CAPACITY OF FILTERS.
Estimating the total additional area of sand filters for which figures are not available at 100 acres, and the maximum capacity of sand filters at three million gallons per acre daily, and of mechanical filters at three million gallons per thousand square feet of filtering area, the total filtering capacity of all the filters in the world used for public water supplies in 1899 is nearly 1600 million gallons daily, of which 15 per cent is represented by mechanical filters and 85 per cent by sand filters. In the United States, including Wilmington, the total filtering capacity is nearly 300 million gallons daily, of which 18 per cent is represented by sand filters, 79 per cent by mechanical filters, and 3 per cent by a special type of filters.
APPENDIX V.
LONDON’S WATER-SUPPLY.
London alone among great capitals is supplied with water by private companies. They are, however, under government supervision, and the rates charged for water are regulated by law. There are eight companies, each of which supplies its own separate district, so that there is no competition whatever. One of the companies supplying 460,000 people uses only ground-water drawn from deep wells in the chalk, but the other seven companies depend mainly upon the rivers Thames and Lea for their water. All water so drawn is filtered, and must be satisfactory to the water examiner, who is required to inspect the water supplied by each company at frequent intervals, and the results of the examinations are published each month.
In 1893 the average daily supply was 235,000,000 gallons, of which about 40,000,000 were drawn from the chalk, 125,000,000 from the Thames, and 70,000,000 from the Lea. Formerly some of the water companies drew water from the Thames within the city where it was grossly polluted, and the plagues and cholera which formerly ravaged London were in part due to this fact. These intakes were abandoned many years ago, and all the companies now draw their water from points outside of the city and its immediate suburbs.
The area of the watershed of the Thames above the intakes of the water companies is 3548 square miles, and the population living upon it in 1891 was 1,056,415. The Thames Conservancy Board has control of the main river for its whole length, and of all tributaries within ten miles in a straight line of the main river, but has no jurisdiction over the more remote feeders. The area drained is essentially agricultural, with but little manufacturing, and there are but few large towns. In the area coming under the conservators there are but six towns with populations above 10,000 and an aggregate population of 170,000, and there are but two or three other large towns on the remaining area more than ten miles from the river. These principal towns are as follows:
| Town. | Population 1891. | Distance above Water Intakes. |
|---|---|---|
| Reading | 60,054 | 49 miles |
| Oxford | 45,791 | 87 miles |
| New Swindon | 27,295 | 116 miles |
| High Wycomb | 13,435 | 33 miles |
| Windsor | 12,327 | 18 miles |
| Maidenhead | 10,607 | 25 miles |
| Guildford | 14,319 | 20 miles |
Guildford is outside of the conservators’ area. All of the above towns treat their sewage by irrigation.
Among the places that are regarded as the most dangerous are Chertsey and Staines, with populations of 9215 and 5060, only 8 and 11 miles above the intakes respectively. These towns are only partially sewered and still depend mainly on cesspools. An attempt is made to treat the little sewage which they produce upon land, but the work has not as yet been systematically carried out. There are also several small towns of 3000 inhabitants or less upon the upper river which do not treat their sewage so far as they have any, but, owing to their great distance, the danger from them is much less than from Chertsey and Staines. Twenty-one of the principal towns upon the watershed have sewage farms, and there are no chemical precipitation plants now in use.
Boats upon the river are not allowed to drain into it, but are compelled to provide receptacles for their sewage, and facilities are provided for removing and disposing of it; and as an additional precaution no boat is allowed to anchor within five miles of the intakes.
The conservators of the river Lea have control of its entire drainage area, which is about 460 square miles, measured from the East London water intakes, and has a population of 189,287. On this watershed there is but a single town with more than 10,000 inhabitants, this being Lutton near the headwaters of the river, with a population of 30,005. The sewage from Lutton and from seventeen smaller places is treated upon land. No crude sewage is known to be ordinarily discharged into the river. At Hereford, eleven miles above the East London intakes, there is a chemical precipitation plant. The conservators do not regard this treatment as satisfactory, and have recently conducted an expensive lawsuit against the local authorities to compel them to further treat their effluent. The suit was lost, the court holding that no actual injury to health had been shown. It is especially interesting to note that of the thirty-nine places on the Thames and the Lea giving their sewage systematic treatment there is but a single place using chemical precipitation, and there it is not considered satisfactory. Formerly quite a number of these towns used other processes than land treatment, but in every case but Hereford land treatment has been substituted.
