In some parts of the country, as, for instance, in New England, the soil and underlying rock are almost entirely free from lime, and rivers from such watersheds are capable of receiving only very small quantities of coagulant without injurious results. The deficiency of alkalinity in raw water can be corrected by the addition to it of lime or of soda-ash. Lime has been used for this purpose in many cases. When used only in moderate amounts it hardens the water, and is thus seriously objectionable. The use of so large a quantity as would precipitate out, as in Clark’s process, has not been employed in practice. If it should be attempted, the amount of lime would require to be very accurately controlled, and the effluent would have to be treated with carbonic acid to make it suitable for supply. Waters so hard as to require the use of the Clark process almost always have sufficient alkalinity, and do not require to be treated with lime in connection with the use of sulphate of alumina. The use of soda-ash is free from the objections to the use of lime, but is more expensive, and would require to be used with caution. Its use has often been suggested, but I do not know that it has ever been employed in practice. In small works the use of a filtering material containing marble-dust, or other calcareous matter, would seem to have some advantages in case of deficiency of alkalinity, although it would harden the water so treated. The alkalinities of a number of waters computed as parts in 100,000 of calcium carbonate (approximately equal to the safe doses of sulphate to alumina in grains per gallon) are as follows: -------------------------------------+--------+--------+-------- |Maximum.|Minimum.|Average. -------------------------------------+--------+--------+-------- Boston water, 1898 | 2.87 | 0.33 | 1.08 Conestoga Creek, Lancaster, Penn. | 12.20 | 3.70 | 6.80 Allegheny River, Pittsburg | 8.00 | 1.02 | 2.90 Mahoning River and tributaries, 1897 | 20.00 | 2.20 | 10.00 Scioto River and tributaries, 1897 | 35.00 | 10.00 | 20.00 Ohio River, Cincinnati, 1898 | 7.00 | 2.00 | 4.50 Ohio River, Louisville | 10.87 | 2.12 | 6.70 Lake Erie, Lorain, Ohio | | | 9.50 Lake Michigan, Chicago | | | 11.50 -------------------------------------+--------+--------+-------- CHAPTER X. MECHANICAL FILTERS.