WATER SUPPLY AT CULCUTTA.
Calcutta, East Indies, is located in the Province of Bengal, on the right bank of the Hugli river, and about forty-five miles inland from the Bay of Bengal. The present population is estimated at 1,100,000. The Hugli river, from which Calcutta derives its water supply, is the most western branch of the Ganges. The drainage-area of the Hugli is about 25,000 sq. mi., although in addition to the runoff of this area considerable excess flow from the Ganges enters the Hugli during July. August and September. The waters of the Hugli river are normally very muddy, and at times, especially in the rainy season, excessively so. A better idea of the effect on the muddiness of the water produced by the heavy rains which occur in the rainy season may be had from the statement that, while the average rainfall in this vicinity amounts to only about 60 ins. annually, as much as 40 ins. has fallen in a single week, and in 1900 the rainfall on two successive days was 14.5 and 10.8 ins. The river water at the intake at Palta is subject to tidal influences. During the dry season, namely, February to June, the water is quite brackish for a part of practically every day, necessitating a cessation in pumping for substantially one-half of the time. There is a good filtration plant here. The pumps here force the water through mains direct to the consumers in the northern part of the city, the remainder being discharged into three reservoirs, two situated in the town and one in the suburbs, from each of which it is again pumped to the remainder of the consumers. The original settling-basins, construction of which was begun in 1867, arc six in number, and are built of masonry. Four are provided with semi-concentric siltcatching pits, and two are plain. The original basins are each 500 x 250 ft., and range in depth from 7 to 0 ft. The total capacity of these six basins is 21.000,000 gals. The “new” settlingbasins, finished about fifteen years ago, are merely earth excavations, puddling being practically dispensed with. Notwithstanding the fact that these basins arc excavated in a sandy clay, the leakage amounts to practically nothing. Two of these basins are each 2,500 x 350 ft., x 9 ft. deep. The remaining two basins are each 2,500 x 250 ft., x 9 ft. deep. The total capacitv of the four basins is, therefore, 84,000,000 gals., making a total settling capacity of 105,000,000 gals. These basins are worked on the fill-and-draw plan, and the period of sedimentation is therefore variable. As the water enters the settling-basins it is treated for approximately three months in the year, namely, during the rainy season of July, August and September. with aluminofcrric. The chemical is applied in the proportion of about 2 grs. per gal., although the method of application precludes accurate measurement. A bag of chemical is suspended directly over the inlet, and is dissolved by the inflowing water. The aluminofcrric costs about $20.50 per ton delivered at the works. The original filters are twelve in number, each 100 x 200 ft., corresponding to a total filtering area of 5-51 acres. The filtering material consists of 30 ins. of fine river sand, supported on 4 ins. of coarse sand and 12 ins. of graded gravel, respectively. The effluent collectors consist of small cross-drains covered with tiles. These drains lead the effluent to main collectors laid along the center of the beds, and these main drains deliver the effluent into collecting wells, from which it flows directly into the conduits leading to the auxiliary pumping station on the outskirts of the city. The new filters are 24 in number, each 200 x too ft., corresponding to a total area of 11.02 acres. In construction these filters are in general the same as in the case of tht original filters, with the exception that the effluent collectors consist of two courses of dry bricks laid sufficiently far apart to provide suitable waterways. The total filtering area of the 36 filters is 16.53 acres. [See Fig. 9 for a general view of the filters.] The average daily water consumption is estimated at 27,000,000 gals. (25 gals, per capita), and the actual rate of filtration at about 2.825,000 gals, per acre per 24 hrs. There are three effluent-collecting wells in the plant, with a total capacity of about 42.000 gals., and from these wells the filtered water flows through two conduits to the outskirts of the city. These conduits are 4 and 3.5 ft. in diameter, respectively, and 12.5 miles long. It is the custom at present to supply filtered water to the entire city under about 30 lbs. pressure at the various distributing stations during the day hours, excepting the hours 10 a. m. to 3 p. m., during which hours water is supplied under low pressure only to the more thickly populated sections of the . city. Filtered water is not used for street sprinkling, sewer flushing or fire-extinguishing purposes, a complete separate unfiltered water system being in use for this purpose, the water being pumped direct from the river into the raw water mains.