In regard to the efficiency of the sewage farms, it is believed that in ordinary weather the whole of the sewage percolates through the land, and the inspectors of the Conservancy Boards strongly object to its being allowed to pass over the surface into the streams. The land, however, is for the most part impervious, as compared to Massachusetts and German sewage farms, and in times of heavy storms the land often has all the water it can take without receiving even the ordinary flow of sewage, and much less the increased storm-flow. At such times the sewage either does go over the surface, or perhaps more frequently is discharged directly into the rivers without even a pretence of treatment. The conservators apparently regard this as an unavoidable evil and do not vigorously oppose it. It is the theory that, owing to the increased dilution with the storm-flows, the matter is comparatively harmless, although it would seem that the reduced time required for it to reach the water-works intakes might largely offset the effect of increased dilution.
The water companies have large storage and sedimentation basins with an aggregate capacity equal to nine days’ supply, but the proportion varies widely with the different companies. It is desired that the water held in reserve shall be alone used while the river is in flood, as, owing to its increased pollution, it is regarded as far more dangerous than the water at other times; but as no record is kept of the times when raw sewage is discharged, and no exact information is available in regard to the times when the companies do not take in raw water, it can safely be assumed that a considerable amount of raw sewage does become mixed with the water which is drawn by the companies.
The water drawn from the river is filtered through 113 filters having an area of 116 acres. None of the filters are covered, and with an average January temperature of 39° but little trouble with ice is experienced. A few new filters are provided with appliances for regulating the rate on each filter separately and securing regular and determined rates of filtration, but nearly all of the filters are of the simple type described on page 48, and the rates of filtration are subject to more or less violent fluctuation, the extent of which cannot be determined.
The area of filters is being continually increased to meet increasing consumption; the approximate areas of filters in use having been as follows:
| 1839 | First filters built |
| 1855 | 37 acres |
| 1866 | 47 acres |
| 1876 | 77 acres |
| 1886 | 104 acres |
| 1894 | 116 acres |
There has been a tendency to reduce somewhat the rate of filtration. In 1868, with 51 acres of filters, the average daily quantity of water filtered was 111,000,000 gallons, or 2,180,000 gallons per acre. In 1884, with 97 acres of filter surface, the daily quantity filtered was 157,000,000 gallons, or 1,620,000 gallons per acre; and in 1893, with 116 acres of filter surface and 195,000,000 gallons daily, the yield per acre was 1,680,000 gallons.
Owing to the area of filter surface out of use while being cleaned, the variations in consumption of water, and the imperfections of the regulating apparatus, the actual rates of filtration are often very much higher and at times may easily be double the figures given.
Evidence regarding the healthfulness of the filtered river-water was collected and examined in a most exhaustive manner in 1893 by a Royal Commission appointed to consider the water-supply of the metropolis in all its aspects with reference to future needs. This commission was unable to obtain any evidence whatever that the water as then supplied was unhealthy or likely to become so, and they report that the rivers can safely be depended upon for many years to come.
The numbers of deaths from all causes and from typhoid fever annually per million of inhabitants for the years 1885-1891 in the populations receiving their waters from different sources in London were as follows:
| Water used. | Deaths from All Causes. | Deaths from Typhoid Fever. |
|---|---|---|
| Filtered Thames water only | 19,501 | 125 |
| Filtered Lea water only | 21,334 | 167 |
| Kent wells only | 18,001 | 123 |
| Thames and Lea jointly | 18,945 | 138 |
| Thames and Kent jointly | 18,577 | 133 |
The population supplied exclusively from the Lea by the East London Company is of a poorer class than that of the rest of London, and this may account for the slightly higher death-rate in this section. Aside from this the rate is remarkably uniform and shows no great difference between the section drinking ground-water only and those drinking filtered river-waters. The death-rate from typhoid fever is also very uniform and, although higher than that of some Continental cities with excellent water-supplies (Berlin, Vienna, Munich, Dresden), is very low—lower than in any American city of which I have records.
