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 col- lected 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. Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] PRELLER ON THE ZURICHWATERWORKS. 259 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,col- lected from about ten groups of small springs in the hills flanking the town, is about 660,000 gallons per day, the minimum yield, however, being only 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 Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 260 PRELLER ON THE ZURICH WATERWORKS. [Selected 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 filtered lake-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 Downloaded by [ University of Liverpool] on [15/09/16].

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