PURIFICATION of WATER at ZURICH. [Selected

PttpCYS.] PRELLER ON THE ZURICH WATERWORKS. 257

(Paper No. 2599.) ‘‘ TheZurich Water-Supply, Power and Electric Works.” By CHARLESSHEIBXER Du RICHEPRELLER, M.A., Ph.D., Assoc. M. Inst. C.E.

PART 1.-THE ZURICH WATERWORKS. THE town of Zurich, containing,with its ten suburbs, nearly 100,000 inhabitants, is the largest in Switzerland, and may also claim the palmfor themagnitude of itspublic works, carried outand managed by the Corporation. Among these, thenew waterworksare especially interesting, not only because the potable-water supply, as well as the motive-power for the waterworks and the electric-installation are derived from the lake, at the lower end of which the town is situated; but also because the experience gained in the management of them leads to important conclusions as regardsthe comparative cost, performance, and action of roofed-in and open filter-beds ; whilst the chemical and bacteriological investigations to which the water-supply is syste- matically subjected, afford valuable information ; bearing, on the one hand,upon the effect of filtrationunder the most varying conditions, and, on theother, upon the vexed question of the self-purification of rivers. By the courtesy of the Zurich authorities, the Author was, in the summer of 1891, for the purposes of this Paper, enabled to inspect the works in detail, as well as to examine the methods of analytical and bacteriological control of the water-supply ; he also visited the whole basin drained by the lake of Zurich, and collected from official sources in each of fourteen other Swiss towns, data as to their water-supply, together with the registered depths of rainfall at each centre ; with a view of placing his researches before the Institution in a comprehensive form. He hastreated the subject of his enquiry in three parts, of which the present Paper forms Part I. These several divisions of the Paper comprise. the following items :- [THE INST. C.E. VOL. ~XI.] S

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 258 PEELELLERON THE ZURICH WATERWORKS. [Selected PARTI. I. The old water-supply. VI. The filter-station. 11. The typhoid epidemic of 1884. VII. The pumping-station. 111. The new water-supply. VIII. Reservoirs and distribution. IV. The Lake of Zurich. IX. Water-supplyand rainfall of V. The intake and main conduit. principal Swiss towns. PART11. I. Working of filter-beds. 111. Self-purification of theRiver 11. Chemioal and bacteriological Limmat. examination of water. 1 PART111.' I. Supply of motive-power. III. Water-supply and electric works 11. The electric works. 1 (summary). I. THEOLD WATER-SUPPLY(Fig. 4, Plate 9). The principle of a mixed water-supply being best suited to local conditions, was originally laid down in 1861, and confirmed in 1873. It was then resolved that the existing spring-water supply should be extended and improved; whilst the supply for domestic, public, and industrial purposes shouldbe taken from the River Limmat (the river that flows from the lake), filtered, pumped up to three reservoirs for distribution, and thence delivered by the Corporation at cost price, afterallowing for interest,sinking- fund and renewals. Theaverage yield of thesprings is only 660,000 gallonsper day, while the pumping-machinery to be erected, and the reservoirs, were designed for a daily supply of, say, 2,200,000 gallons of Limmat water; the total representing, for the then population of 62,000, about 44 gttllons per head per day, which at that timewas considered ample for all requirements. Lake- Water Supply.-The pumping-station was originallyplaced on the left bankof the Limmat, about1,045 yards below the point of outflow of the river from the lake, water-power amounting to 24 HP. being used for driving two smallpumps, to which, in 1870, a steam-engine of 64 HP. and two newpumps were added. In 1871 a sand filter-bed, of 1,360 square yards filtering-surface, was constructed'in the bed of the river, at a point about 275 yards below the outflow from the lake; whence the filtered water passed through a concrete conduit 23.6 inches in diameter to the pumping-station. This conduit was, in 1873, extended to an additional pumping-station erected about 550 yards lowerdown the river, near the Central Railway Station, containing a steam-engine of 59 HP. and two new pumps. This had againto be increased, This Part iii will appear in a subsequent YOlUme.-sEC. INST.C.E.

in 1875, by the addition of an engine of 68 HP. and two pumps. Thus the motive-power amounted, in 1874, to 215 HP., whilst the consumption of filtered Limmat water already reached 2,860,000 , @Ions per day (exceeding the estimatedmaximum by 440,000 gallons), with about 40 miles of mains and 800 hydrants. Four years later, in 187S, owing to a further increase in the consumption, the pumping-machinery was removed to a new site on the right bank of the river, locally known as the “ Letten ” quarter, where the motive-power was furnished by turbines; the steam- enginesbeing left atthe old stationsand utilized for other purposes. The additional length of main conduit, made of cast- iron, was about 1,210 yards; so that the total length of conduit from the submerged filter-bed to the Letten pumping-station was 2,530 yards; the latter being situated about2,805 yards below the outflow of the Limmat from the lake, and 715 yards below the confluence of the Rivers Limmatand Sihl. Spring- Water Supply.-The average quantity of spring water,collected from about ten groups of small springs in the hills flanking the town, is about 660,000 gallons per day, the minimum yield, however, beingonly about 190,000 gallons; whilst the ratio of minimum to maximum yieldranges between 1 : 4 and 1 : 26. These fluctuations are due to the thinness of the superficial sand and gravel strata. With the yield of the spring, varies also the velocity of the water; whence, after heavy rain, it is liable to turbidit,y, arising from the change of velocity, whereby the sand is stirred up ; nor does this turbidity disappear until the velocity has again become constant. Generally, the more the temperature of a spring varies with the seasons, the less constant and more superficial is the spring itself. The percentage of rainfall which percolates the soil of the Zurich hills, that is, collects into springs, iscarefully measured by pluviometers and percolation gauges.’ The Author hasdeduced the following Table of the average annual quantities from the monthly statisticsof the last 5 years :-

1 These infiltration- of absorption-gauges, which have been in use since 1866 are constructed on the principle of the Dalton “ lgsimeter ” ; and consist of a square tin box with vertical sides, 1 metre in depth, open at the top and closed at thebottom, sunk into the soilto its full depth, and having at lowest its point a drain-pipe whichconveys the percolated water by gravitation to a lower point, where the outflow can be observed and measured. The tin box is filled with the excavated soil, and the surface iscovered with turf, so as torestore both soil and surface to their original condition. The quantity of percolated water is measured and determined on the assumption that at the depth of the outflow there is no evaporation, whether the water be in motion or at rest. S2

Rainfall per Month Percolation per Month.

Average ' ~ Altitndc In Plantations. Year. is::: 1 Altitude Avernae. 480 660 Metre ~ 1 Metres In I in the on the cupu- Meadow. Town. Hills. Ilferous. ferous. l ---- _- -

Gallons. ~ Inches. Inches. Inches. Inches. Inches. 1885 372 4.1 3.6 1.3 2.2 1.8 1886 477 4.3 3.6 1.8 1.4 1.6 1887 417 3.5 3'0 1.4 1.7 1.5 1888 582 4.9 4.5 2.4 2'6 2.4 1889 490 I 3.9 3.3 2.0 2.5 1.8

Average of 5 years

(Mini- (Aver- 50 58 50 I 52 numof, age 25 , of 23 -J years.) , years.) Per cent. of rainfall. 238 4.09

As will be observed, the monthly rainfall in the town (at the Observatory) is 15 per cent. more than on the hills (the difference of altitude being about 590 feet) ; the mean annual fall in the former locality is 54 inches, and in the latter, 43 inches; whilst the rate of percolation is l5 per cent. more in coniferous than in cupuliferousplantations or meadow land.The meanpercentage of percolation or absorption for 5 years is 52 per cent., and for 21 years, 66 per cent., of the registered rainfall; and the greater part of 39 percent. must be evaporated; as, owing to the nature of the ground, only a small percentage of water flows off the surface. The water of the three principal contiguous groups of springs aboveZurich, representingabout two-thirds of the entirespring-water supply, passes, beforedistribution, through acovered filter-bed adjoining the low-pressurereservoir for the filtered lake-water ; and is thuseffectually freed from the turbidity to whichat times it isliable. These springs feed, in addition to house-supply, about 600 public and private fountains; about 25 small and 5 large ornamental fountains being supplied with filtered lake-water. Several of the high-lying suburbs, moreover, have a spring-watersupply of theirown; and this accounts for the comparativelylimited distributing-area of thehigh- pressuredivision of the filteredlake-water; although here, too, the consumption of the latter is constantly increasing, owing to the fluctuations, alreadyreferred to, of the spring-water.The total cost of the spring-water supply, so far as it is underthe

control of the Corporation, including the intake-works, filter-bed, public fountains, and 7 miles of pipes, has been up to the present time dt15,OOO; equivalent to 51 98. 3d. perlineal yard. The spring-watersupply represents about 14 per cent. of thetotal water-supply of the town and suburbs.

11. THETYPHOID EPIDEMIC OF 1884. In the early spring of 1884, Zurich was visited by a virulent outbreak of typhoid fever, which, beginningin the month of March, reached its maximum intensity in April, and practically disappeared by the end of June, fully 2 per cent. of the population having been attacked, and 9 per cent. of the cases proving fatal. The Commission appointed to inquireinto the cause of this epidemic arrived at the conclusion that the infection could not be traced to abnormal meteorological or sanitary conditions ; but that thefiltered Limmat water, althoughclear and chemically satisfactory, contained an abnormal quantity of bacteria. It was subsequently discovered that, owing to the dredging operations in connection withthe newquay-works, theimpure matter deposited at the bottom of the river had been stirred up; and, not being effectually retained by the submerged filter-bed, had found its way into the concretemain, which was by no means water-tight,and had been damaged duringthe blasting and removal of an erratic block from the bed of the Limmat. The reason why the defective condition of both filter-bed and conduit did not arouse suspicion until after the outbreak of the epidemic was twofold : first, the Limmat water was generally so clear and chemically purethat filtration was considered of altogether secondaryimportance ; and, secondly, thepermeability of the concrete, laidat 2 to 3 metres depth below the river-bed, was regarded asan advantage rather thana defect, inasmuch as thesand in which the conduitwas imbedded was supposed to actas a filtering medium. The bacteriological investigations, in conjunction with the proved percolation of impure matter through the filter and conduit, stamped the Limmat wateras the vehicle of infection ; the more so because the typhoid epidemic in Zurich coincided with a similar outbreakat Geneva, where similar dredgingoperations were in progress, and where the water taken from the lake was, and is to this day, used unfiltered. Hence, at Zurich, the necessity was promptly recognized of providing an entirely new water-supply for the town and suburbs; the dredging-area being in the meantime enclosed, and a cast-iron conduit substituted for the defective concrete main of the old supply.

