Water purification

Mar., 1895. ]

~tvr WATER

Purificatiott.

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PURIFICATION.*

BY RUDOLPH HERING.

[ Omchlde~tfro~n p. I44.]. W e now come finally to the process of purification b y

/i/tratio~z, which is more effectual than a n y of those h i t h e r t o mentioned in effecting a p e r m a n e n t removal of the impurities. It allows the w a t e r to pass t h r o u g h a porous m a t e r i a l which, first, on account of the s t r a i n i n g action of its pores, frees it from s u s p e n d e d particles which have m a d e it turbid. Then, owing p a r t l y to a chemical purification a n d p a r t l y to the action of living bacteria, the dissolved organic m a t t e r is transformed into n i t r a t e s or mineral compounds. T h i s transformation is eKected by reason of the large i n t e r s t i t i a l area existing in the m a n y pores of the filtering material. W e m a y divide the processes into two elasses: those where the filtration is rapid, and those where it is slow. I n the former case we obtain merely'a clarification of t h e water, because a certain time and special conditions are necessary to allow of a complete destruction of the organic matter by the filter, w h i c h conditions are fulfilled only b y slow filtration. In the case of rapid filtration, the pores are c o n t i n u a l l y filled with w a t e r flowing t h r o u g h them under more or less pressure. W h e n t h e pores are of nearly equal size, as for instance in the P a s t e u r filter, or in filters m a d e of certain other artificially prepared materials, there obtains a n e a r l y fixed relation b e t w e e n the a m o u n t of clear w a t e r p r o d u c e d and the pressure exerted upon the filter. But where t h e pores are of u n e q u a l sizes, an increased pressure is apt t o force out the r e t a i n e d substances from the smaller into t h e larger pores, and thus, to some extent, to allow t h e m to escape a n d l e a v e the water in a less clear condition. Filters of natural gravel, where the pores are of different sizes, h a v e * A lecture delivered before the Franklin Institute, February 2, I894,

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been known to run quite t u r b i d for a while after the pressure upon them has been increased. T h e d e g r e e of clearness depends .upon the degree of fineness of t h e pores, a n d the tiner the pores the g r e a t e r will be the pressure n e c e s s a r y to deliver the same (plantity of clear water. \,Ve have, therefore, a fixed relation b e t w e e n the d e g r e e of clearness ()f the water, the fineness of the m a t e r i a l , t h e pressure of the water whc'n p a s s i n g t h r o u g h the filter a n d the q u a n t i t y of clear w a t e r delivered by it. T h e a c t i o n of rapid filtration is a h n o s t wholly mechanical, so far as practical purposes are concerned. T h e first action of a filter is to r e t a i n the coarse p a r t i c l e s of suspended m a t t e r upon the surface. Some of t h e m e n t e r the pores for a s h o r t distance and these particles t h e m s e l v e s , when lodged, act as a filtering m a t e r i a l and assist in s t r a i n ing out the finer particles. T h e most effective p a r t of t h e filter is, therefore, t h a t near its surface. T h e a c c u m u l a t i o n of the finer particles g r a d u a l l y c a u s e s the filter to become o b s t r u c t e d ; and, if the tilter is to r e m a i n e ~ c i e n t , these r e t a i n e d particles m u s t be p e r i o d i c a l l y removed. T h e depth of m a t e r i a l necessary for efficient filtering depends on its fineness; the finer it is, the shallower the filter m a y be. T h e pores of a filter are n o t fine e n o u g h to r e m o v e t h e bacteria by straining. In all s y s t e m s of r a p i d f i l t r a t i o n t h e y are r e t a i n e d only by a film w h i c h is g r a d u a l l y f o r m e d on the surface of the filter, e i t h e r by the a c c u m u l a t i o n of organic m a t t e r or b y a c o a g u l a n t in the w a t e r . A s it always takes some time for this fihn to form, a n d as its c o n t i n u i t y can readily be d e s t r o y e d at a n y time, it offers no security a g a i n s t the possible r e a p p e a r a n c e of a l a r g e proportion of the bacteria in the e~ttuent water. R e g a r d i n g the materials which are used for r a p i d filtration we m a y classify t h e m as b e i n g fibrous, g r a n u l a r a n d porous. T h e fibrous material used for filters consists of felt, cloth, straw, cotton, or the tike. T h e use of s p o n g e s is also common. Mineral wool m a d e of fine spun glass is likewise used.