In this connection, it was shown by the Registrar-General that there is only a very small amount of typhoid fever on the watersheds of the Thames and Lea, so that the danger of infection of the water as distinct from pollution is less than would otherwise be the case. Thus for the seven years above mentioned the numbers of deaths from typhoid fever per million of population were only 105 and 120 on the watersheds of the Thames and the Lea respectively, as against 176 for the whole of England and Wales.
| LONDON FILTERS, 1896. | |||||||
|---|---|---|---|---|---|---|---|
| Twenty-sixth Annual Report of the Local Government Board, pages 206-213. | |||||||
| Company. | Amount of Storage Raw Water, Days. | Average Thickness of Sand, Feet. | Average Rate of Filtration. | Bacterial Efficiency. | |||
| Imperial Gallons per Square Foot per Hour. | Millions U. S. Gallons per Acre Daily. | Maximum. | Minimum. | Average. | |||
| Chelsea | 12.0 | 4.0 | 1.75 | 2.19 | 99.92 | 99.62 | 99.86 |
| West Middlesex | 5.6 | 2.75 | 1.25 | 1.56 | 99.94 | 91.48 | 99.79 |
| Southwark & Vauxhall | 4.1 | 2.5 | 1.5 | 1.88 | 100.00 | 84.33 | 97.77 |
| Grand Junction | 3.3 | 2.25 | 1.63 | 2.05 | 99.98 | 84.03 | 99.31 |
| Lambeth | 6.0 | 2.8 | 2.08 | 2.60 | 99.97 | 96.45 | 99.81 |
| New River | 2.2 | 4.4 | 1.89 | 2.37 | 100.00 | 77.14 | 99.07 |
| East London | 15.0 | 2.0 | 1.33 | 1.67 | 99.93 | 97.03 | 99.56 |
APPENDIX VI.
THE BERLIN WATER-WORKS.
The original works were built by an English company in 1856, and were sold to the city in 1873 for $7,200,000.
The water was taken from the river Spree at the Stralau Gate, which was then above, but is now surrounded by, the growing city. The water was always filtered, and the original filters remained in use until 1893, when they were supplanted by the new works at Lake Müggel. Soon after acquiring the works the city introduced water from wells by Lake Tegel as a supplementary supply, but much trouble was experienced from crenothrix, an organism growing in ground-waters containing iron, and in 1883 this supply was replaced by filtered water from Lake Tegel. With rapidly-increasing pollution of the Spree at Stralau the purity of this source was questioned, and in 1893 it was abandoned (although still held as a reserve in case of urgent necessity), the supply now being taken from the river ten miles higher up, at Müggel.
The watershed of the Spree above Stralau, as I found by map measurement, is about 3800 square miles; the average rainfall is about 25 inches yearly. At extreme low water the river discharges 457 cubic feet per second, or 295 million gallons daily, and when in flood 5700 cubic feet per second may be discharged. The city is allowed by law to take 46 million gallons daily for water-supply, and this quantity can be drawn either at Stralau or at Müggel.
Above Stralau the river is polluted by numerous manufactories and washing establishments, and by the effluent from a considerable part of the city’s extensive sewage farms. The shipping on this part of the river also is heavy, and sewage from the boats is discharged directly into the river. The average number of bacteria in the Spree at this point is something over ten thousand per cubic centimeter, and 99.6 per cent of them were removed by the filters in 1893.
The watershed of the Spree above the new water-works at Müggel I found by map measurement to be 2800 square miles, and the low water-discharge is said to be 269 million gallons daily. The river at this point flows through Lake Müggel, which forms a natural sedimentation-basin, and the raw water is quite clear except in windy weather.