111. THE:NEW WATER-SUPPLY. The daily quantity to be supplied having been provisionally fixed at about 66 gallons per head of population (100,000 inhabitants), the question to be determined was whether this supply could be best obtained from spring, underground or lakewater. In its geological structure, the districtof Zurich (Fig. 3, Plate 9) is composed chiefly of the Molasse formation, upon which rests the so-called ‘‘ Nagelfluh,”-a conglomerate which is inpart derived from disintegrated molasse andthe detritus of ancient watercourses, but principally from the moraine-matter of ancient glaciers. Of the 13 groups of availablesprings within and immediately beyond the watershed of the lake of Zurich, there was only one Alpine spring sufficiently copious, situated in the upperWaeggi valley, Fig. l, at a distance of30 miles from Zurich, at an altitude of 2,788 feet above sea-level, or 1,444 feet above Zurich, which drains an area of about 3.86 square miles ; this, at the considerable rainfall in Alpine valleys of 79 inches perannum, less 25 per cent. for absorption and evaporation, should yield 6,340 gallons per minute, sufficient for a minimum supply of 4,400 gallons perminute at adistance of 30 miles. It was, however, found that the actual total yield of the spring varied betweena minimum of 1,320 gallonsper minute in the dry season, and a maximum of 52,800 gallons per minute in the wet seas0n.l TheAuthor mentions this as an example of the fallacious nature of an estimate of the yield of a spring based simply upon the mean rainfall in a district that embraces several drainage-areas. The h priori objection of insufficient yield of the various groups of spring-water applied equally to the under-ground water, which in and below Zurich, parallel to the Limmat and Sihl, is found at a depth of between 65 feet and 32 feet below the surface of theground, as is shown byabout 500 wells now used chiefly for agricultural purposes. Forthis purpose theavailable area is quite inadequate ; nor is the water itself of sufficient purity, subjectas it is to infiltrations of impurematter from sewers, from the river Sihl, and other sources. In view of these facts, in

Spring watcr risingin the upper Alpinereaches is, in spite of its crystalline clearness, peculiarly liable to pollution by the scattered droppings of Gazing cattle, unless the whole drainage area is enclosed. Although the water purifies itself to a great extent in thecourse of its flow, it can produce epidemics by the droppings of diseased cattle, of which cases are recorded in the upper Rhine valley, at Neuchatel and at Appenzell.

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] PRELLER ON THE ZURICH WATERWORKS. 263 conjunction withthe chemicaland notably the bacteriological inferiority of most of thesprings, as well as of theunder- ground water, compared even with unfiltered Limmatwater, it wasdecided to derive the newwater-supply entirely from the lake, retaining the existing spring-water supply, and utilizing the original Limmat-water supply delivered by the old conduit exclusively for the supplyof motive power.

IV. THE LAKEOF ZURICH.

The drainagearea of the lake of Zurich, Fig. 1, Plate 9, is about423 square miles. Its principaltributaries are the rivers Seetz and Linth, both of which rise in the Glarus Alps, and discharge first into the Wallen lake; this latter being in its turn drained by the famous Linth canal, which was constructed in 1822 by Escher von der Linth, for the purpose of regulating the discharge of the Linth and the Wallen lake into the lake of Zurich, and draining the intermediate tract of marshy land. The Wallen lake, which has a superficial area of 9 square miles, and a mean depth of 492 feet, is situated 46 feet above the lake of Zurich. It acts as a vast storage-reservoir for water descending from the Alps; and its purifying action is rendered more effectual by the equable discharge of the Alpine rivers, due to their origin in the glaciers. The lake of Zurich,' in addition to the discharge of the Wallen lake, receives the water of the sub-Alpine hills, including the greater part of the yield of those springs, which, if adopted for thesupply of Zurich,would have had to be conveyed longdistances through costlyconduits. Its mean depthis 230 feet. Its meandischarge throughthe river Limmat is 1,777,000,000 gallonsper day. Thepurifying action of such a lake as this is especially exercised in the precipitation of impure matter, favoured by the slow motion of the water; in extensive oxidation,promoted by the large surface of waterwhich is in contact withthe air; by the watersbeing kept in motion by navigation and by occasional storms; by solar heat which extracts oxygen from aquatic plants ; by an aquatic faunaof great variety, ~- ..~

1 The lake of Zurich owes its origin in the first instance not, as is generally held, to glacial erosion, but to deep rents in the molasse formation which later on facilitatedthc intrusion and erosive action of the greatLinth glacier, advancingand receding in the course of glacialand genial epochs. This is proved by the fact that even at the pointof its greatest depth of 475 feet, which coincides with thc rent-line, the bottom consists only of soft mud, the rock or true bottom being at a far greater and unknown depth.

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 264 PRELLER ON THE ZURICH WATERWORKS. [Selected which collects at the outflow of drainsand streams, and feeds on theimpure matter contained inthem; and,lastly, by the innumerablequantities of microbeswhich live atthe bottom of the lake, and voraciously devour and destroy the excrements which fall into it from steamersand barges, or may be thrown into it by the riparian population. How effectual is this process of purification in Swisslakes, may be gathered from thefact that the normal quantity of bacteria in the lake of Zurich, at the lowerend, has beenfound to average 170 per cubiccentimetre ; in Lucerne 50 ; in Geneva 38 ; and in Constance 58 per cubiccentimetre ; whereasspring-water, especially after rain, oftencontains as many as 2,000 bacteriaper cubic centimetre. Moreover, the water of the lake of Zurichis essentially a soft water ; its mean alkalinity, or hardness, being only 8-75 English degrees; whereas the spring-water of the district is 23.1 English degrees, and is therefore suitable only for drinking, and not for cookingpurposes. For a townsituated like Zurich,hygienic, engineering, and financial considerations alike point to the lake as the propersource of apure, adequate, and unfailing water- supply;and this is amply attested by the success of the new waterworks, the construction and working of which are described below.

V. THEINTAKE AND MAIN CONDIJIT. (1) Intake.-The point of intake of the lake-water, marked by a buoy, is 919 feet from the nearest quay, the depth of the lake at ordinary water-level being there 56 feet. At first the intake- apparatus consisted of a vertical pipe35 inches in internal diameter, the upper orifice of which was covered by a screen and surrounded by a wire cage, placed at a depth of 13 feet below ordinary water- level. Withthe view,however, of obtainingthe .water at the lowest possible temperature, the intake was subsequently lowered to 46 feetbelow the surface,or 10 feetabove thebottom; for which purpose theupright orifice was closecl, and a trumpet- shaped orifice, pointing downwards, was attached to the horizontal mainconduit, Figs. 5 and 6, Plate 9. At a, lowerdepth, the proximity of the impure matter deposited at the bottom of the lake was found to impair the quality of the water; whilst, con- trary to what on theoretical grounds might have been expected, the water at the depth of 46 feet was as pure as that at 13 feet depth.The comparativetemperatures at different depthsare shown by the following Table, abstracted by the Author from a series of almost daily tests in1886, 1887, and 1888 :-

&pt.h of Intake. Temperature in Degrees Centigrade. 13 feet. I 39 feet. 1 46 feet. 1 52 feet.

~ ~ ___.------

Minimum . . . 2O.6 2O.8 2O.9 3O.O Maximum . . . 17O.3 13O.8 11O.6 1 9O.4 Mean . . . . 9O.9 1 1 7O.9 1 7O.1 1 6O.3 The increase in the temperatureof the water from the intake to the reservoirs, after passing through the main conduit, filter-beds, and pumping-station, is, in the summer months, 2" C. The mean temperature of the water distributed is 9" to loo C. (2) Maila ColzduiL-The main conduit, from the intake to the filter-beds, andthence to the pumping-station,consists of four sections of pipes, as follows :- 1st section 351 yards 35 inches in diameter, from the intake to the moat connecting the lake with the river Sihl. 2nd ,, 2,556 ,, 36 inches in diameter, from the moat tothe Sihl and the filter-station. 3rd ,, 26 ,, 24 inches in diameter, inthe filter-station. 4th ,, 698 ,, 36 inches and 18 inches in diameter, from the filtcr- station to the pumping-station. Total, 3,631 yards.

The three last sections consist of cast-iron pipes ; the first is composed of wrought-iron pipes, which wereacquired from the St. Gothard Railway, where they hadbeen used for the compressed- air installation of the great tunnel. These pipes rest on a timber frame, composed of three rows of piles connected by cross- beams, the whole containing 11,975 linealfeet of timber. The pumping- and excavation-works of the second section-being the most difficult, owingto the main having to be laidrapidly at an unfavourable season of the year, in a trench 8 feet below the bed .of 'the moat and of the changeable river Sihl-were carried out by Messrs. Locher and Co., of Zurich. Thefourth section was carried from the filter-station to the suction-wells of the pumping-station, across the river Limmat, by afoot-bridge 400 feet 3 inches in length; the conduit thus acting as a siphon, in which the water is set flow to in less than ten minutes bymeans of an. air-pump actuated by hydraulic power. This expedient was resorted to owing to the heavy cost of reaching the suction-wells of thepumping-station by gravitation;which would have involved a trench at considerable depth, not only under the river- bed, but under the water-power supply-canal and the foundations of thebuilding. The total cost of theintake works (including

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 266 PRELLER ON THE ZURICH WATERWORKS. [Selected theintake-apparatus, 5272) andmain conduits,amounted to 516,514; theaverage cost of mainconduit of the different sections laid down being, therefore, 908. per yard.