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The g r a n u l a r materials are such as sand, gravel, finelybroken stone, charcoal, coke, cinders, and some artificial materials for which special a d v a n t a g e s are claimed. Porous material m a y be unglazed porcelain, b u r n t clay, certain volcanic minerals, or some artificial product. But porous stone soon becomes d o g g e d , and t h e n it is almost impossible to clean it. It is useful only when its pores are very fine, so t h a t the m a t t e r which is strained out is kept so near the surface t h a t it can be r e m o v e d by washing. Some filtering m a t e r i a l s act also chemically upon the m a t t e r which is dissolved ~n the w a t e r ; for instance, animal charcoal will absorb gases, such as oxygen, nitrogen, carbonic acid and s u l p h u r e t t e d hydrogen, and will remove some mineral salts. It will also destroy some of the albuminous compounds, as is proven by the fact t h a t free a m m o n i a and n i t r a t e s are f o u n d in the effluent water. Both of these are formed b y decomposition while the water is passing t h r o u g h the charcoal. Charcoal is known to be more effective t h a n sand in r e m o v i n g dissolved substances, particularly at first, and also in r e m o v i n g coloring m a t t e r s contained in the water. B u t it is not as p e r m a n e n t in its operation, and its efficiency ceases in time. E x p e r i m e n t s m a d e with fine charcoal and coke for rapid filtration, show that besides a c t i n g as a strainer, and besides having a very slight chemical action, t h e y have removed, for several days at a time, over ninety-five per cent. of the bacteria contained in the water. T h e y are specially well adapted for rapid filtration, because, after they have t)ecome charged with organic matter, t h e y can be dried and will have lost b u t little of their original value as fuel. We shall now describe a few appliances which have been ~sed for the rapid filtration of water. A m o n g filters used in the household, we m a y m e n t i o n the P a s t e u r filter as one of the best. T h i s consists of tubes made of unglazed porcelain, so a r r a n g e d t h a t the water, introduced into a comp a r t m e n t where these tubes are erected, filters into them, the clear w a t e r bein K drawn out from the bottom. It has been found t h a t this m a t e r i a l isl for a considerable time, highly efficient in k e e p i n g out all living organisms, btlt i t

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should not be forgotten that the tubes n e e d f r e q u e n t cleaning, as a ~,limy coat forms on the outside and e v e n t u a l l y impairs die effectiveness of the filter. Besides c l e a n i n g t h e m perii)dicallv the f r e q u e n c y of the period d e p e n d i n g on the c h a r a c t e r of the w a t e r and upon the q u a n t i t y use'd, t h e y should occasionally receive a boiling, so t h a t a n y bacteria that m a y h a v e e n t e r e d t h e m a t e r i a l m a y be destroyed. ()ther efticient forms of domestic filters use charcoal o r coke as their filtering material. Others h a v e fine sand a n d some have: finely divided iron. T h e difference in their v a h t e depends upon the proper disposition of the materials a n d upon the facilities for cleaning them. 3.11 such house f i l t e r s very soon h)se their efficiency b y b e c o m i n g clogged. T o insure a }air d e g r e e of h e a l t h f u l n e s s of the water, it is almost es:sential to h a v e t h e m cleaned once a day, e v e n t h o u g h the water still runs clear. T h i s m a y be n e c e s s a r y merely wi)h r e g a r d to the surface layers, t h e entire filter being cleaned a t l o n g e r intervals. T h e r e are, in the m a r k e t , filters so a r r a n g e d that, w h e n e v e r desired, t h e y can v e r y easily be cleaned, by m e r e l y r e v e r s i n g the c u r r e n t of w a t e r . This drives out the s u s p e n d e d m a t t e r w h i c h has been collected, and washes the material. S u c h filters, while gener-. ally more expensive, are vastly superior to those w h i c h donot allow Of such f r e q u e n t washing. As alr(}ady m e n t i o n e d , a filter is i m p r o v e d in its a c t i o n b y a d d i n g to the w a t e r a precipitant. A t t a c h m e n t s can b e added to most filters, which allow a certain a m o u n t of Mum,_ and, perhaps, other precipitants to be mixed v~ith t h e water, and thus cause a coagulation of t h e o r g a n i c m a t t e r , w h i c h is i h e r e b y m o r e easily r e t a i n e d and itself assists i n r e t a i n i n g also large q u a n t i t i e s of microbes. A m o n g the larger filters we m a y m e n t i o n those f u r n i s h e d b y the New York Filter C o m p a n y a n d others, and u s e d extensively in our own country. T h e y are b u i l t a c c o r d i n g to variou> patterns, as t h e c i r c u m s t a n c e s m a y r e q u i r e . T h e s e filters act in a similar m a n n e r to those a l r e a d y m e n tioned. T h e filtering material g e n e r a l l y consists of fine: sand or cdke. J