There were 16 towns on the watershed with populations above 2000 each in 1890, and an aggregate population of 132,000, which does not include the population of the smaller places or country districts. None of these places purify their sewage so far as they have any. Fürstenwalde with a population of 12,935, and 22 miles above Müggel, has surface sewers discharging directly into the river. Above Fürstenwalde the river runs through numerous lakes which probably remove the effect of the pollution from the more distant cities. There is considerable shipping on the river for some miles above Fürstenwalde (which forms a section of the Friedrich Wilhelm Canal), but hardly any between Müggel and Fürstenwalde. The raw water at Müggel contains two or three hundred bacteria per cubic centimeter, and is thus a comparatively pure water before filtration. It is slightly peaty and the filtered water has a light straw color.
Lake Tegel, which supplies the other part of the city’s supply, is an enlargement of the river Havel. The watershed above Tegel I find to be about 1350 square miles, and the annual rainfall is about 22 inches. The low water-discharge is said to be 182 million gallons daily, and the city is allowed by law to take 23 million gallons for water-supply.
There were ten towns upon the watershed with populations above 2000 each in 1890, and with an aggregate population of 44,000. Of these Tegel is directly upon the lake with a population of 3000, and Oranienburg, 14 miles above, has a population of 6000 and is rapidly increasing. The shipping on the lake and river is heavy. The lake water ordinarily contains two or three hundred bacteria per cubic centimeter. The lake is shallow and becomes turbid in windy weather.
There are 21 filter-beds at Tegel with a combined area of 12.40 acres to furnish a maximum of 23 million gallons of water daily, and 22 filters at Müggel with a combined area of 12.7 acres to deliver the same quantity. Twenty-two more filters will be built at Müggel within a few years to purify the full quantity which can be taken from the river. All of these filters are covered with brick arches supported by pillars about 16 feet apart from centre to centre in each direction, and the whole is covered by nearly 3 feet of earth, making them quite frost-proof. The original filters at Stralau were open, but much difficulty was experienced with them in winter.
The bottom of the filters at Tegel consists of 8 inches of concrete above 20 inches of packed clay and with 2 inches of cement above, and slopes slightly from each side to the centre. The central drain goes the whole length of the filters and has a uniform cross-section of about 1⁄7300 of the area of the whole bed. There are no lateral drains, but the water is brought to the central drain by a twelve-inch layer of stones as large as a man’s fist; above this there is another foot of gravel of graded sizes supporting two feet of fine sand, which is reduced by scraping to half its thickness before the sand is replaced. The average depth of water above the sand is nearly 5 feet. The filters are not allowed to filter at a rate above 2.57 million gallons per acre daily, and at this rate with 70 per cent of the area in service the whole legal quantity of water can be filtered. The filters work at precisely the same rate day and night, and the filtered water is continuously pumped as filtered to ample storage reservoirs at Charlottenburg. The pumps which lift the water from the lake to the filters work against a head of 14 feet. The apparatus for regulating the rate of filtration was described on page 51.
As yet no full description of the Müggel works has been published, but they resemble closely the Tegel works. Both were designed by or under the direction of the late director of the water-works, Mr. Henry Gill.
The average daily quantity of water supplied for the fiscal year ending March 31, 1893, was 29,000,000 gallons daily, which estimate allows 10 percent for the slip of the pumps. Of this quantity 9,650,000 was furnished by Stralau and 19,350,000 by Tegel. The greatest consumption in a single day was 43,300,000 gallons, or 26.6 gallons per head, while the average quantity for the year was 18.4 gallons per head. All water without exception is sold by meter, the prices ranging from 27.2 cents a thousand gallons for small consumers to 13.6 cents for large consumers and manufacturers. The average receipts for all water pumped, including that used for public purposes and not paid for, were 15.4 cents a thousand gallons, against the cost of production, 9.8 cents, which covers operating expenses, interest on capital, and provision for sinking fund. This leaves a handsome net profit to the city. On account of the comparatively high price of the city water and the ease with which well-water is obtained, the latter is almost exclusively used for running engines, manufacturing purposes, etc., and this in part explains the very low per-capita consumption.