VI. THEFILTER-STATION. The site of the filter-station, on the left bank of the Limmat, was selected, on the one hand, because the ground forms part of a tract of land owned by the Corporation, and now chiefly occupied by factories, whichutilize the hydraulic motive-power of the waterworks;and on theother, because the construction of the filter-beds near the reservoirs above the town would have involved both inconvenience and unnecessarily heavyexpenditure. The system of sand-filtration was adopted in preference to other more or less experimental and costly methods. Nor is there any doubt that sand filter-beds are at once economical and efficient, provided they are constructed, (a> so as to give not only pure water, but a givenquantity in a giventime ; (bj so asto freely admit atmospheric air, whereby, especially in covered filters, the danger of the development of injurious germs is reduced to a minimum ; so as to be water-tight and as high above ground as possible, thus preventing the infiltration of under-ground water; and (dj so as to admit of easy draining, cleansing, aerating and renewal of the filtering-strata at alltimes. (1) The Filter-beds(Figs. 7,8,9,10, Plateg).-The five filter-beds first constructed, including the sand-depot (765 yards X 27 yards) in front of them, below which are placed the eight different conduit-pipes, as well as the clear-water basin and the scour-well for emptying the filters, occupy an areaof 7,076 square yards.l It was first intendedto cover allthe filter-beds witharched concrete roofs and a layer of turf; inorder, however, to test thecomparative action of covered and open filter-beds, the former were limited to three,the two others remaining uncovered for thetime being. The excavated material amounted to 19,097 cubic yards, and was used for forming raised pleasure-grounds on one of the new quays of the lake. The filter-beds areall of the same dimensions, viz., 143 feet 1 inch X 60 feet 10 inchesinclusive, and 132 feet 10 inches X 57 feet 5 inches, exclusive of the outer walls; the effective filter- surface being 810 square yards per filter-bed. The average depth

’ Two roofed filter-beds, of the same dimensionsas the existing ones, are now in progress-making a total of seven-and the two beds now open will also be roofed shortly.

of the floor below the surface of theground is 10 feet. The partition walls between the beds are 4 feet 5 inches in thickness at the footings, and 1 foot 10 inches at the copings; the floor is 14 inchesthick, and has an inclination of 1 in 40 inthe direction of the two drain-pipes running throughout the length of the bed. On the floor rests the drain-grate, composed of two alternate layers of brick, on which are supported the following filtering strata from bottom to top (Fig. 8) :- Inches. One layer of coarse gavel (to fill up spaces between bricks) . 6 .... fine ...... 4 coarse sand .... coarse ...... 6 .... fine sand (the filteringmedium) .....-32 Total ...... -48 Thequantity of sand used is about 1,046 cubic yards for each filter-bed;material specially suited for the purpose being obtained from theupper end of the lake. Thesorting, clean- ingand washing of the sand, whichwas at first performed by manual labour, is now effected by a sand-washing machine, actuated by a hydraulic motor, the water from the latter being also used for the washing operation. By means of this machine, 39 cubic yards of fresh sand can be prepared per day for immediate use, at a cost of 8 - 74d. per cubic yard, being two-thirds of the cost bymanual labour. The cost of thefiltering-material, including setting and stamping, is 5300 per filter-bed, equal to 1s. lO$d. per cubic yard. The roofs of the covered filter-beds are formed of goined arches of 14 feet 9 inches span, 4 feet 1 inch in height, and 8 inches in thickness, resting on 24 concrete piers, theaverage height of which is 11 feet 6 inches;their mean cross-section being 2 feet 11 inches by 2 feet 3 inches. A.t the points of intersection of the arches, circular skylights, 2 feet l1 inches in diameter,are provided (36 to eachchamber) for light and ventilation. The masonry throughout consists of Port- land cement concrete, the proportion of cement, sand, and gravel being, for the walls and floors, 1 to 3 to 6, and for the arches 1 to 3 to 4. All the surfaces in contact with water are carefully lined with cement mortar (1 part of cement to 3 parts of sand) 1 inch thick; and over this surface is spread a thin coating of cement powder, faced, after setting, with a skin of cement-milk. In this way an impervious lining was obtained. When the filters arein operation, the head of water over the uppermostsand stratum is about 3-3 feet ; the water-level being about 279 inches below the coping-stones, of whichthe upper edge is atthe

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 268 PRELLER ON THE ZURICH WATERWORKS. [Selected ordinary water-level of the lake. The filtered water is collected by the two drain-pipes already referredto, and is by them delivered into the return-flow well, whence it passes to the outlet- well. The four wells required for working the filter-beds are all placed inthe front wall, which is widenedfor the purpose (Figs. 9, 10 and 11); while between the return-flow and outlet- wells a sluice-valve is provided for the purpose of regulating the rate of filtration. (2) The Clear-Water Basin (Figs. 11 and 12).-This tank, situatedin front of the filter-beds, in the centre of andunder the sand-depot, collects the water which has passed from all the filter-chambers throughthe return-flow andontlet-wells; it is thence delivered through the main to the pumping-station on the oppositeside of theriver. It issquare in plan, with rounded corners, and is 32 feet 10 inches wide by 14 feet 9 inches deep, its capacitybeing 83,000 gallons. A gauge inthis basin indicates atall times whether the rate of percolation throughthe filter-beds corresponds with the work simultaneously performed by the pumps, and enables the watchman to equalise the supply and demand. Like the filter-chambers, this basin is built in concrete, with animpervious lining ; and is surmounted by anarched iron-girder-and-asphalt roof, supported by six columns. The addition of a second basin of equal size is contemplated, so that one may be available when the other is beinginspected or cleaned. (3) The Suction- Well.-The water flowing by gravitation from Fiq. 13.

SUCTIONWELL. Scale &. the intake in the lake to the filter-station is first discharge& into the suction-well, Fig. 13, which is placeddose to the filter-station on the slope of theriver bank. It is 27 feet in depth,and is divided into three chambers. The water coming from the lake is discharged into the middle chamber, out of which it either over-

flows into a second, upper, chamber, whence it is conveyed to the various filter-beds by a pipe 36 inches in diameter ; or it can be run to waste direct into the river, when the supply of the filter- beds is tobe shut off. Thethird chamber, which was to serve as the foundation of a wire-rope power-transmission tower, now contains a30-HP. high-pressureturbine, which actuates acen- trifugal pump fitted in a chamber on the top. This is provided for the purpose of liftingthe lake-water into the filter-beds, when, at a time of exceptionally low water-level,the supplyof lake- water by gravitationis insufficient to meet an increased demand. (4) The Filter-Sump.-This is 15 feet in depth, and is situated under thesand-depot, betweenthe clear-water basinand thesuction- well; it consists of two sections connected by a valve, whereby the filter-beds can be emptied by pumping, when this cannot be effected by the drains discharging into the river, owing floods.to (5) The Conduits (Figs. 7 and ll).-Thesepipes, whichare laid under the sand-depot, in front of the filter-beds, are eight in number,all but one being of cast-iron :--No. (1) is the inlet-main, 36 inches in diameter,conveying the water from the suction-well tothe filter-beds; No. (2), the overflow pipe, 24 inches in diameter, discharges the overflow from thefilters intothe river; No. (3) isthe filter scour-pipe, 14 inchesin diameter, by means of which the filter-beds can be emptied for cleaning ; No. (4), the outlet-main, 24 inches in diameter, delivers the filtered water into the clear-water basin; No. (5), 36 inches in diameter, conveys the water from the clear-water basin to the pumping-station; No. (G), 4 inches in diameter, conveys potable water from the low-pressure distributing-pipes in the town, and, by a connection with thereturn-flow well, serves to fill the filter-beds from below after cleansing ; Eo. (7), the hydraulic-pressure pipe, 2.8 inches in diameter, supplies hydraulic power for driving the sand-washing machine ; and No. (8), a cement conduit, 12 inches in diameter, drains the sand-depot. The total length of pipes in the filter-station is 1,084 yards, with 43 T-branches and stop-valves. 33 (6) Cost of Construction.-The construction of the entire filter- station,including 54,800 for Corporation landdebited to the waterworks, intake conduit-pipes, wells, and accessory work, was ;E22,%O--equal to an averagecost of about 55 10s. per square yard of covered and open filtering-surface. The cost of the filter-beds alone, viz., masonry andfiltering-material, was &7,360 for the three covered, and $3,280 for the two open, beds-equal to about 53 1s. per square yard of covered, and about 52 per square yard of open, filter.

VTI. THEPUMPING-STATION. (1) Power.-The waterworks, in common with the power-supply and electric works to be described in the third part of the Paper, use as motive-power thewater of theriver Limmat, which is partially dammed at a point about 2,000 feet above the pumping- station, immediately above the confluence of the Limmat and the Sihl, and conducted into a supply-canal running parallel to the river.The canal is about 75 feet 6 inches inwidth, has a fall of 1 in 2,000, and is separated from the river by an earth-work dam. A sluice-lock, 82 feet 3 inches inlength, and 14 feet 9inches wide, connects the head-race with the tail-race, which latter is about 951 feet in length.The cost of the canals, including weir, lock, and sundries, was about S61 per lineal yard. The fallof the Limmat, and thecorresponding volumes of water available for motive power, are as follows :-

(a) At high-water level in summer, fall 4.92 feet,volume 2,295 cubic feet per second ; equal to 1,300 HP. (b) At mean-water level in summer, fall 8.20 feet, volume 1,660 cubic feet per second ; equal to 1,570 HP. (c) At low-water level in winter, fall 10.5 feet, volume 1,059 cubic feet per second ; equal to 1,250 HP.

