1 Schiedam This town is on the River Maas, which is turbid and subject to pollution from the Rotterdam sewers. The water first passes through sedimentation basins. It then passes through preliminary filters of moderately coarse river sand and thence through final filters of very fine dune sand, having 50 per cent more area than the preliminary filters. Results have apparently been satisfactory during the past thirty years or more. Bremen Several of the larger German rivers contain at intervals turbidity too high for slow sand filters. At Bremen coagulants have occasionally been used for aiding sedimentation of River Weser water before passage through open slow sand filters. Water filtered shortly after a filter has been scraped is known to suffer in quality. To remedy this, a system of siphons was arranged at Bremen many years ago whereby the first-filtered water could be applied to another filter having well ripened sand. This double filtration arrangement which is described in detail in the Transactions of the American Society of Civil Engineers (liii, 1904, p. 210), proved satisfactory. Altona This slow sand plant attained wide publicity following the outbreak of cholera in Hamburg, 1892-3. Although supplied from the river Elbe below the Hamburg sewers, it protected the Altona consumers from infection. These filters, as at Bremen, delivered water of poorer quality just after cleaning. For years the first-filtered water was pumped back to the inlet, mixed with unfiltered water and passed through ripened filters. This arrangement was later superseded by use, prior to the slow sand filters, of preliminary mechanical filters of the Bollmann type, a patented arrangement used at Luebeck, and also at the Wulheide works of Berlin for the removal of iron from ground water, and described in Engineering News-Record (September 13, 1923). Zurich The slow sand plant built in 1886 treated lake water practically free of turbidity, but subject to occasional heavy growths of organ isms. Rates reaching 7 million gallons per acre daily were frequent, but prefilters were necessary to remove microscopic organisms. As the city grew a complete new plant was installed in 1912-14, having an intake in very deep water and arranged to draw from approximately mid-depth, thus avoiding both surface and bottom growths. The water first passes, at a rate of about 50 million gallons per acre daily, without any coagulant, through prefilters resembling American mechanical filters, and cleaned by applying air and water together for about ten minutes and then water alone. Depending upon the extent of plankton growth, they are washed from once or twice daily to once or twice weekly. Their effluent is applied to covered slow sand filters, operated at about 3 million gallons per acre daily. The prefilter effluent is so clear that the final beds. sometimes run four hundred days or more between scrapings. The quality of the final effluent is considered very high, although the final filters receive so little organic matter for providing gelatinous coatings around the sand grains that the bacterial content of their effluent is measurably above normal for some time after scraping the filters. British practice Great Britain has several dozen filter plants of the rapid sand or mechanical type. The more recent are quite similar to American plants. Generally these serve as prefilters, to lessen the load on slow sand filters. In some cases coagulants are used, chiefly for removing deep vegetable stain from moorland waters. The function of double filtration in these instances is, therefore, partly to improve the quality of the final effluent and partly to lessen the investment for supplementing existing overloaded filters. London experiences Rapid sand filters, used without coagulants, have been installed as prefilters by the Metropolitan Water Board, apparently to increase the output of slow sand filters and particularly to guard against loss of capacity in the latter at times of unusual plankton growths. It seems to be the view that the output may thus be increased sufficiently to justify the investment in prefilters. Mechanical filters have been studied for some years under the direction of Sir Alexander Houston. In 1921 the Board authorized the construction at Barn Elms of mechanical filters of 7.5 million United States gallons daily capacity. They treat Thames River water after storage in a reservoir holding about 3 weeks' supply and before it goes to the slow sand filters. These filter units resemble American mechanical filters, and operate at about 160 million U. S. gallons per acre daily. The effluent looks quite clear, although it is said that bacterial removal is little over 50 per cent. The uncovered final sand filters sometimes have very noticeable algae growths in the water over them. Operating at about 2.5 million U. S. gallons per acre daily, the sand filters frequently run several hundred days without cleaning. Although algae growths seem to be the clogging element, a factor to be considered is whether there is enough organic matter in the prefilter effluent to ripen fully the final filters. Such prefilters, used without coagulant on water stored for three weeks or so, are sufficiently well thought of so that the Board authorized in 1923 the installation of two additional prefilter plants above Hampton, of 