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Mar., ~895.]

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The a p p a r a t u s of A n d r e w H o w a t s o n for s o f t e n i n g very hard water, is s o m e w h a t e x t e n s i v e l y u s e d in E n g l a n d and France. T h e s o f t e n i n g is effected by a continuous addition of lime and soda, and the a p p a r a t u s requires a daily cleansing. t t o w a t s o n ' s industrial filters, h a v i n g c r u s h e d silex, charcoal er polarite as filtering materials, are also used. W i t h respect to all of these rapid filters, we m a y say that what is gained in r a p i d i t y is lost in efficiency. It is true that the character of the w a t e r m a y be greatly i m p r o v e d ; in fact, we m a y r e m o v e all t h e s u s p e n d e d matter, if the pores of the material are fine e n o u g h ; it is true likewise that we may cause chemical c h a n g e s w h i c h will improve the water, converting some of the o b j e c t i o n a b l e minerals or gases, and some of the organic matter, into u n o b j e c t i o n a b l e compounds; and finally, it is true that we can r e m o v e a v e r y high percentage and occasionally all of t h e microbes which inhabit the impure water. But we cannot place entire confidence in rapid filtration for always t u r n i n g o u t pure water. W e have a l r e a d y observed t h a t rapid filters require frequent a t t e n t i o n to keep t h e m clean. If this attention be omitted, the w a t e r is worse than in its original state. T h e accumulation of organic m a t t e r in the filter a n d the equally great a c c u m u l a t i o n of bacteria, m a y then render t h e filter a positive d a n g e r b y h a v i n g b e e n converted into a diseasebreeding mass. T h i s m a y occur after a few days of service. W e cannot, therefore, too u r g e n t l y insist u p o n knowing the composition and construction of filters that we use, and upon giving t h e m the attention w h i c h is a b s o l u t e l y necessary to g u a r d us a g a i n s t the d a n g e r which can otherwise arise from them. N o rapid filter will remain safe for many days w i t h o u t cleaning, and if m u c h w a t e r i s drawn from them, t h e y m u s t b e cleaned almost d a i l y in order t h a t they may continue to furnish pure water. W e now come to the last division of m y subject, viz.: that of slow filtration, b y which w e u n d e r s t a n d a percolation of w a t e r t h r o u g h t h e filtering m a t e r i a l w i t h o u t completely filling its pores. This m a y b e called the n a t u r a l filtration of water, b e c a u s e in n a t u r e this same process trans-