Hence, at 80 per cent. efficiency of the absolute water-power, the mean effective motive-power, in respect of the turbine-shafts, is 1,100 HP., and is utilizedfor the following purposes :- HP. (l) For pumpingthe potable-(filtered) watersupply . . 237 (2) For supplying motive-power byhydraulic pressure . . 128 (3) For supplying motive power by wire-ropetransmission 227 (4) For supplying hydraulic motive-power for the electric- ) 444 lightinginstallation . , ...... Total ...... 1,036- In theevent of theavailable water-power, at exceptionally low water-level of the Limmat, being reduced to 592 HP. effective (when the fall is 10.5 feet and thevolume only 883 cubic feet per second), the deficiency is supplied by two reserve steam-engines of 296 HP. (effective) each. The total hydraulic power will meet the demand for the next five or six years. The additional power then required for the water-supply, which increases at the rateof 5 per cent. per annum, and for extension of electric lighting, may readily be obtained by electric transmission at a high potential,

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] PRELLER ON THE ZURICH WATERWORKS. 271 either from the river Limmat below Zurich, from the river Reuss, or from the Rhine, the distance from these points being about 20 miles. (2) Buildings and Machinery.-The entire hydraulic machinery, as well as the recently-addedelectric-plant, including reserve steam-engines, are contained in a brick building 377 feet 4 inches X 75 feet 6 inches, the foundation of which is in part formed by twelve turbine-chambers,each 31 feet 6 inches indepth, composed of three superposed arched concrete divisions, of which the middle one constitutes the inlet, and the lowest the outlet, chamber. The iron roof of the building is supported by two rows of iron columns inside the machine-room, in which are three overhead travelling-cranes. The building is heated by two stoves with ventilators, and is lighted by electricity; whilst an electric regulator and the four automatic indicators, registering the differences of water-level in the variousreservoirs down to about 2 inches, are fixed on the wall. The steam-boilers, as well as the sheds, smithy, and offices, are placed in a separate wing adjoiningthe electric installation. The area of thebuilding, including the new electrical section, outside wing, and depots, is 40,904 square feet. Thetotal cost of theland and buildings, including turbinschambers andfittings, was 556,120. The pumps are actuated by 10 turbines; a reserve of power being afforded by 2 reaction turbines, of 172 HI?. each, recently put downin connection withthe electric-supply. All these turbines are of the Jonval type, built as reaction turbines, with three or twoconcentric rings. In eight of them, thediameter is 11 feet 9 * 73 inches, the number of revolutions increasing at times from 25 to 37 per minute, their tested efficiency in work, measured on the vertical shaft, beingas follows :-

(a) At maximum fall and minimum volumeof river Limmat-75 per cent. (a) minimumAt fall and maximum ,, 99 ,, -851per cent.

Hence, their effective power is :-

in case (U) 94.68 brake HP. (b) 108'49 brake HP. The power developed by each pair of turbines is transmitted from the vertical shaft by bevel-gearingto a horizontal shaft, which, in the case of the first 8 turbines, makes 50 revolutions, and in that of the 2 latest, 66 revolutions per minute; thence, the power is transmitted by pinions to the main shaft, which is of steel, 7.6 inches in diameter, making 100 revolutions per minute. This shaft

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 272 PRELLER ON THE ZURICH WATERWORKS. [Selected is about 328 feet in length,and passes throughoutthe length of thebuilding; so thatthe whole of theturbine-, pumping-, steam-, and electric-machinery can be coupled to it separately or jointly, as and when required. The collective effective power of the 10 turbines is 1,130 HP. They were all supplied by Messrs. Escher Wyss and Co., of Zurich, at a cost of S1,220 for each of those developing 98 HP., and $1,260 for each of those developing 172 HP., including gearing and horizontalshaft. The total cost was 512,320. Thereare at present in operation 7 sets of horizontal com- pound Girard pumps, each set comprising two pumps, and having its ownsuction air-vessel, while 3 of thesets have each two pressure air-vessels, andthe other 4 setshave one pressureair-vessel each. The dimensions of the several setsare as follows :-

- First Type Second Type Third Typr Three Sets. , ThreeSets. One Set. - - ~ ~ ~~- ____ Inches. Incbes. Inches. Diameter of piston...... 1 11.41 I 9.05 11.41 Sq. Ins. Sq. Ins. Sq. Ins. Area ,, ,, ...... 102.3 ' 64.33 102.3 . j Inche-.Inclles. Inches. Length of stroke ...... 1 23.6235.43 23-62

Number of double strokes per minute . . ' 25 ~ 25 37.5

Thetotal pumping-capacity of the 7 setsis 8,143,600 gallons per day. An eighth set of pumps of the third type is now being put down as reserve, with space for 2 additional sets. The cost of the pumps of the first type was &1,616 per set, and of the second 51,928 per set ; whilst the third type, which delivers 50 per cent. more water, and works with ease up to 50 revolutions per minute, onlycost 2E2,024 complete, or 10 per cent. more than the others. The total cost of the pumping-machinery, including main shaft, cranes, gauges,indicators, and sundries, which were also supplied by Messrs. Escher Wyss and Co., of Zurich, was 325,160.

VIII. RESERVOIRSAND DISTRIBUTION. (1) Reservoirs (Fig. 4, Plate 9).-From the suction-wells of the pumping-station, the filtered lake-water is pumped up to 3 covered reservoirs (low, medium, and high pressure) for distribution to the low-, mediuln-, and high-pressure divisions respectively of the town and suburbs; whilst the Limmat water of the old

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] PRELLER ON THE ZURICH WATERWORKS. 273 conduit is pumped up to a fourth open tank for the supply of hydraulic power. The differences of level between the pumping-station and these 4 reservoirs, the mean lift (suctionand pressure), thelengths and diameters of pipes, and the cubic capacities of the reservoirs are as follows :-

To be Reseut. crease^

Feet.Feet. Feet. ~ Feet. Inches. Cubic Feet. .1 Feet. Mean water-level of 1,342.0 ...... I .- l Coping - stones of l filter-bed walls .) ...... ' .. .- Floor of filter-beds . 1,329.0 ...... l .~ Water-leveliupump- suctionwells . .) 1,325.48 . . I Water-level in low- 1,479.66154'20229.66 8,858 12.16204,836 .. pressure reservow ) Water-level in me- l dium-pressure re- 1,624'03298'56374.02 7,8741 10.18 68,5143141,266 servoir . . . l 1,611.05485.57526.22 9,186 8.10 10,5951 21,190

1,855-70'528.22554.47 5,577 18.00353,170'529,750

The power employed in pumping is 420 HP.' The lower- and medium-pressure reservoirs are built of brick, and the recently-constructed extension of the former is of concrete, the thickness of wallsbeing 59 inches,while the arches are 10 inches in thickness, 14 feet 9 inches wide between the springings, and 27 inches in height. The depth of the reservoirs is between 13 feet and 14 feet 9 inches; the lower reservoir has 26, and the upper 8, ventilating shafts ; and both are covered with a layer of turf 3 feet 3 inches in depth. The intermediate walls, 20 inches in thickness, which support the groined arches, divide the reservoirs into oblong chambers, 11 feet to 14 feet .9 inches in width ;

The power required to pump 1 ton of water to the fourreservoirs works out respectively at 0-24,0.45,0.48, and 1-3HP. per hour. The filtered lake-water is to a great extent distributed direct without passing through the reservoirs, which consequently serve, in the main, only to store the surplus water. The quantity pumped per day to the four reservoirs is 13,000, 5,000, 250, and 3,500 tons respectively. [THE INST. C.E. VOL. CXI.] T

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 274 PRELLER ON THE ZURICH WATEB,WORKS. [Selected and thus circulation of the water is ensured-the play of valves being so arrangedas to cause thewater, after entering the reservoir, to flow through 8 chambers in the lower, and 13 in the upper,reservoir before it reaches the outlet.The third, or high-pressure,reservoir, which is likewise built of cement concrete, is circular in plan, 32 feet 2 inches in diameter. In addition to the extensions of these reservoirs now being carried out,another small low-pressure reservoir of 10,595cubic feet capacity is to be constructed on the Uetliberg side of the lake, at an altitudeof 1,444 feet above sea-level, or 118 feet higher than the pumping-station,and at adistance of 14,436 feet from the latter. The open reservoir or hydraulic accumulator will be referred to in the third partof the Paper on the supply of power. The cost of the threereservoirs, of an aggregate capacityof 1,800,000 gallons, was 212,920. (2) Distribution.-The distributionin the IOW-, medium- and high-pressure divisions is on the '' trunk-ancl-branch " principle, withmain and subsidiary pipesand connecting-valves; butis worked for the most part on the circulation system, in order to prevent the supply from running short in a,ny one part of the town or suburbs. The pipes are all laid 5 feet below the surface of the ground,and the large number of stop-cocks allows the shutting-off of thesupply, when necessary, to be restricted to the smallest possible area. The length of main and service-pipes, which vary between 16 inches and l 6 inch ind.iameter, is 72 miles, .owing to the extensive area of distribution in the suburbs. The fire-hydrants are placed about 66 yards apart, close to the edge of the curb, and can be used for scouring purposes, in addition to the .ordinary flushing-valves. The totalcost of the distribution system, including 752 valves and 1,294 hydrants, was ;E81,000.

