Experience with the Use of a Nonbasic Alum in Connection with Mechanical Filtration.

Abstract

The chemical which is used most commonly for the coagulation of waters, in connection with the mechanical system of filtration, is basic sulphate of alumina, more often referred to as "sulphate of alumina," or merelyas "alum." In a case where so much depends upon the proper chemical treatment of the water, it is evident that the quality of the chemical used is a matter of fundamental importance. Most of the products which have been furnished for this purpose have been reasonably satisfactory in this respect. Recently, however, an instance has occurred where inferiority in the quality of the alum has caused a noticeable diminution in the efficiency of the filter. The writers have thought that a brief description of this experience might be of interest to others. Theoretical sulphate of alumina has the formula A12(S04)3. 18 H20; that is, it contains 48.64 per cent of water, 15.32 per cent of alumina (Al,^), and 36.04 per cent of sulphuric acid (SO3). The aluminum sulphate of commerce, however, differs somewhat from these theoretical proportions. It usually contains somewhat less water of crystallization and more sulphuric acid and alumina. The excess of alumina over the theoretical amount is generally greater than that of the acid, in the case of alums which have been furnished for filtration purposes. This is what has given rise to the trade name "basic sulphate of alumina." The excess of alumina (Al203) over the theoretical amount necessary to combine with the acid (SO3) usually varies from 1 to 5 per cent, although it sometimes is as high as 10 per cent. The ratio of alumina (A\203) actually present in an alum to the amount theoretically necessary to combine with the acid (SO3) present has been sometimes termed the ratio of basicity. While in the case of most alums which have been furnished for filtration purposes the basicity ratio has exceeded unity, alums are sometimes manufactured for other purposes which have a basicity