the action is very attractive, as the oxygen liberated by their decomposition oxidizes some of the organic matter of the water, thereby purifying it in part, while the manganese is precipitated as a flocculent precipi- tate having all of the advantages pertaining to a precipitate of hydrate of alumina, and without the disadvantage of adding acid to the water, as is the case with the compounds of alumina and iron. These chemicals, when used in comparatively concentrated condition, have powerful germicidal actions, but in water purifica- tion the amounts which can be used are so small that no action of this kind results. The amount which can be applied to a water is limited to the amount which can be decomposed by the organic matters present in the water, and is not large. The Use of Metallic Iron and Aluminumy with the Aid of Electricity. — Elaborate experiments were made at Louisville with metallic iron and aluminum oxidized and made available by the aid of electric currents. The use of iron with electric currents was tried in sewage purification some years ago, under the name of the Webster process, but was never put to practical use. The theory is to oxidize the iron or aluminum in contact with the water, with the formation of ilocculent hydrates, by the aid of an electric current, thereby securing the advantages of the application of salts of these metals to the water without the disadvantage of the addition of acid. Other Chemicals Employed. — A solution containing chlorine produced by electrical action has been suggested. Chlorine is a powerful disinfectant, and when used in large quantities kills bacteria. It is not possible to use enough chlorine to kill the bacteria in the water without rendering it unfit for human use. The nature of this treatment has been concisely described by Digitized by VjOOQIC 150 FILTRATION OF PUBLIC WATER-SUPPLIES. Dr. Drown,* who shows that the electrically prepared fluids do not differ' in their action in any way from well-known chemicals, the use of which would be haidly considered. The use of ozone and peroxide of hydrogen have also been sug- rgested, but I do not know that they have been successfully used •on a large scale. The same is true of many other chemicals, the consideration of which is hardly necessary in this connection. COAGULANTS WHICH HAVE BEEN USED. In actual work sulphate of alumina is practically the only coagulant which has been employed, excepting the alums, which are practically its equivalent in action, differing only in strength. Nearly all important experiments upon the coagulation of water have been made with sulphate of alumina, and in the further dis- cussion of this subject only this coagulant will be considered. AMOUNT OF COAGULANT REQUIRED TO REMOVE TURBIDITY. In the coagulation of turbid waters a certain definite amount of coagulant must be employed. If less than this amount is used either no precipitate will be formed, or it will not be formed in sufficient bulk to effect the desired results. It is necessary that the precipitate should be sufficient, and that it should be formed practically all at one time. The amount of coagulant necessary to accomplish this purpose is dependent upon the turbidity of the raw water. With practically clear waters sulphate of alumina of the ordinary commercial strength, that is to say, with about 17 per cent soluble oxide of aluminum, used in quantities as small as 0.3 or 0.4 of a grain per gallon, will produce coagulation. As the turbidity increases larger amounts must be employed. A special study was made of this point in connection with the Pittsburg experiments. t As an average of these results it was found that two grains per gallon of sulphate of alumina were * Jour, of the New England Water Works Assoc, Vol. viii, page 183. t Report of the Pittsburg Filtration Commission, 1899, page 55. Digitized by VjOOQIC THE COAGULATION OF WATERS. 151 required to properly coagulate waters having turbidities of i.oo, so that they could be filtered by the Jewell filter, and 2.75 grains were required for the Warren filter. Aside from the amount required to produce a precipitate in the clearest waters, the amount of coagulant required was pro- portional to the turbidity. As an average for the two filters the Turbidity of Raw Wcrter. Fig. 20. — Amount of Coagulent Required to Remove Turbidity. required quantity was approximately 0.30 of a grain, and in addition 0.02 of a grain for each 0.0 1 of turbidity. Thus a water having a turbidity of 0.20 requires 0.70 of a grain per gallon; a water having a turbidity of 0.50 requires 1.30 grains; of i.oo, 2.30 grains; of 2.00, 4.30 grains, etc. These are average minimum results. Occasionally clear effluents were produced with smaller quantities of coagulant, while at other times larger quantities were necessary for satisfactory results. Digitized by VjOOQIC 152 FILTRATION OF PUBLIC WATER-SUPPLIES. The amount of coagulant required for clarification at Cincinnati has been stated by Mr. Fuller in his report. A number of his results are brought together in the following table, to which has also been added a column showing approximately the correspond- ing results at Pittsburg. ESTIMATED AVERAGE AMOUNTS OF REQUIRED CHEMICAL FOR DIFFERENT GRADES OF WATER. Chemical Required , Grains per Gallon. Suspended Matter, Parts in icx},ooo. Raw Water for Subsided Water for Subsided Water for Minimum for Sand Filters. Sand Filters. Mechanical Filters. Raw Water for Cincinnati Cincinnati Cincinnati Mechanical Filters^ Report, Page ago. Report, Page 290. Report, Page 341. Pittsburg. I.O 0.75 0.40 2.5 1.25 0.50 5.0 1.50 0.70 7-5 1.30 1.95 0.90 10. 1.50 1.60 2.20 1. 00 12.5 1.60 1.80 2.45 1. 15 15.0 1.70 2.00 2.65 1.30 17.5 1.80 2.10 2.85 1.40 20.0 1.95 2.20 3.00 1.60 30.0 2.25 2.45 3.80 2.00 40.0 2.50 2.75 4.40 2.50 50.0 2.80 60.0 3.05 75.0 3.40 100. 4.00 120.0 4.75 Mr. Fuller's results seem to show that a greater amount of coagulant is required for the preparation of water for mechanical filters than is necessary in connection with sand filters. The results with sand filters indicate that settled waters and raw waters containing equal amounts of suspended matters are about equally difficult to treat. The results at Pittsburg indicate that the raw waters required much smaller quantities of coagulant for given amounts of suspended matters than was the case with subsided waters at Cincinnati, the results agreeing more closely with the amounts required to prepare raw water for sand filters at Cin- cinnati. Digitized by VjOOQIC THE COAGULATION OF WATERS. 