Ix. \vATER-sUPPLY AND RAISFALLOF PRINCIPAL SWISS TOWNS. For the purpose of comparison with the Zurich water-supply, as well as with a view to giving a general idea of waterworks in Switzerland, the Author has obtained from the local authorities, data of the potable-water supply of the principal towns ; and from these data he hasprepared Table I, Appendix, giving foreach town the daily consumption, the cost of construction of the works, and otherinformation. Table I1 presents theannual rainfall of each town, of the principal lakes on both sides of the Alps, and the monthlyrainfall atZurich; being compiled from statistics

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] PRELLEE ON THE ZURICH WATERWORKS. 275 obtained from the Central Swiss Meteorological Office at Zurich. The data for the annual rainfall of the towns and lakes refer to 1888, an unusually rainy year, whilst the a'verage rainfall over a period of 20 years is also given. An examination of the Tables shows : (1) That the average consumption of potable water is 53 gallons per head per day; the highest rates of consumption being 100 to 125 gallons per head per day at Montreux, Neuchiitel, and Bienne. Thehigh rate in the lastthree cases is accountedfor bythe fact that, in those towns, the water is chie0y supplied by rate, instead of by measurement ; this latter system being exclusively adopted only at Basle, where the measured consumption in 1890 was 31 gallons per head of total population : (2) That the average length of mainis about 10 miles, the greatest lengths of main being at Basle,Lucerne, Lausanne, and St. Gall: (3) That the cost of works per head of population is over 53 ; the lowest being 51 Ts., at Montreux; and the highest at Geneva and Neuch$tel, viz., 54 16s. and S7 6s. respectively: (4) That the average return on thecapital invested is 8 per cent., thehighest being at Basle, Montreux, and Bienne, viz., 9.5, 10.5 and 12 per cent. respectively: (5) That of the fifteentowns referred to, tenare suppliedexclusively withspring-water ; one (Fribourg)with river-water;and four(Zurich, Geneva, Lausanne and St. Gall) with lake-water ; the mean temperature being 48O.2 F., and that of the spring-water being not only not lower, but in several cases actually higher than that of river- and lake-water : (6) That at Zurich, Fribourg, and St. Gall, the lake- or river-water is filtered, whereas at Geneva it is used unfiltered : (7) That all theprincipal townshave not only anadequate but an abundant supply of potablewater, withthe single exception of Lugmo, which, althoughhighly favoured byNature, and a favourite resort of visitors and tourists, has not, up to the present time, a regular or uniform water-supply : (8) That the annual rainfall of Lugano is 42 per cent. greater than that of Zurich, that of the lakes south of the Alps being 35 per cent. greater than that of the northern lakes ; and (9) that the greatest monthly rainfall in Switzerland generally takes place in the summer. The earnings upon whichthe returns arebased, are equalto about 3.3,. per 1,000 gallons ; the highest being those of Basle, although even there the receipts do not represent more than 5.6d. per 1,000 gallons. With the exception of Lausanne, Montreux, and Herisau, in the two former of which the water is supplied by companies, and in the latter by a local syndicate, the waterworks of all the T2 Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 276 PRELLER ON THE ZURICH WATERWORKS. [Selected towns are owned and worked bythe Corporations, thesupply being given atcost price after covering interest, sinking-fund and renewals ; and the fact that, even at the low rates charged, the yield of the capital invested is as high as 9 per cent., in itself affords proof that the working of waterworks is enlinently the province of municipalauthorities. Moreover, the considerable expense incurred by the various Corporations in procuring good water, either from more or less distant sources of supply, or by costly hydraulic works such as those of Zurich and Geneva, shows how keenly alive they are to the hygienic importance of a good and adequate supply, not only in their own local interest, but in that of the numerous visitors to Switzerland. The most eloquent proof of the beneficial effect of the filtered lake-water supply on the sanitary condition of the town of Zurich, is afforded by the fact that the number of ordinary typhoid cases, which, under the old supply, was, in 1880,4 per 1,000 of the population, has now, in 1891, dropped to 0-4per 1,000-representing a diminution of 90 per cent. From a hygienic, no less than from a scientific and engineering pointof view, the Zurich Waterworks are undoubtedly, not only among Swiss towns, but on the Con- tinent generally,a model of how such works should be constructed andworked; and, looking tothe fact that t.he lake of Zurich provides not only an inexhaustible supply of pure potable water, but also motive-power for delivering that supply, aswell for a variety of industrial works and for electric-lighting, it may with truth be said that that lake is atonce the ornament and thesource of health and wealthof the town andits suburbs. For information kindly furnished him, the Author hasto express his obligations to Dr. W.Burkhard, Engineer-in-Chiefto the Corporation ; and to Dr. A. Bertschinger, Director of the Municipal Laboratory; also to Dr. A. Biirkli, Engineer-in-Chief of the Cor- porationQuay Works, and to Dr. R. Billwiller, Director of the Central Swiss Meteorological Office in Zurich ; and he is further indebted to the courtesy of the engineers of the fourteen other Corporations for the data which enabled him to prepare the Table of Water-supply of Swiss Towns.

Part I of thePaper is illustrated by drawings from which Plate 9 has been prepared.

APPENDIX.

TABLEI.-WATER-SUPPLY OF PIUNCIPALSWISS Towss. ~- Consumptior Alti- i tude Total I'opu- Per 1889. ~ aboveSea- lation. Total He%

~ Level. per per Day. i Daj - 1-1- -- Feet. ' $g& jGallr Zurich . . 1,3511 93,000 4,730 50 Spring and . filtered lake. Basle l . . . 1 869/ 72,500 2,270, 31 13-79'125,600 '1 149.5 50' 1' Spring. Geneva 1,240~52,640 2,780; 50 7.77' 226,800 4 7'9.0 Unfiltered * .I 48' :( lake. Berne . . . '1,758' 47,150 2,220' 48 Spring. Lausannea. . ,1,68634,050 2,220,59 Lake Bret. St. Gall4 . . 2,198 27,840 790 28 Spring and filtered lake. Chaux de Fonds '3,274 25, S40 7SO( 30 Spring. Lucerne . . 11,437, 21,300 1,050, 50 , v~~~~:'~n-.}1,24019,110 1,920 100 7.89) 26,400 1 7110*5145°*5 , , Neuchfttel5 . 1,43416,500 1,660 100 12.43120,000 '7 6; 6.5!46'*41 Winterhur . . 1,450 15,960 96060 5.90 60,000 3 13 7.0150O ' Bienne . . 1,430 15,410 1,920'124 2.48 25,500 11 1312*0,50° 1 if Henmu' . . 2,546' 12,970 450' 35 3'11 18,000 '1 8 9.5'46O.41 SchaEhausen 670' 54 2.0518,200 ;l 10' 9.049O . I1,29G; 12,400 Fribourg* . . 2,100;12,240 1,050, 86 Lugano9 . . , 902 7,169 ...... ------__-- Totals . 15,470855 44.09 .. 1 .. ~ .. j .: -- - -- __-----1 .. Averages . .. ~ .. .. 1 53 916 .. .. ~ 8 ,48' 21 'r7G l , 1 Bask ; undcr-ground water pumped when springwater is insufficient. * Geneva; between 8,000,000 and 13,200,000 gallons of lake-water are delivered per day for industrial and public purposes, in addition to theabove potable supply. Lausanne: a spring-water supply by various companies of 1,992,000 gallons per day, in addition to theabove supply by '' Lac de Bret " Company. St. Gall ; above figures include 528,000 gallons per day of water to be derived from Lake Constance and pumped to the towvn-reservoirsdistance 13.7 miles, differenceof level between lake-level and town 892 feet-by water-power of the River Goldach. Temperature of water at 164 feet depth, 39' F. 5 NeuchLtel ; the spring of the Reuse valley, in the Jura, gives normally 1,900,800 gallons per day ; but the yield has of late years fallen about 17 per cent. 6 Bienne; the normal yield of the spring, in the Jura, is 6,380,000 gallons per day; the average of the last four years is 60 per cent. less than that quantity. 7 Herisau (Appenzell); the above data comprise Supply from private springs and water delivered by a local syndicate. 8 Fribouig;the water of theRiver Saline is also used for motive-power by ropetransmission. 9 Lugano : there are several schemes for a regular potable water-supply ; but none has yet been carried out.

TABLE11.-RAINFALL IN SFITZERLAXD. (1) Towns.

864 to 1883. __ Altitude l above Sea- Average Level. per __-Annum. -- Feet. Feet. Feet. Zurich . . . . , 153 5.00 3 .'TO 1 1,351 Lake Zurich (water) 4.95 1,342 shed) . . . .I, 131 Basle . . . . 130 2.25 2.75 869 Geneva. . . . 132 3.31 2'79 1,240 Berne . . . . 153 3.66 3.60 1,758 Lausanne . . . 160 4.00 1,686 St.Gall . . . 170 6.15 +:is 2,198 Chaux de Fonds . l57 4.80 3,274 Lucerne . . . 152 4.35 .. 1,437 Montreux . . . 137 3.78 1,240 Neuchktel . . . 145 3.15 3.20 Winterthur . . 158 4.93 .. 1;450 Bienne . . . . 150 4.10 1,430 Herisau . . . 154 4.sj 2 ,546 Hchafiausen . . 127 3.2i .. 1,296 Fribourg . . . 137 3.86 2,100 Lugano . . . 146 , 8.15 5.40 902

(2) Lakes.

In Twenty Years. ___ Average --per Annum. __ ___- -- 1 Feet. Feet. !Square Mllt?S.! Feet. Coostance, at Trogeu St.Gall 4.56 1,306 207.85 905 ' . l Zurich, at Zurich . . . . 3.79 1,342 1 34.00 ~ 4662 Lucerne, at Altdorf . . . . I 3.74 1,434 43'62 1 853% NeuchLtel, at Neuohatel . 3-20 1! 1,427 92.05 1 4T.Y . i Geneva, at Geneva . . . . i 2-79 1,230 221.11 984 2 Maggiore, at Locaruo . . . , 4.90 l 646 82 .G1 , 1,1303 Lugano, at Lugano . . . . ' 4.85 889 19.30 i 91g3 Como, at Castasegna . . . 5.40 697 l 60.21 1 ,m3 - Average . . . . 4.15 1 ..