The volume of sewage, however, for the same year, including rain-water, except during heavy showers, was only 29 gallons per head, showing even with the private water-supplies an extraordinarily low consumption.
The friction of the water in the 4.75 miles of 3-foot pipe between Tegel and the reservoir at Charlottenburg presents an interesting point. When well-water with crenothrix was pumped, the friction rose to 34.5 feet, when the velocity was 2.46 feet per second. According to Herr Anklamm, who had charge of the works at the time, the friction was reduced to 19.7 feet when filtered water was used and after the pipe had been flushed, and this has not increased with continued use. He calculated the friction for the velocity according to Darcy 15.0 feet, Lampe 17.8 feet, Weisbach 18.7 feet, and Prony 21.5 feet.
APPENDIX VII.
ALTONA WATER-WORKS.
The Altona water-works are specially interesting as an example of a water drawn from a source polluted to a most unusual extent: the sewage from cities with a population of 770,000, including its own, is discharged into the river Elbe within ten miles above the intake and upon the same side.
The area of the watershed of the Elbe above Altona is about 52,000 square miles, and the average rainfall is estimated to be about 28 inches, varying from 24 or less near its mouth to much higher quantities in the mountains far to the south. On this watershed there are 46 cities, which in 1890 had populations of over 20,000 each, and in addition there is a permanent population upon the river-boats estimated at 20,000, making in all 5,894,000 inhabitants, without including either country districts or the numberless cities with less than 20,000 inhabitants each. The sewage from about 1,700,000 of these people is purified before being discharged; and assuming that as many people living in cities smaller than 20,000 are connected with sewers as live in larger places without being so connected, the sewage of over four million people is discharged untreated into the Elbe and its tributaries.
The more important of these sources of pollution are the following:
| City | Population in 1890. | On what River. | Approximate Distance, Miles. |
|---|---|---|---|
| Shipping | 20,000 | —— | —— |
| Altona | 143,353 | Elbe | 6 |
| Hamburg | 570,534 | Elbe | 7 |
| Wandsbeck | 20,586 | Elbe | 8 |
| Harburg | 35,101 | Elbe | 11 |
| Magdeburg | 202,325 | Elbe | 185 |
| Dresden | 276,085 | Elbe | 354 |
| Berlin and suburbs | 1,787,859 | Havel | 243 |
| Halle | 101,401 | Saale | 272 |
| Leipzig | 355,485 | Elster | 305 |
| Chemnitz | 138,955 | Mulde | 340 |
| Prague | 310,483 | Moldau | 500 |
The sewage of Berlin and of most of its suburbs is treated before being discharged, and in addition the Havel flows through a series of lakes below the city, allowing better opportunities for natural purification than in the case of any of the other cities. Halle treats less than a tenth of its sewage. Magdeburg will treat its sewage in the course of a few years. Leipzig, Chemnitz, and other places are thinking more or less seriously of purification.
The number of bacteria in the raw water at Altona fluctuates with the tide and is extremely variable; numbers of 50,000 and 100,000 are not infrequent, but 10,000 to 40,000 is perhaps about the usual range.
The works were originally built by an English company in 1860, and have since been greatly extended. They were bought by the city some years ago. The water is pumped directly from the river to a settling-basin upon a hill 280 feet above the river. From this it flows by gravity through the filters to the slightly lower pure-water reservoir and to the city without further pumping. The filters are open, with nearly vertical masonry walls, as described in Kirkwood’s report. The cross-section of the main underdrain is 1⁄2800 of the area of the beds.