1 53 AMOUNT OF COAGULANT REQUIRED TO REMOVE COLOR. The information upon this point is, unfortunately, very inade- quate. In some experiments made by Mr. E. B. Weston at Providence in 1893 with a mechanical filter,* with quantities of sulphate of alumina averaging 0.6 or 0.7 of a grain per gallon, the removal of color was usually from 70 to 90 per cent. The stand- ard used for the measurement of color is not stated, and there is no statement of the basis of the scale, consequently no means of determining the absolute color of the raw water upon standards commonly used. At Westerly, R. I., with a New York filter, the actual quantity of potash alum employed from Oct. 10, 1896, to March I, 1897, was 1.94 grains per gallon, the amount being regulated to as low a figure as it was possible to use to secure satisfactory decolorization. There is no record of the color of the raw water. A very rough estimate would place it at 0.50 upon the platinum scale. The chemical employed in this case was alum, and two thirds as large a quantity of sulphate of alumina would probably have done corresponding work, had suitable apparatus for applying it been at hand. At Superior, Wisconsin, the water in the bay coming from the St. Louis River, having a color of 2.40 platinum scale, was treated experimentally with quantities of sulphate of alumina up to 4 grains per gallon, by Mr. R. S. Weston in January, 1899, but even this quantity of coagulant utterly failed to coagulate and decolorize it. At Greenwich, Conn., during 1898 the average amount of sulphate of alumina employed, as computed from quantities stated in the annual report of the Connecticut State Board of Health for 1898, was about 0.44 of a grain per gallon, and this quantity sufficed to reduce the color of the raw water from 0.40 to 0.30, platinum standard. This reduction is very slight, and it is * Rhode Island State Board of Health Report for 1894. Digitized by VjOOQIC 154 FILTRATION OF PUBLIC WATER-SUPPLIES, obvious that this quantity of coagulant was not enough, for decolorization. Some experiments bearing on color removal were made at East Providence, R. I., by Mr. E. B. Weston, and are described in the Proceedings of the American Society of Civil Engineers for September, 1899. In this case the color is reported to have been reduced from 0.58 to o.io platinum standard by the use of one grain of sulphate of alumina, containing 22 per cent of effective alumina, equivalent to about 1,30 grains of the ordinary article per gallon. The various experiments seem to indicate that a removal from 80 to 90 per cent of the color can be effected by the use of a quantity of sulphate of alumina equal to rather more than two grains per gallon for waters having colors of i.oo, platinum standard, and proportionate quantities for more and less deeply colored waters. With much less sulphate of alumina decoloriza- tion is not effected, and even larger quantities do not remove all of the color. The data are much less complete than could be desired, and it is to be hoped that experiments will be undertaken to throw more light upon this important subject. SUCCESSIVE APPLICATION OF COAGULANT. Mr. Fuller, in his experiments at Louisville, has ascertained that when sulphate of alumina is added to extremely muddy water the sediment absorbs some of the chemical before it has time to decompose, and carries it to the bottom, and so far as this is the case, no benefit is derived from that part of the coagulant which is absorbed. In other words, it is necessary to add more coagu- lant than would otherwise be necessary because of this action. The data showed that different kinds of suspended matters took up very different amounts of coagulant in this way. With only moderately turbid waters the loss of chemical from this source is unimpertant. Hardly any trace of it was found at Pittsburg with Digitized by VjOOQIC THE COAGULATION OF WATERS. 155 the Allegheny River water. At Louisville, however, it was an important factor, as shown by Mr. Fuller's results. To avoid this. loss of chemical Mr. Fuller has suggested the removal of the greater part of the suspended matters by sedimen- tation, without chemicals, or with the aid of a small quantity of chemical, followed by the application of the final coagulant prior to filtration. With the worst waters encountered at Louisville the saving in coagulant to be effected in this way is very great. Mr. Fuller states in ** Water Purification at Louisville," p. 417: ** The practical conclusions to be drawn from this experi- ence are that with preliminary coagulation, followed by subsidence for a period of about three hours, the application of coagulants may be divided to advantage, and a considerable portion of the suspended matter kept off the filter, when the total amount of required coagulant ranges from 2 to 2.5 grains or more of ordinary sulphate of alumina per gallon. In the case of a water requiring more than this amount of coagulating treatment, a proper division of the application would increase the saving of coagulants and would diminish the frequency of washing the filter." In his final summary and conclusions, page 441, Mr. Fuller estimates the amount of sulphate of alumina required for the clari- fication of the Ohio River at Louisville at 3.00 grains per gallon of water filtered if all applied at one point, or at 1.75 grains by taking advantage of subsidence to its economical limit prior to the final coagulation. The saving to be effected in this way is suffi- cient to justify the works necessary to allow it to be carried out. With less turbid waters, or waters highly turbid for only short intervals, the advantages of double coagulation would be less apparent. 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 Digitized by VjOOQIC 15^ FILTRATION OF PUBLIC WATER-SUPPLIES. is added to water it is decomposed, as explained above, with the formation of alumina, which is alone useful in the work of purifica- tion, and sulphuric acid, which combines with the calcium car- bonate 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 sul- phuric acid, and the decomposition of the coagulant is not com- plete, and a portion of it goes undecomposed into the effluent. The effluent then has an acid reaction, and is unfit for domestic supply. WheYi distributed through iron pipes, it attacks the iron, rusting the pipes, and giving rise to all the disagreeable conse- quences of an iron containing water. 