" Rainy days" are those on which the rainfall is 0 * 02 inch or more. Average rainfall of lakes north of Alps is 44 inches per annum in 20 years. Average rainfall of lakes south of Alps is 60 inches per annum in 20 years.

-1 T r

01 m 3

lgI l-

PART 11.-PURIFICATION OF WATER AT ZURICH.

I. WORKING OF FILTER- BE^. (I) FiZtra,tion at Varying Rates.-In the course of the last few years,a series of investigationshave been made at the Zurich Waterworks for the purpose of testing the effect of filtration at variousrates upto 3,700 gallonsper square yardper twenty- four hours; with a view to ascertain whether the existing rate (1,100 to 1,800 gallonsper square yard per twenty-four hours) could be safely increased ; or whether additional filter-beds should at once be constructed to meet the constantly growing demand. The average results which the Author has compiled from about 300 analyses are as follows :-

Rate of Percolation per Twenty-four Hours. -. l Upto of 1 upto ChemicalConstituents the Filterec 50 Gallons 900 Gallon Water are stated in parts per 1,000,oao. )er Square]per Squar Yard. I Yard.

Organicmatter . . . . . 17.0 17.0 20.2

Ammonia ...... 0.004 0.004 , 0.006 dlbuminoid ammonia . . . l 0.027 0.0'27 0.037 Bacteria per cubic centimetre 20 31 22 15 18

These results show, therefore, that, provided the filter-beds are in efficient working-order, neither the chemical nor the bacteriological purity of the filtered water is impaired by increasing the rate of percolation from 1,025 gallons to 2,800 gallons per day, a fact which is at variance with the view advanced elsewhere, that the mean rate of percolation for sand-filters should be limited to 550 gallons per square yard per twenty-four hours. (2) Filtration under Exceptional Conditions.-In the year 1880, when, after an exceptionally severe winter, the ice on the lake broke up, there was a considerable increase of the normal number of typhoidcases; and this wasnot unreasonably attributedto

Municipal Laboratory, Zurich, Dr. A. Bertschinger, 1589.

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] PRELLER ON PURIFICATION OF WATER AT ZURICH, 281 probable contamination of the Limmat water passing through the oldsubmerged filter and concretemain. It wastherefore feared that a similar phenomenon might recur after the still more severe frost of the winter 1889-90, during which the entire lake was frozen over from the end of January to the end of March; and thatthe impurematter accumulated on the surface of the ice might, after the thaw, pass into and contaminate the new water- supply.A series of about 50 specialanalyses were therefore made between January and May, with the view of ascertaining whether, and how far, the quality of the unfiltered and filtered water would be affected during the frost, and during and after the thaw. From the results of these analyses,l the Author has deduced the statement given in Table I, covering five consecutive periods : viz., immediatelybefore thelake was frozen over;during the frost; upon the thaw setting in; after the thaw; and until the lake-water returned to its normal condition :-

TAEILEI.

15 Nov., ~ :ss$; ~ 20 March, Chemical Constituents espressedin ,lpj;E I 19 March, Ita jy:ril, parts per 1,000,000 ...... 1890 Lake Norm;l. 1 Frozen 1 ;$:, over. , Thaw. -~-~-___~ Before filtration- l Organicmatter . . . . ' 16.918.318.0 17.7 18'3 Ammonia ...... ' 0.007 0.009 0.007 ~ 0.005 0.005 albuminoid ammonia . . 0.028 0'032 0.030 ' 0.029 ' 0.033

Bacteriauer cubic centimetre 122 667 I 038 632 I 171 ,, (maxima) . . 202 2,179 2,152 ' 1,425 ~ 229 Meantemperature of p. +410,4 water . . . +37O.2 +39' +40° , k48O.2 Mean temperature of ,, -15O -16O +40° +45O +57O-2 Air . . . . ] After filtration- Organicmatter . . . . 14.1 14.4 1 14.014.1 l 14-9

Ammonia ...... 0 ~ 0 0'0 0

Slbuminoid ammonia . . 0.018 ~ 0.019 0.019 0'020 ~ 0.019 Bacteria per cubic centimetre 10 13 9 11 I 16 20 27 29 ' ,, (maxima) . . . 22 ~ 21

The above indicates: (1) A sudden andlarge increase of bacteria a fortnight after the lake was' frozenover, then alter-

' ;Municipal Laboratory, Zurich, 1891.

nately rising and falling until it returned to the normal number : (2) That theseviolent fluctuations were quiteindependent of thetemperature either of theair or of thewater: (3) That the great increase of bacteria during the frost is probably due to thenatural process of oxidationbeing impeded bythe ex- clusion of air from the water; whilst the high rate of bacteria duringthe thaw is due to theimpure matter collected on the icebecoming mixed withthe water when the icemelted : (4) That, throughout these bacteriological fluctuations, the chemical composition of thewater shows butslight and inappreciable changes from its normal condition; hence the importance of bacteriological investigations : (5) That, throughout the fluctuations in the qualityof the unfiltered lake-water, the filtered water continued of good quality ; showing that the filter-beds performed their duty effectively. (3) Cornparatice Cost and Working of Covered tsnd @er& Filter-beds. -The action of both the covered and open filter-beds is subjected toconstant tests; and the working of thelast 4 years affords interesting information respecting theircomparative working-cost and efficiency. The loss of head, due to the resistance of the filtering strata, is measured three times a day at every filter-bed ; and the filter-beds are cleaned when thedifference of level between the unfiltered and the clear water attains 24 to 32 inches. The water is then drawn off by the filter-scour conduit and well; a layer of about ji inch is, by means of iron spades, carefully removed from the surface of the sand, the thickness of which, when new or renewed, is 32 inches. This cleansing operation has to be effected after about 50 days’ work in the covered, and about 40 days’ work in the open, filters ; and when, after successive cleans- ings, the sand stratum hasbeen reduced to a minimum of 12 inches, the filter-bed is either replenished with carefully-washed sand, or the entire filtering-material is renewed. After this operation, the filter-bed is refilled from below with filtered water through the return-flowwell ; and,upon the filter-bed beingthus again rendered efficient, the filtered water is at first allowed to flow to waste, until it is proved to be chemically and bacteriologically pure. Thislast period occupies, inthe case of entire renewal, between l1 and 20 days, and, in the case of cleansing, about 12 hours. In 4 years, the sand was renewed in 3 filter-beds after 2 years’ working; and, in theother 2 filter-beds, theentire filtering-material was renewed after 3 years’ working. The average cost of cleansing and partial re-filling with sand is about 51 14s. 5d. exclusive of, and ;E2 3s. 27. inclusive of, the

sand ; that of renewingthe entire layer of sand is S34 ; and that of renewing the whole filtering-material is S100 per filter- bed-equivalent to 18 pence and 22 pence respectively per cubic yard of filtering-material. In comparingopen with covered filter-beds (Table I, Appendix), it is found that the maintenance andcleansing of the open filter-beds is attendedwith much greater difficulty thanthat of the covered ones. Owing to solar action during the summer, a fil111of green algae forms on the surface of the sand, and increases the loss of head through thefiltering-strata; hence the open filter-beds require more frequentcleansing than the covered ones, whichlatter, though ventilated,are kept dark. Again, in winter,the ice that forms on the surface of the open filter-beds rendersthe cleansing of these beds both more costly and troublesome thanthat of the covered ones. The Author draws thefollowing conclusions from the statistics furnished inTable I of theAppendix :-(l) The open filter- beds require 50 per cent. more cleansing than the covered ones : (2) The covered filter-beds filter 14 per cent. more thanthe open ones: (3) The cost of filtration is 10 per cent. less by the covered than by the open beds; whilst the cost of construction of the covered beds exceeds that of the open ones by 27 * 5 per cent. Hencethe lower cost of construction of the open filter- beds does not compensatefor theirhigher cost of maintenance and their lower yield as compared with covered beds. The Zurich authorities have, therefore, decided to roof not only the two open filter-beds, but also the five new ones. It should be added, however, that in point of quality of the filtered water, no appreciable difference has been found between covered and open filter-beds.

