WATER-SOFTENING FOR MUNICIPAL SUPPLIES.
In a paper read at the Boston convention of the American Water Works association, George W. Fuller treated of the above subject, which he prefaced by stating that, while limestone shale and other rocks front which water dissolves salts of lime and magnesia are comparatively rare in New England, in the middle West such rocks abound, and the waters frequently contain ten times as much mineral matter as those along the Atlantic coast. This hard water, while within ordinary limits, it has little, if any evil effects on the health of the consumer, is less satisfactory than soft water for practically all purposes, whether domestic or steam-raising, and the use of boiler compound for softening the water is only a step in the right direction, and more complete and sys tematic treatment of these hard waters for boiler use is advisable. Industrial softening plants date from about 1891, before which time a few small plants were in use. The last decade has seen great improvements and developments, and now the aggregate capacity of such plants exceeds 50, 000,000 gals, daily. * * * One large plant is at Southampton, on the south coast of Eng land—a city of about 75,000 inhabitants, whose public water supply comes from wells and galleries in the chalk cliffs. The town has softened its water since 1888. It is treated with lime water to reduce the calcium carbonate, and, after a short period of settling, is clarified by passing through patented cloth filter presses. The plant has a nominal capacity of 5,000,000 imp. gals. About 4,000,000 gals, are treated daily. The lime water tanks are four in number, about 9 ft. in diameter and u ft high, with conical bottoms. They are operated in pairs, each serving for a 12-hour shift. As each shift begnis, enough milk of lime is made to serve for a 12-hour watch. After draining out the contents of the tanks used during the preceding watch they are charged with milk of lime and filled with water to be used during the following 12-hour shift. Softened water is delivered into these tanks at the bottom by a pump attached to the low-lift pump delivering well water to the softening plant. The lime water in this way is delivered in a quantity proportional to the volume of water to be treated and it takes about 1.5 to 2 hours to pass from the bottom of the lime tanks to the top, where it overflows as a fairly well clarified solution of saturated lime water. There are no stirring arrangements in the tanks. The hard well water, with its proportionate quantity of lime water, is thoroughly mixed at the entrance of the sedimentation basin, holding an average blow-period of about forty minutes, each six minutes are deposited about 2 ft. of carbonate of lime on the floor of the basin, which is about 76 ft. long and 44 ft. wide. The water is very milky as it leaves the settling basin and enters the ten cloth filters, each of which is a tank 4 by 8 ft. with 500 sq ft. of cloth filtering surface. Each unit is rated at a capacity of 500.000 Imp. gals, daily—equal to a flow of 0.84 U. S. gallons per sq. ft. per minute. These filters are cleaned every ten to twelve hours hv moving up and down a pipe with jet sprinklers between each pair of filtercloiths. It takes about 2.5 minutes to wash a filter. The cloths last about one year and prove a very satisfactory means of removing the carbonate of lime, ot which there are about eighteen grains per imp. gallon -equal to alkalinity of about 225 parts per 1,000.000. with carlxmate of magnesia equal to about seven parts per 1.000,000. The inerttstants (permanent hard-ess) are so small that they are disregarded. As it needs more treatment to remove the gelatinous, sticky magnesia precipitant than the familiar carbonate of lime, the Southampton water is the easiest type to soften. Its hardness is generally reduced to about eighty parts per 1.000,000. The reaction is sufficiently completed in the forty minutes, so that the filtered and softened water contains no caustic alkalinity—an important improvement over 1902. * * * In 1881.) tests were made at Cincinnati for coagulating the Ohio river water. The scale was 250,000 gallons per twenty-four hours. In this plant lime wafer was_ applied to the river water in quantities to give excess of lime water from about three to four grains per gallon in the water, after settling for twelve hours. The water was then coagulated so that it could be filtered at a rapid rate, and the clarification and the removal of bacteria and organic matter were satisfactory. Owing to the presence of the caustic alkalinity of the excess of lime water, the clay particles were coagulated. In this process no magnesia is utilised as a result of lime application to coagulate waters. The amount of magnesia in the Ohio river water at Cincinnati is too small to effect this result, and there was no economy in this style of treating th: local water, when due allowance was made for re-carbonating the effluent so as to eliminate the caustic alkalinity due to the excess of lime water. * * * The largest municipal water-softening plant now in operation in this country is at Winnipeg, Man. Its capacity is nearly 3,000,000 U. S. gailons daily. * * * This water is obtained from deep wells and contains in parts per 1,000,000 about twentytwo parts of free carbonic acid and 203 parts of carbonate of lime and 113 parts ot carbonate of magnesia. Permanent hardness or sulphate hardness amount to about 173 parts per 1,000,000; no attempt is made to reduce this by soda ash. Ordinarily the carbonate hardness is removed so as to leave a residual alkalinity of about eighty parts per 1,000,000. Lime is applied as a saturated lime water. The hard water enters a weir-box and, by means of a series of proportional weirs, any desired percentage of the hard water is diverted to a pipe leading to the lime saturatortanks. rvs the water flows to the latter, it meets a stream of cream of lime in such volume as, on an average, is sufficient for the purpose intended. This cream of lime is dissolved as the water passes through the saturator-tanks and leaves the latter as a fairly clear lime water. This arrangement insures the proper quantity of lime water at all times, regardless of the fluctuations in operating the low-lift pumps. The lime water, on leaving the saturators, flows to a baffled channel, where it mixes with the main body of hard water; thence to the settling tanks, with a capacity of about 2-hours flow and operated 011 the continuous plan. In these a majority of the carbonate of lime is deposited, and the remainder is removed by passing the settled water through filter presses. This water, owing to the relatively high amount of magnesia present, is a far more difficult water to treat than the one at Southampton, as is shown by a tendency of the filtered water to deposit carbonate of lime in the high-lift pumps—spoken of generally as “after reactions.’’ To eliminate the latter the water is carbonated by passing through it gas obtained by burning coke. The plant has been in operation since July, 1901. * * * The first municipal softening plant in the United btates was put in service in December, 1903. at Oberlin. Ohio. The plant ordinarily furnishes about 165.000 gallons of softened water daily. The water is surface, and usually contains about 175 parts per 1.000.000 of alkalinity, far the greater portion of which is lime rather than magnesia. The sulphate hardness ordinarily ranges from about too to 150 parts per 1.000,000. There is much more variation in the hardness of this surface water than in ordinary ground waters. Lower temperatures in winter also add to the difficulty in completing the reactions. This was the first municipal softening plant in the United States in which the sulphate hardness is regularly removed by adding soda. The chemicals needed are weighed out every three hours and applied to both lime and soda tanks—• the strength of the solutions from the tanks, in spite of mixing, depending upon the quantity in them. After receiving the chemicals, the hard water is passed through a mixing box about fortyfeet long, with many baffles, then successively through two reservoirs, the total capacity equaling about 4 days’ supply. The water then passes through mechanical filters of the pressure type. The alkalinity is said to be reduced ordinarily to about thirty parts per 1,000,000. The long storage in the reservoirs is helpful, and no after reactions to speak of have been encountered. The cost of the softening works is given as $12,000. Quantities of applied lime range from six to seventeen grains and of soda from two to six grains per gal. * * * The water at Freeport, 111., is from deep wells, and is very hard. It contains much iron in solution in the shape of ferrous carbonate, and crenothrix grows abundantly in the pipes. A 2,000,000-gal., mechanical filter plant was installed in October, 1903, and sufficient lime (two to four grains per gal.) is applied to remove the vein. The hardness is thus reduced about thirteen to fifteen per cent. * * * At St. Louis, Mo., since March 22, 1904, the turbid Mississippi river water has been completely clarified by the use of iron and lime and sedimentation basins holding an average flow of somewhat more than two days. Incident to the proper behavior of the sulphate of iron, lime is used in sufficient quantities to cause precipitation of a part of the lime and magnesia in the water. * * * An interesting feature consists in adding chemicals in dry solid form at 5-minute intervals to tanks in which water (lows under conditions to insure a practically uniform rate of their application. * * * A water-softening plant of 30,000,000 gallons daily capacity is being built at Columbus, Ohio, to treat the very hard and often turbid Scioto water, and remove bacteria, turbidity, carbonate hardness and the sulphate hardness. Lime water will be applied as a saturated solution as at Winnipeg. After the hard water has received this lime, it will pass through a highly baffled basin holding about an hour’s flow. Shortly after the entrance of the water into this basin it will receive a solution of soda. This basin will produce a thorough agitation, to complete the reaction in the presence of the resulting precipitates, so far as feasible. The water will enter settling basins holding 12-hours’ average flow. The water may be withdrawn from these basins after varying periods of sedimentation, to apply the softened water to mechanical filters for final treatment, without having to use a coagulant. It is expected that sulphate of iron will he added during much of the time, either to complete coagulation to prepare the water for filtration or to eliminate traces of caustic alkalinity. For the latter purpose raw water will be conveyed to the settling basin to be mixed with the softened water. This plant mixes very thoroughly the water with the chemicals (applied with automaticcontrol) to complete the reactions promptly. Fairly long periods of sedimentation are used, also raw water and sulphate of iron to remove caustic alkalinity, and mechanical filters for final clarification. * * * The cost of this plant is set down at about $250,000. Messrs. Hering and Fuller are consulting engineers. * * * New Orleans will have a 40.000.000-gallon purification plant for the purification and softening of the Mississippi river water. Here lime water will he used to soften the water to enable the precipitates of lime and magnesia to coagulate the water so far as practicable. Soda will not be used, as the sulphate hardness of the local water is fairly low in amount. The general features of this plant resemble quite closely those at Columbus, Ohio, with many important differences in the details. Some are very ingenious, and the plant will be unusually well equipped from a standpoint of controling automatically its operation. Messrs. Hering and Fuller are the consulting engineers. Speaking generally, the cost of chemicals for softening will be from half a cent to I cent and one-half per 1.000 gallons, depending on the hardness of the raw water, the price of chemicals and, especially, on whether or not soda is used. The chemicals may be applied in various ways to be determined by experience on a large scale. The thorough mixing of the chemicals with the water will apparently be helpful in several ways, including the reduction of “after reactions.” Raw water and sulphate of iron applied to the water, after settling in relatively large basins, are feasible methods of eliminating caustic alkalinity and after deposits of lime. Mechanical filters seem to afford the best means for the final clarification of the softened water.