Considerable trouble has been experienced from frost. With continued cold weather it is extremely difficult to satisfactorily scrape the filters, and very irregular rates of filtration may result at such times. In the last few years, with systematic bacterial investigation, it has been found that greatly decreased efficiency frequently follows continued cold weather, and the mild epidemics of typhoid fever from which the city has long suffered have generally occurred after these times. Thus a light epidemic of typhoid in 1886 came in March, following a light epidemic in Hamburg. In 1887 a severe epidemic in February followed a severe epidemic in Hamburg in December and January. In 1888 a severe epidemic in March followed an epidemic in Hamburg lasting from November to January. Hamburg’s epidemic of 1889, coming in warm weather, September and October, was followed by only a very slight increase in Altona. In 1891 Altona suffered again in February from a severe epidemic, although very little typhoid had been in Hamburg. A less severe outbreak also came in February, 1892, and a still slighter one in February, 1893. In the ten years 1882-1892, of five well-marked epidemics, three broke out in February and two in March, while two smaller outbreaks came in December and January. No important outbreak has ever occurred in summer or in the fall months, when typhoid is usually most prevalent, thus showing clearly the bad effect of frost upon open filters (see Appendix II). With steadily increasing consumption the sedimentation-basin capacity of late years has become insufficient as well as the filtering area, and it is not unlikely that with better conditions a much better result could be obtained in winter even with open filters.[63]
The brilliant achievement of the Altona filters was in the summer of 1892, when they protected the city from the cholera which so ravaged Hamburg, although the raw water at Altona must have contained a vastly greater quantity of infectious matter than that which worked such havoc in Hamburg.
From these records it appears that for about nine months of the year the Altona filters protect the city from the impurities of the Elbe water, but that during cold weather, with continued mean temperatures below the freezing-point, such protection is not completely afforded, and bad effects have occasionally resulted. Notwithstanding the recent construction of open filters in Hamburg it appears to me that there must always be more or less danger from open filters in such a climate. Hamburg’s danger, however, will be much less than Altona’s on account of its better intake above the outlets of the sewers of Hamburg and Altona, which are the most important points of pollution at Altona.
APPENDIX VIII.
HAMBURG WATER-WORKS.
The source and quality of the water previously supplied has been sufficiently indicated in Appendix II. It was originally intended to filter the water, but the construction of filters was postponed from time to time until the fall of 1890, when the project was seriously taken up, and work was commenced in the spring of 1891. Three years were allowed for construction. In 1892, however, the epidemic of cholera came, killing 8605 residents and doing incalculable damage to the business interests of the city. The health authorities found that the principal cause of this epidemic was the polluted water-supply. To prevent a possible recurrence of cholera in 1893, the work of construction of the filters was pressed forward much more rapidly than had been intended. Electric lights were provided to allow the work to proceed nights as well as days, and as a result the plant was put in operation May 27, 1893, a full year before the intended time. Owing to the forced construction the cost was materially increased.
The new works take the raw water from a point one and a half miles farther up-stream, where it is believed the tide can never carry the city’s own sewage, as it did frequently to the old intake. The water is pumped from the river to settling-basins against heads varying with tide and the water-level in the basins from 8 to 22 feet. Each of the four settling-basins has an area of about 10 acres, and, with the water 6.56 feet deep, holds 20,500,000 gallons, or 82,000,000 gallons in all. The works are intended to supply a maximum of 48,000,000 gallons daily, but the present average consumption is only about 35,000,000 gallons (1892), or 59 gallons per head for 600,000 population. This consumption is regarded as excessive, and it is hoped that it will be reduced materially by the more general use of meters. The sedimentation-basins are surrounded by earthen embankments with slopes of 1:3, the inner sides being paved with brick above a clay layer. The water flows by gravity from these basins to the filters, a distance of 11⁄2 miles, through a conduit 81⁄2 feet in diameter. The flow of the water out of the basins and from the lower end of the conduit is regulated by automatic gates connected with floats, shown by Fig. 11, page 60.
The filters are 18 in number, and each has an effective area of 1.89, or 34 acres in all. They are planned to filter at a rate of 1.60 million gallons per acre daily, which with 16 filters in use gives a daily quantity of 48,000,000 gallons as the present limit of the works. The sides of the filters are embankments with 1:2 slopes. Both sides and bottoms have 20 inches of packed clay, above which are 4 inches of puddle, supporting a brick pavement laid in cement. The bricks are laid flat on the bottom, but edge-wise on the sides where they will come in contact with ice.
The main effluent-drain has a cross-section for the whole length of the filter of 4.73 square feet, or 1⁄17000 of the area of the filter; and even at the low rate of filtration proposed, the velocity in the drain will reach 0.97 foot. The drain has brick sides, 1.80 feet high, covered with granite slabs. The lateral drains are all of brick with numerous large openings for admission of water. They are not ventilated, and I am unable to learn that any bad results follow this omission.
The filling of the filters consists of 2 feet of gravel, the top being of course finer than the bottom layers, above which are 40 inches of sand, which are to be reduced to 24 inches by scraping before being refilled. The water over the sand, when the latter is of full depth, is 43 inches deep, and will be increased to 59 inches with the minimum sand-thickness. The apparatus for regulating the rate of filtration was described page 52. The cost of the entire plant, including 34 acres effective filter-surface, 40 acres of sedimentation-basins, over 2 miles of 81⁄2-foot conduit, pumping-machinery, sand-washing apparatus, laboratory, etc., was about 9,500,000 marks, or $2,280,000. This all reckoned on the effective filter area is $67,000 per acre, or $3.80 per head for a population of 600,000.
The death-rate since the introduction of filtered water has been lower than ever before in the history of the city, but as it is thought that other conditions may help to this result, no conclusions are as yet drawn.
| DEATHS IN HAMBURG FROM ALL CAUSES, AND FROM TYPHOID FEVER, BEFORE AND AFTER THE INTRODUCTION OF FILTERS. | |||
|---|---|---|---|
| Year. | Deaths from all Causes per 1000 Living. | Deaths from Typhoid Fever per 100,000 Living. | |
| 1880 | 24.9 | 26 | |
| 1881 | 24.1 | 30 | |
| 1882 | 23.7 | 27 | |
| 1883 | 25.2 | 25 | |
| 1884 | 25.1 | 26 | |
| 1885 | 25.3 | 42 | |
| 1886 | 29.0 | 71 | |
| 1887 | 26.6 | 88 | |
| 1888 | 24.5 | 54 | |
| 1889 | 23.5 | 43 | |
| 1890 | 22.0 | 27 | |
| 1891 | 23.4 | 24 | |
| 1892 | 41.1 | 34 | Cholera year. |
| 1893 | 20.2 | 18 | Filtered water from May 28. |
| 1894 | 17.9 | 7 | |
| 1895 | 19.0 | 11 | |
| 1896 | 17.3 | 6 | |
| 1897 | 17.0 | 7 | |
| 1898 | 17.5 | 5 | |
Average for 5 years, excluding cholera year, beforefiltration, 1887 to 1891 | 24.0 | 47.2 | |
Average for 5 years withfiltration, 1894 to 1898 | 17.7 | 7.2 | |
APPENDIX IX.
NOTES ON SOME OTHER EUROPEAN WATER-SUPPLIES.
Amsterdam.—The water is derived from open canals in the dunes. These canals have an aggregate length of about 15 miles, and drain about 6200 acres. The water, as it enters the canals from the fine dune-sand, contains iron, but this is oxidized and deposited in the canals. The water after collection is filtered. It has been suggested that by using covered drains instead of open canals for collecting the water, the filtration would be unnecessary; but, on the other hand, the cost of building and maintaining covered drains in the very fine sand would be much greater than that of the canals, and it is believed, also, that the water so collected would contain iron, the removal of which might prove as expensive as the present filtration. In 1887 filters were built to take water from the river Vecht, but the city has refused to allow the English company which owns the water-works to sell this water for domestic purposes, and it is only used for public and manufacturing purposes, only a fraction of the available supply being required. Leyden, the Hague, and some other Dutch cities have supplies like the dune supply of Amsterdam, and they are invariably filtered.
Antwerp is also supplied by an English company. The raw water is drawn from a small tidal river, which at times is polluted by the sewage of Brussels. It is treated by metallic iron in Anderson revolver purifiers, and is afterward filtered at a rather low average rate. The hygienic results are closely watched by the city authorities, and are said to be satisfactory.
Rotterdam.—The raw water is drawn from the Maas, as the Dutch call the main stream of the Rhine after it crosses their border. The population upon the river and its tributaries in Switzerland, Germany, Holland, France, and Belgium is very great; but the flow is also great, and the low water flow is exceptionally large in proportion to the average flow, on account of the melting snow in summer in Switzerland, where it has its origin.
The original filters had wooden under-drains, and there was constant trouble with crenothrix until the filters were reconstructed without wood, since which time there has been no farther trouble. The present filters are large and well managed. There is ample preliminary sedimentation.
Schiedam.—The filters at Schiedam are comparatively small, but are of unusual interest on account of the way in which they are operated. The intake is from the Maas just below Rotterdam. The city was unable to raise the money to seek a more distant source of supply, and the engineer, H. P. N. Halbertsma, was unwilling to recommend a supply from so doubtful a source without more thorough treatment than simple sand-filtration was then thought to be. The plan adopted is to filter the supply after preliminary sedimentation through two filters of 0.265 acre each, and the resulting effluent is then passed through three other filters of the same size. River sand is used for the first, and the very fine dune sand for the second filtration. The cost both of construction and operation was satisfactory to the city, and much below that of any other available source; and the hygienic results have been equally satisfactory, notwithstanding the unfavorable position of the intake.
Magdeburg.—The supply is drawn from the Elbe, and is filtered through vaulted filters after preliminary sedimentation. The pollution of the river is considerable, although less than at Altona or even at Hamburg. The city has been troubled at times by enormous discharges of salt solution from salt-works farther up, which at extreme low water have sometimes rendered the whole river brackish and unpleasant to the taste; but arrangements have now been made which, it is hoped, will prevent the recurrence of this trouble.
Breslau is supplied with filtered water from the river Oder, which has a watershed of 8200 square miles above the intake, and is polluted by the sewage from cities with an aggregate population of about 200,000, some of which are in Galicia, where cholera is often prevalent. In recent years the city has been free from cholera, and from more than a very limited number of typhoid-fever cases; but the pollution is so great as to cause some anxiety, notwithstanding the favorable record of the filters, and there is talk of the desirability of securing another supply. Until 1893 there were four filter-beds, with areas of 1.03 acres each, and not covered. In 1893 a fifth bed was added. This is covered by vaulting and is divided into four sections, which are separately operated, so that it is really four beds of 0.25 acre each. The vaulting is concrete arches, supported by steel I beams in one direction.
Budapest.—A great variety of temporary water-supplies have at different times been used by this rapidly growing city. The filters which for some years have supplied a portion of the supply have not been altogether satisfactory; but perhaps this was due to lack of preliminary sedimentation for the extremely turbid Danube water, and also to inadequate filter-area. The city is rapidly building and extending works for a supply of ground-water, and in 1894 the filters were only used as was necessary to supplement this supply, and it was hoped that enough well-water would be obtained to allow the filters to be abandoned in the near future. The Danube above the intake receives the sewage of Vienna and innumerable smaller cities, but the volume of the river is very great compared to other European streams, so that the relative pollution is not so great as in many other places.
Zürich.—The raw water is drawn by the city from the Lake of Zürich near its outlet, and but a few hundred feet from the heart of the city. Although no public sewers discharge into the lake, there is some pollution from boats and bathers and other sources, and, judging by the number of bacteria in the raw water, this pollution is increasing. The raw water is extremely free from sediment, and the filters only become clogged very slowly. The rate of filtration is high, habitually reaching 7,000,000 gallons per acre daily; but, with the clear lake water and long periods between scrapings, the results are excellent even at this rate. The filters are all covered with concrete groined arches.
Filtration was commenced in 1886, and was followed by a sharp decline in the amount of typhoid fever, which, up to that time, had been rather increasing; for the six years before the change there were sixty-nine deaths from this cause annually per 100,000 living, and for the six years after only ten, or one seventh as many; and this reduction is attributed by the local authorities to the filtration.[64]
St. Petersburg.—The supply is drawn from the Neva River by an English company, and is filtered through vaulted filters at a very high rate.
Warsaw.—The supply is drawn from the Weichsel River by the city, and is filtered through vaulted filters after preliminary sedimentation at a rate never exceeding 2,570,000 gallons per acre daily.