11. CHEMICALAXD BACTERIOLOGICALEXAMINATION OF WATER.

Both the unfiltered and filtered lake-water, as well as the spring- water supply, is subjected to fortnightly, andoften weekly, chemical and bacteriological examination in the Municipal Laboratory. The samples of the unfiltered water are taken in the lake, at a depth of 46 feet, by means of exhaustedglass bnlbs, each of

* The cleansing of the old submerged Limmat filter and removal of a layer of about 3 inches of mnd, which can only be effected by divers, costs about $80.

which has its neck drawn out into a loop closed at the end, and is:aopened to admit the water by breaking theloop at the required depth. The samples of the filtered water are taken at thepumping- station before the water is distributed, and also at various points ,of the-distributing-area. Samples of the spring-water are similarly taken before andafter filtration. Besides thisregular examination,special analyses areconstantly made of the lake-water, takenat depths varying between 13 feetand 52 feet; of the water immediately before it enters, and after it leaves the filter- beds; and also whenever the quality of the water is liable to be ,affected by special phenomena, such as severe frosts. The ordinary chemical and bacteriological tests average about 250 per annum, .and, to ensure uniformity,are conducted according to fixed ,methods, which in some respects differ from, andgive more accurate results than, those usually employed, and of which the Author therefore gives a short outline, as follows :- (l) Chemical AnaZysis.-(a) Dry residue, obtained by evaporating the water and drying the residue at 100' C. (b) Glow residue, obtained by heating the dry residue to white heat;after moistening with carbonate of ammonia, it is dried again at 150" C. (c) Hardness. The glow residue istreated with hydrochloric :acid and methyl orange; the degree of hardness of the water is determined by the quantity of carbonate of lime and magnesia found in it. (a) Organic matter, determined by titration. This determination, to which generallyso much importance is attached,is, however, no *criterionas to whether the organic matter isharmless or injurious; although water containing much organic matter shows that mi- crobes, which feed upon it, are more likely to multiply in it than in water containing less organic matter. (e) Ammonia, and (f) Albuminoid ammonia,---Wanklyn's method. (g) Nitrous acid, qualitative test, by iodide of potassium and .starch, with a slight addition of sulphuric acid. (h) Nitric acid, qualitativetest, by diphenylamine reaction in concentrated sulphuric acid. (i) Sulphates (sulphate of lime), qualitativetest, by treating 100 cubic centimetres of the water with a few drops of hydrochloric acid and solution of chloride of barium. (k) Chlorides (chloride of potassium), qualitative test, by nitric acid and nitrate of silver. Good wat,ershould contain less than 35 parts in 1,000,000 of organic matter, no nitrous acid, less than 5 parts of nitric acid,

less than 0.02 part of ammonia,and less than 0.05 part of albuminoid ammonia. AS maybe seen from the standardanalysis, Table 11, the lake-water contains only slight traces of chlorides, sulphates and nitric acid,and no nitrousacid; thence the regularanalysis is quantitative fororganic matter, ammonia arid albuminoidammonia;the other substancesbeing tested onlyqualitatively,, nnless the tests point to their being present in any appreciable quantity. (2) Bacteriologiccl Control.-This is limited to determining the number of bacteria per cubic centimetre of water ; the differentia- tion according to species being, for the purposes of water-supply, both unnecessary and unreliable, unless an abnormal number of’ bacteria, in conjunction with an unfavourable chemical analysis, points to probablecontamination. The cultivation of bacteria is conductedaccording to Professor Koch‘s method, modified by ProfessorCramer, of Zurich,‘ the salientfeatures being as follows :- The bottles,or Erlenmeyer receivers, used for the cultivation, are of the thinnest white glass, with cotton stoppers, 12 centimetres highand 9 centimetres in diameter atthe bottom.These receivers, andthe small bottles and sink-bulbscontaining thewater to be tested, as well as the gelatine test-cylinders and the pipettes for measuring the water, are sterilised by being heated for two hours to 150” C. The gelatine for cultivating the bacteria is preparedaccording to Buchner’smethod (50 to 100 grammes, according tothe season, of whitegelatine, with 5 grammes of: Liebig’s meat-extract, 5 grammes of peptone, and 20 grammes of sugar), latterly also, according to Liiffler’s direct meat-extract and, peptone method ; it is then dissolved in distilled water, made dis- tinctly alkaline, boiled with water so as to bring the liquid upto 1 kilogramme, filtered hot,and poured into the test-tubes, in which it is intermittently sterilised inDr. Eoch‘s sterilising-apparatus, and kept for 10 minutes at boiling-point at least 6 times in the space of 10 days. Of the gelatine so prepared, 3 to 4 cubic centimetres are then introduced intoeach glass receiver, to which is added 0 * 5.

Professor Cramer’s method (of using “ receiver-bottles ”) has over Professor Koch’s so-called “plate ” method the advantage of greatersimplicity and, rapidity of cultivating bacteria, and offers, moreover, greater protection against. theintrusion of extraneous germs. For purposes of bacteriological tests of water, viz., counting, and not differentiating, the bacteria, Professor Cramer’s. method ie also preferred to Petri’s method of using glass boxes, and toEsmarch‘a ‘‘ tcst-tubes.”

centimetre, carefully measured by a pipette, of the water to be tested ; whereupon each receiver is closed with its cotton stopper, and is gently agitated until the gelatine and water a,re thoroughly mixed. After cooling, the receivers are suspended for observation bottom upwards, the bacteria being then counted by means of a microscope. Thebacteria are countedevery day from their firstappearance, and the counting is stoppedwhen the gelatine begins to liquefy. Thelast counting, generally between the fourth and eighth days-according to the season-is decisive, and from it thenumber of bacteriaper cubic centimetre is determined. For the purpose of comparison, two samples of each water arealways taken and tested simultaneously. The water is put nnder cultivation with as little delay as possible, being kept in a refrigerator at from 43" to 50' F., in order to prevent the germs from multiplying. Before adding the water to the gelatine in the reeeivers by means of pipettes, it is thoroughly shaken, so as to distribute the germs equally; whilst the gelatine (which melts at 80" F., and solidifies at 68" F.) is cooled down to 86" F.; as certain species of bacteria are known not to live in temperatures exceeding 10.4' F. Thetemperature of the closedroom, in which the cultivation of the bacteria t,alres place, is kept day and night at W0*8and 71O.6 F. (3) Comparative Chemical and Bacteriological Analyaes of Un-$1- tered and Filtered Water (Graphical Table 11, Appendix).-From the Departmental Returns, the Author has deduced the compara- tivedata and average results embodied in Tables I1 to VI, as follows :-

TABLE~~,-CHEMICAL ANALYSES OF UNFILTERPDAND FILTEREDLAKE-WATEK, 13EING TEE MEANOF A SERIES OF ANALYSESFROM DIFFERENTPARTS OF THE LAKE. Part8 per 1,000,000. Inorganic Parta per 1,000,000, __ Constituents. -_ Unfiltered. Filtereil. Filtered. Unfiltered. Dryresidue . . 154.0152.5 Alkali(Na,O) . . . 2.52.5 Glowresidue . 140.3143.2 Magnesia (MgQ) . . 9.7 9.8 Organicmatter . 18's 15.2 Lime (CaO) . . . 61.6 62.3 Ammonia . . traceslight trace ~ Ironand alumina . . 1.2 2.0 o.039 o.023 ' Silica , . . . . 3'4 4.0 A1bumin:idjammonm Chlorine (Cl) . . . 1'4 1.3 Nitrousacid . . 0 0 Nitricacid(N,O,) . . 1.5 1.5 Nitricacid . . tracetrace Sulphnric acid (SO,) . 9.1 9.4 Chlorides . . tracetrace Carbonic acid,deter- Sulnhates . . reaction reactionmined from hardness Haiddness, En- ) 8.92 -- - glish degrees Total mineral matter . 140.7 143.8

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] PRELLER ON PURIFICdTION OF WATER AT ZURICH. 287 Thisanalysis shows that the hardness of thewater increases about 4 degree by filtration.