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forms the more or less po]luted rain and surface water i n t o a pure spring or g r o u n d water. It accomplishes the purification n o t only by r e m o v i n g the s u s p e n d e d and dissolved organic matter, which serves as food for bacteria, b u t also b y r e m o v i n g these bacteria themselves, n o t by a process o f straining, as s o m e t i m e s takes place in rapid filtration, b u t , we m a y say, b y a process of starvation. In the earliest a t t e m p t s at purification by slow filtration, t h e effort was merely to clarify t h e l i q u i d ; there was n o e n d e a v o r to remove the dissolved and invisible s u s p e n d e d matter. L a t e r it was f o u n d necessary to pay a t t e n t i o n to these matters, and p a r t i c u l a r l y to the r e m o v a l of particles of an organic nature. P a s t e u r first proved t h a t f e r m e n t a tion was caused not b y d e e o m p o s i n ~ organic matter, b u t b y living o r g a n i s m s , and t h a t living o r g a n i s m s were also t h e cause of many, and perhaps all, z y m o t i e diseases. I t w a s t h e n discovered that filtration r e m o v e d m o s t of these o r g a n isms or bacteria. It is now k n o w n t h a t a perfect filter m a y , u n d e r certain conditions, cause a complete d e s t r u c t i o n o f t h e organic m a t t e r in the presence of these bacteria. It is n o t known in j u s t w h a t m a n n e r t h e y effect this d e s t r u c t i o n , b u t t h a t t h e y do so in some w a y is proven by t h e fact t h a t , w h e n we destroy the bacteria t h e m s e l v e s in the i m p u r e water, this passes t h r o u g h a filter m e r e l y s t r a i n e d and n o t deprived of its dissolved organic ma~ter. W e m a y d e s t r o y t h e bacteria, for instance, b y m i x i n g w i t h the w a t e r s o m e poison, such as chloroform. A g a i n , we find t h a t w h e n t h e t e m p e r a t u r e is reduced to the f r e e z i n g point, the w a t e r issues in an impure state. It has also been observed t h a t in darkness the purification is not so efficient as in the I i g h t . All of this shows t h a t the c o n d i t i o n s conducive to o r g a n i c life are necessary for the c o m p l e t e purification of water. W e must, therefore, provide c o n d i t i o n s w h i c h will e n a b l e t h e bacteria to do their work as efficiently as possible. A f t e r t h e y have done so a n d h a v e deprived the w a t e r of a l l of their food, t h e y will, of course, perish from s t a r v a t i o n . One of the i m p o r t a n t c o n d i t i o n s for the life of t h e bacteria is t h e presence, in the water, of a c e r t a i n q u a n t i t y o f o x y g e n , to be used in the conversion of the o r g a n i c i n t o

Mar., I895.]

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mineral compounds. It is, therefore, necessary to replenish the oxygen b y at~ration until the purification has been completed. This r e q u i r e m e n t explains the fact that a contin. uous filter in which the pores are completely filled with water, e v e n t u a l l y p r e v e n t s the bacteria from completing their work. A supply of the air necessary for their action can be furnished only by w h a t is t e r m e d " i n t e r m i t t e n t " filtration. Many filters have, however, proven effective for a while w i t h o u t this intermittency, this effectiveness b e i n g due to the formation, on top of the filter bed, of a film or skin, consisting of tile retained organic matter, most of which is, in some cases, in a c o a g u l a t e d state. T h e bacteria are then kept o u t b y a straining process similar to that which we m e n t i o n e d as taking place to some extent by the precipitation m e t h o d of r e m o v i n g organic m a t t e r and bacteria. B u t in time this layer of coagulated m a t t e r will o b s t r u c t the filter completely, and it requires to be periodically removed. Experience has s h o w n that, i m m e d i a t e l y after cleaning, a large n u m b e r of bacteria will pass t h r o u g h into the effluent water. After a while, again a fresh skin forms and the w a t e r becomes purer, b e c a u s e there is once more a fine strainer to keep the o b j e c t i o n a b l e particles from passing through.' A permanently effective filter, therefore, requires intermitteney, b y which the b a c t e r i a are d e s t r o y e d through the more or less complete oxidation of the organic matter, caused b y a liberal contact with air in the interstices of the material. In certain cases, in which the w a t e r naturally contains a'suf2 ficient q u a n t i t y of oxygen, continuous filtration produces excellent results. T h e carbon is converted into carbonic acid and the nitrogen into nitric acid, which at once combines w i t h the bases contained in the liquid or the soil, and forms h a r m l e s s nitrates. English, G e r m a n and F r e n c h scientists have m a d e many investigations on this subject, and have reached some vMuable conclusions; b u t it r e m a i n e d for the M a s s a c h u s e t t s State Board of Health, t h r o u g h the i n v e s t i g a t i o n s mainly of Messrs. Mills, Sedgwick, Drown, H a z e n a n d Fuller, to investigate t h e s u b j e c t systematically on a large scale and

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to formulate its principles in a definite, t a n g i b l e and p r a c tical manner. T h e results of the M a s s a c h u s e t t s i n v e s t i g a tion have been published in tile a n n u a l reports of t h e S t a t e Board of H e a l t h . T h e y h a v e also been s u m m a r i z e d in several papers read before the I n t e r n a t i o n a l E n g i n e e r i n g a n d Public t I e a l t h Congress in Chicago last s u m m e r , b y Messr3. H i r a m F. Mills and George \V. P~uller. "~" From these we m a y q u o t e as follows: " T h e circumstances t h a t b r i n g m o s t clearly to m i n d the essential conditions of i n t e r m i t t e n t filtration are these : A bed of g r a v e l stones, as large as robin's eggs, h a v i n g an a b u n d a n c e o f n i t r i f y i n g organisms a t t a c h e d to each stone, has s e w a g e poured over it for a short t i m e ; after the sewage has s e t t l e d away, every stone it has reached is covered w i t h a thin f i l m of liquid in contact w i t h air and e n v e l o p i n g t h e n i t r i f y i n g organisms. T h e n nitrification takes place r a p i d l y and w i t h g r e a t completeness. A single h o u r will make a m a r k e d c h a n g e in the c h a r a c t e r of the a d h e r i n g liquid. " A f t e r a few hours a n o t h e r c h a r g e of sewage d i s t r i b u t e d o v e r the filter will mingle s o m e w h a t w i t h the f o r m e r a d h e r i n g liquid and cause it to m o v e d o w n w a r d . If this c h a r g e is too a b u n d a n t , a p a r t of it m a y flow too r a p i d l y t h r o u g h t h e large interstices b e t w e e n the stones a n d reach the o u t l e t drains w i t h o u t being completely nitrified. T h i s c o n d i t i o n limits the a m o u n t t h a t can be purified b y coarse m a t e r i a l s . T h e air spaces are larger t h a n necessary, and the area o f surface is small. By decreasing t h e d i a m e t e r of the s t o n e s to one-tenth of their p r e s e n t diameter, we m a y still h a v e t h e same a m o u n t of air and w a t e r space a n d ten t i m e s t h e surface to which the water m a y adhere. In t h i s ease t h e air spaces b e t w e e n the films, t h o u g h very m u c h smaller t h a r t before, are sufficient to s u p p l y t h e o x y g e n n e c e s s a r y f o r nitrification if we allow sufficient t i m e b e t w e e n a p p l i c a t i o n s for the sand to become drained. It is, however, e v i d e n t t h a t if we continue d i m i n i s h i n g t h e d i a m e t e r of the g r a i n s * " Purification 0fSewage and of Water by Filtration." By Hiram F. Mills, A . M , C.E.; " R e m o v a l of Pathogenic Bacteria by Sand Filtration." B y George W. Fuller.; Papers prepared for the International Engineering Congress of the Columbian Exposition. r893.

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of material, we shall reach a condition in which the films of water a d h e r i n g to the grains will occupy the whole space; there is then no room for air, and nitrification cannot take place. Sand in a filter bed, t h a t remains s a t u r a t e d after d r a i n i n g twenty-four hours, is of little value, and m a y be very objectionable." Up to the p r e s e n t time the experiments of the State Board of H e a l t h indicate t h a t the sand of even grain, that presents the conditions m o s t favorable for the complete purification of sewage, has a d i a m e t e r of grain of a b o u t o'2 of a millimeter, or a b o u t o'oo8 of an inch. The q u a n t i t y of s e w a g e that m a y be permanently, t h o u g h intermittently, applied to filters of such sand, is a b o u t ioo,ooo gallons per acre daily. " T h e very c o m p l e t e purification here referred to produces an effluent chemically as good as m a n y drinking waters, h a v i n g in ioo,ooo parts a b o u t 0"oo2o part of free ammonia, and a b o u t o'o13o part of a l b u m i n o i d ammonia, the former b e i n g b u t one-tenth of one per cent. and the latter b u t t w o per cent. of the a m o u n t in t h e sewage. Nearly all of the organic m a t t e r of the s e w a g e is oxidized, forming, in t h e effluent, soluble nitrates to t h e extent of a b o u t ~'8 parts in Ioo,ooo. Bacteriologically, the results are even more r e m a r k a b l e , there remMning in the effluent less t h a n i in io,ooo of the n u m b e r applied in the sewage. The a p p e a r a n c e of the effluent is that of a bright, clear spring water." The surface of the filter should, in the intermissions between t h e applications, r e m a i n uncovered for a period much longer t h a n t h a t d u r i n g which it is covered. Polluted w a t e r supplies m a y be purified in a similar manner. As they, however, contain some free oxygen dissolved t h e y do not require so m u c h air in the sand as does sewage. Hence, m u c h larger quantities, say, t e n to t w e n t y times as much, or b e t w e e n I,ooo,ooo a n d 2,ooo,ooo gallons daily p e r acre, can b e passed t h r o u g h the filter. In this ease, the a c c u m u l a t i o n of s u s p e n d e d m a t t e r is more rapid, and, in order to maintain t h e efficiency of the filter, t h e deposits s h o u l d be r e m o v e d at more f r e q u e n t inter-

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ffc~'ing .'

[J. F. I.,

vals. It was found t h a t 6o,ooo,ooo gallons of water, p a s s i n g t h r o u g h one acre of the sand, as m e n t i o n e d above, r e q u i r e d the removal of one-eighth of an inch of sand and s e d i m e n t . Therefore, when filtering 2,oo0,ooo gallons daily, abol:t onee i g h t h of an inch should be r e m o v e d every t h i r t y days. " M o s t of the tests which the State Board of }{ealth relics upon, as g i v i n g the efficiency of different filters in r e m o v i n g disease germs, h a v e been m a d e by a p p l y i n g to the filters water c o n t a i n i n g large n u m b e r s of the t y p h o i d bacillus or of the more easily recognized/~aciNzfs 2rvdi£~iosus. "q'h:,se tests show t h a t t h e best sands c o n t a i n i n g l a r g e n u m b e r s of these bacilli m a y have from 1,5oo,ooo to 3,ooo,ooo gallons of water filtered daily per acre, w i t h the cons t a n t removal of more t h a n 99.{ per cent. of their n u m b e r . " In actual practice such results can only be o b t a i n e d b y c o n s t r u c t i n g the filters with g r e a t care, so t h a t the s a n d forms a h o m o g e n e o u s mass w i t h o u t stratification or c h a n nels w i t h coarser grairis, allowing a l a r g e r q u a n t i t y of w a t e r to pass t h r o u g h the interstices. Mr. Mills has r e c e n t l y designed and built a filter at L a w r e n c e , Mass., for the p u r i f i cation of the Merrimac R i v e r w a t e r w h i c h supplies the c i t y , based upon the results o b t a i n e d by the i n v e s t i g a t i o n s of t h e S t a t e Board of t I e a l t h . It is the first filter for w a t e r s u p plies t h a t has been b u i l t in this country, w h e r e the principle of i n t e r m i t t e n e y has been c o n s i s t e n t l y and intelligentl)~ applied, and the results so far h a v e a m p l y r e w a r d e d t h e care taken in the construction, a n d confirmed the r e s u l t s . o b t a i n e d in the smaller e x p e r i m e n t a l filters of the S t a t e Board of Health. It covers an area of two a n d o n e - h a l f acres and has a capacity of 5,ooo,ooo gallons daily. " I n s u m m i n g up our p r e s e n t k n o w l e d g e of the r e m o v a l o f p a t h o g e n i c bacteria from d r i n k i n g water, we n : a y s t a t e t h a t in addition to the experience of certain E u r o p e a n cities, t h e L a w r e n c e investigations, covering more t h a n five years a n d i n c l u d i n g the bacterial e x a m i n a t i o n s of more t h a n I I,OOO, samples of water, indicate t h a t it is e n t i r e l y p r a c t i c a b l e t o c o n s t r u c t filters which will p u r i f y w a t e r economically, a n d r e m o v e more t h a n ninety-nine per cent. of t h e b a c t e r i a in. t h e unfiltered water."