21.6 and 38.4 million U. S. gallons capacity respectively, and generally similar to those at Barn Elms. American experiences Preliminary filters or scrubbers to aid in the preparatory treatment of water have been adopted at a number of places since 1900, to remove microscopic growths or to lessen turbidity where the use of coagulants was thought undesirable or inexpedient. Some prefilters were built to lessen total cost of the final filtered water, through savings effected in sedimentation basins or final filters. Experiences have ordinarily shown no very pronounced advantages for prefilters as compared with other preparatory treatment. But prefilters have attained more prominence recently at several places where they are needed to reduce polluting matter so as not to over-tax the final filters. Philadelphia experiences In 1902 extensive tests were made of various types of scrubbers to remove impurities prior to filtration, principally to remove turbidity. These tests resulted in installing at the Belmont plant so-called scrubbers of the Maignen type consisting of layers of sponge and coke. Coke was found more serviceable, and sponge has now been eliminated. At the Torresdale plant, on the Delaware, mechanical prefilters were built and are still operated. Coagulants were not used until recently. Filtering material averages about 0.80 mm. effective size. The rate of filtration is nominally 80 million gallons per acre daily. At the Queen Lane plant, receiving Schuylkill River water passed through a sizeable reservoir, mechanical prefilters were built to remove turbidity and other impurities prior to slow sand filtration at a nominal rate of 5 million gallons per acre daily. Most of these prefilters were recently converted into mechanical filters for single filtration. In recent years both rivers at the present intakes have become too much polluted for satisfactory treatment by single filtration. The first filtration provides an effluent nearly equal in quality to unfiltered upland streams. Second filtration completes the task and renders the water safe, while chlorination gives an added factor of safety. This is the present method of protection at Torresdale and one which can be continued as long as pollution is not too great and industrial wastes do not interfere. The Schuylkill River is double filtered only in part and the opportunities for it are not. as satisfactory as at Torresdale. (See Jour. Amer. Water Works Assn., 1923, vol. 10, p. 351.) Other experiences At Wilmington, Del., scrubbers preliminary to treating Brandywine Creek water in an unbaffled reservoir and in slow sand filters were abandoned as incapable of sufficiently removing turbidity during freshets. At New Castle, Pa., an arrangement of the mechanical filters in two groups so that the effluent from one group might be coagulated again and passed through the other group at times of unusually heavy pollution was abandoned some years ago because results did not warrant the expense and trouble involved. Montreal experiences This plant was installed with both prefilters and slow sand filters to be used without coaguiant or preliminary sedimentation. The purpose was to protect the slow sand filters from plankton growths which might occur in the Bay of St. Louis a new miles above the intake in the St. Lawrence. Such growths have never occurred and turbidities have not been sufficient to justify double filtration. The normal rate per acre daily in million Imperial gallons is 115 for the prefilters and 6.5 for the final filters. (See Jour. Amer. Water Works Assn., 1920, vol. 7, p. 898.) Albany and Poughkeepsie experiences The well known slow sand filter plants at these two places were installed to treat Hudson River water which at these intakes contained substantial pollution as well as turbidity and vegetable stain. As the loading increased, prefilters of the mechanical type were installed to remove suspended matter. Recently these prefilters have been so equipped as to make them mechanical filters for treatment preparatory to slow sand filtration. Complete double filtration has no doubt proved well worth while, notwithstanding that the quality of the Hudson water has become such that steps are being taken at Albany to secure an unpolluted upland supply. (See Jour. Amer. Water. Works Assn., 1920, vol. 10, p. 97, and Jour. N. E. Water Works Assn., 1923, vol. 27, p. 238.) Summary Enough has been said to outline the trend of progress in keeping the load on ordinary filters within reasonable limits. Double filtration has undoubtedly proved helpful generally, though not always. The individuality of local conditions, particularly of existing filter plants, makes double filtration a special problem such that no general rules are applicable. COVERS FOR RESERVOIRS FOR FILTERED WATERS AND GROUND WATERS These are four principal reasons why it is advantageous to provide covers for reservoirs used for storing ground waters and filtered waters: (1) to keep out the light and thus prevent the growth of algae; (2) to keep out atmospheric dust, air-blown spores and seeds; (3) to protect the supply against human tampering; and (4) to prevent growths of pipe-moss in the distribution system. These may be combined in the statement that the primary object of reservoir covers is to preserve the cleanliness of the water. But there |