TABLEIII.-PRINCIPAL CHEXICAL CONSTITUENTS IN PARTSPER 1,000,000 AND NUXBEROF BACTERIAIX UNFILTERED LABE-WATER AT VARYING DEPTHSOF INTABE.

~~~

- ~ At 13 Veet. At 39 Feet. ~ At 46 Feet. At 52 Feet.

~ ~ I--_-

Organic matter ..... 21.1 ~, 21.6 ' 21.621.7 Ammonia ...... 0.006 0'011 0.0110.011 Albuminoidammonia ... 0.046 0.044 0.044 0.048 Bacteria per cubic centimetre 149 160 175 352

This Table shows that the quality of the water is not impaired by greater depth of intake up to alimit of 46 feet; whilst at 52 feet, the increased quantity of bacteriapoints to the water being affected by micro-organisms living at the bottom (56 feet).

TABLEIV.-PHINCIPAL CHEMICALCONSTITUENTS IN PARTS PER 1,000,000, AND NUMBEROF BACTERIAIN UNFILTEREDAND FILTEREDLAKE-WATER AT THE FILTER-QTATIOX,PUXPING-STATIOX, AND POINTSOF DISTRIBUTION,m 1889.

__ Before After At Dis- 1 Less after ' Filtration. I Filtration. 1 tribution. I Filtration. l Per cent. Organic matter . . .. . 1 18.2 l 14.5 14.5 ' 20

Ammonia ...... 0 ,- 01 Oi O Albuminoidammonia .... 0.034 0,020 I 0.021 1 40 I Bacteria ...... ! 175 j 17 27 ~ 90

TABLEV.-AVERAGE NUXBEROF BACTERIAFOR FOURYEARS.

After At Dis- Less after Per cubic Centimetre * '{' Fi;%n. Filtration. ! tribution. ~ Filtration. - -- , Per cent.

1886 ..... 17s 1 1 25 27 ~ 86 1887 .....39 239 18 93

1888 ....., 188 19 29 ~ 90

1889 .....~ 175 . l7 27 1 90 I-- I-- ___---__- Average ...1 195 1 20 31 1 90

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 288 PRELLER ON PURIFICATION OF WATER AT ZURICH. CSelectea The effect of filtration is seen from a glance at the two graphical Tables A and B, Appendix 11, whichgive the chemical and bacteriological results for theyear 1890, andthe bacteria for the period of 4 years as above. These Tables, in conjunction with the above results, show that the filter-beds retain 90 per cent. of the bacteria ; and also reduce the organic mat,ter and albuminoid ammonia by 20 per cent. and 40 pes cent. respectively; although, during distribution, there is again an insignificant increase, when the water occasionally remains at rest instead of circulating in the distributing-pipes.

TABLEVI.-LAKE-WATER AND SPRING-WATER COMPARED, 1889.

~~ I Before Filtration. I Filtration.After Chemical Constituentsin Parts per 1,000,000. Spring

.. ~~ ~~ ~ --,- _._.__-,--

Organic matter . . . . . 13.2 ' 18.2 11.3 ~ 14.5 i Ammonia ...... , 0

Albuminoid ammonia . . 0.0130.034 i 0.013 ' 0.020 '1 l i

Bacteria per cubic centimetre 323 ' 175 ~ 41 1 17

Hardness, French degrees . 33.0 33.012.5 ~ 12.75

ThisTable shows that,whilst the chemical quality of both waters is practically the same, the spring-water contains nearly double the number of bacteria contained by the lake-water, and that, therefore, the filtered lake-water is superior as potable water to that derived from the springs.

111. SELF-PURIFICATIONOB THE RIVERLINMAT. The new drainage system of the town and suburbs of Zurich dates from theyear 1883, and wasconstructed toutilize the collected sewage in irrigating a tract of about 300 acres of land situatedabout 2 miles below the town,specially acquired for that purpose bythe Corporation at a cost of over $40,000. Owing, however, to local opposition, the irrigation was deferred; and, in the meantime, the sewage-main discharges into a collect- ing-well close to the left bank of the Limmat, at a point about 1,090 yards below the pumping-station of the waterworks; and thence discharges into the river by three 25-inch pipes, which are

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] PRELLER ON PURIFICATION OF WATER AT ZURICH. 289 laid obliquely to the course of the river, the outfall taking place in midstream, 66 feet from either bank. The average delivery of sewage is 4,400,000 gallons, the maximum being 11,000,000 gallons, per day; whilst the average flow of the Limmat, including the River Sihl, is 1,980,000,000 gallons per day; the sewage, therefore, represents about 0.2 per cent. of the average daily volume of the river. The pollution of the Linlmat by this outfall naturallyraised great objections on thepart of theriparian population inthe immediate vicinity ; not only because it was likely to damage iron and timber structures and water-wheels by the actionof ammonia and deposit of mud; but on account of the injury to be appre- hended to the public health and pisciculture. The inquiry which was instituted into this matter demonstrated the fact of neither chlorides, sulphides, nitrogen, nor ammonia being present in any excessive quantity; and showed that the impure matter contained in the sewage is largely decomposed before it reaches the point of outfall.Subsequently, a series of bacteriological investi- gationswere undertaken by the Hygienic Institute of Zurich,l withthe view of ascertainingwhether the changes whichthe Limmatwater undergoes owing to the sewage mixingwith it, renders it unfit for pisciculture and the domestic purposes for which it is used lower down the river. The inquiry was conducted by weekly investigations extending from January to April, 1889 ; a season when the volume of the river is lower and more constant than at any other time of the year.Samples of thewater were taken at different stations (rangingaccording to the velocity of theriver on each day overa distance of about 10 miles), not simultaneously, but in succession; and were, within an hour or two after being taken, putunder cultivation according tothe bacteriologicalmethod already referred to. In the Appendix, Table 111, the Author has workedout, extended, and arranged in an intelligible form the results of these investigations ; together with the meteorological data, and the volumes and velocities of the river as far as they relateto the 10 days on which samples were taken atall the 9 stations in succession ; deducing therefrom the average number of bacteria and rateof self-purification at each station, as the only satisfactory method of arriving at a reliable conclusion. From the Table referred to it will be seen: (I) That 96 per cent. of the precipitation takes place within 0.3 mile below the sewage outfall : (2) Thatwithin 6 miles of the sewage outfall,the number of

1 Hygienic Institute, Zurich, Dr. C. Schlatter, 1890. [TEE INST. C.E. VOL. CXI.) U

bacteriafalls to the number immediately above thatpoint : (3) That the greater the volume and velocity of the river, the slower is the rateof self-purification : (4) That, so far as concerns the sewage, therate of self-purification is not influenced by meteorological changes. The River Limmat, after itsconfluence with the Sihl, andbelow the Zurich waterworks, has a fairly uniform width of about 98 feet, and a depth of about €4 feet, its discharge being 317,850,000 cubic feet per day; hence its mean velocity is about 4 miles per hour. Its fall from Zurichto the well-known sulphurbaths of Baden, a distance of 18 miles, is 10-5 feet per mile ; and the time the water takesto travel from the outflow of the lake to Baden is about 54 hours. Afterpassing the waterworks, the river only receives a few insignificant streams, which, although after rain they carry sand in suspension, do not materially affect the rate of self-purification of the river. Taking, therefore, the mean decrease of bacteria between two stations at 40 per cent. within G miles, it follows that within an additional distance of G miles, the process of self-purification will becomplete; i.e., thebacteria will be reduced to the normal number of the lake-water, or about 170 per cent. And that this is actually the case, is attested by the fact that at Baden the Limmat water is freely used for domestic purposes, andthat fish livein it in abundance.These considerations therefore lead to the conclusion that, under the conditions described, and provided thereare no intermediate sources of pollution, a riversuch as theLimmat, flowing atthe mean velocity of about 4 miles per hour, will purify itselfwithin distance of about 16 miles from the point of pollution.

APPENDIX.

TABLEI.-ROOPED AND OPEN FILTER-BEDS. Cost of Construction and Worleing and the Action of Filtep-beds.

Three Roofed Filter- Two Open Filter- beds. beds. - Cost of Filtering Cost of Filtering Surface. Surface.

. ~ .

~ Per SquareYard. ____-/-Per-- Sq=rd. (l) Cost of constructioK- 1 L S. d. € S. d. Land,intake,andmains. ....1 8 5 2-1 582 Filter-bedsand wells .....~ 3 1 0 119 5 Conduits, outer wells, and sand depot i 0 17 7 I 0 17 -7 Total ...... 177-i ~ :-52~-

~~ ~ ~ ~~~~ ~ ~ ~- ~~~~~ ~ ~~~~ ~ ~~ - i Per 1,ooo.ooo Gallons.! Per 1,ooo,ooo Gallons. (2) Cost of working- Pence. 1 Pence. Superintendence...... 9.22 , 11.22 Clcansing a.nd replenishingsand . . 10.15 16.65 Breaking ice ...... Renewal of filtering-strata .... Maintenance,intake, and mains . . Repairs and sundries .....

(;.+er cent. interest and sinking-fund. Total ...... l-

. ~. ~~ ~ - ~ ~ - (3) Action of filter-beds- , Total filtering-surface ..... 89. yds. 2,410 sq. yds. 1,663 Filtering surfaceper filter-bed . . 803 j ,, 832 Totaldelivery per annum average ;alls:'984,793,000 galls. 565,878,000 ,, ,, per filter-bedper annum) averageJ ,, 328,264,000 1 ,, 188,626,000 ,, ,, per unit of filtering-sur-) ,, 488,400 ,, 406,260 face per average)annum ... , One filter-bed requiresrenewal of\ (after 3 years} (after 2 years} material ...... j 4 months. 5 months. Empt,yiug, cleansing, renewal, andre- 30 days. fillingrequircs ...... ! 35 daps. Wiltration to waste ...... 11 ,, 20 3, Total of renewal period .... 41 ,, I 55 ,, - U" Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 292 PRELLER ON PURIFICATION OF WATER AT ZURICH. [Selected

TABLEI1 (GRAPHICAL). d.

AFTER FlLTRATlON

LAKIPWATERBEF'OKE AND AFTER FILTRATION: CHEMICAL AND BACTERIOLOGICAL TA~LE,1890; INTAKE IN LAKE46 FEET BELOW MEAN WATER-LEVEL.

TABLEI1 (GRAPHICAL). B.

AFTER 'FILTRATION

LAKE-XATERBEFORE AND AFTER FILTRATION; COMPARATIVE BACTERIOLOGICAL TABLE,1887-90; INTAKE IY LAKE46 FEET BELOW MEANWATER-LEVEL.

l Mean j Temperatun 1 1890. in Uegrecs Centigrade.

-- -_ Cubia '0 Yeet. l Yeet. 6 February ...... ' - 2.8 883 1.47 13 ...... I -10.7 954 1.61 18 ,, 1,236 1.90 20 ,, 1,130 1.805 25 ,, , 646 1,201 1 .93 27 646 1,165 l .so5 4 M&h . 592 918 1.57 6 March . . 26 April . . . 30 ,, . . 904 . _-- Average 664 -i-

Increase or decreaseper cent. . ~ l -I- Distance between stations

Distances from Station 1 . . .

SELF- Distance from Distance from Lake.- Sewage- Outfall. Miles. Miles. 13.5 6.21 15.5 7.46 17.5 8.70 19.5 9.94 21.5 11.18 23.5 12.42

At station 14, 12 miles from sewage-outfall

THE RIVERLIMMAT.

P Number ofBacteria per Cubic Centimetre.

Decrease between Bridge. Stations 3 and 4.

:tation 1 ltation 8. Station 9.

l l Per cent. 5,050 2,220 2,000 98.0 1,430 900 800 96.0 400 1,050 1,700 96.0 2,950 2,470 1,080 84'0 800 1,980 510 89'0 250 250 250 99.4 7,500 3,600 3,900 95.0 5,700 1,940 3,130 97'0 2,540 5,100 5,100 94.0 5,560 3,950 3,550 91.0 .-- -__--- -- 225 1 1,731 1 296,670: 12,870; 10,8921 5,902 4,218 2,346 2) 100 .. - 28 -45 - 10 .. ------l 1 Meandecrease between two stations = 40 per ccnt. 44,292 feet 8.078 miles

--p-__-

v J + 2,100 = 0.7; 100- 0.7 = 99-3per cent. ; decrease 99.3 per cent. in 6 miles.

PURIFICATIONIN 12 MILES.

bacteria 2,100 per cubic centimetre. ,, 1,260 ,> ,, - 40 per cent. ,, 756 ,, - 40 ,, .. 452 ,. - 40 ., ,, 272 ,, - 40 ,, ,, 164 ,, , - 40 ,, (Bacteria at outflow from lake = 225.) the water is as pure as at its issue from the lake

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. *ART PLAN OF ZURICH 8HowINo MAINS t RESERVOIRS. Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved.