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.