The most common chemicals which are employed in coagulating waters previous to rapid sand filtration are compounds of aluminum, lime, and iron. The alums, namely, ordinary potash or ammonia alum, or sulphate of alumina, are perhaps the most efficient coagulants known. If the alum is omitted and sulphate of iron (copperas) is used only, certain modifications are necessary, and especially the introduction of the lime compound, which is a necessary ingredient when the coagulation is produced by ferrous sulphate (copperas) alone.

Attention is called by the report of the Geological Survey to the fact that to obtain satisfactory results from the use of lime when used with iron sulphate as a coagulant it is necessary to make use of sufficient lime to neutralize and precipitate the iron. The use of too little lime results in poor coagulation, caused by the incomplete precipitation of the iron, some of which is usually left in solution and appears in the effluent of the filters. The use of too much lime results in the formation of lime incrustants, which deposit in the air and strainer systems and cause much trouble through clogging.

Further, water which is treated with lime and iron will show an increase in permanent hardness, as compared with the effect of the use of compounds of aluminum. Aside from sanitary reasons the aluminum salts are considered more satisfactory as coagulants; they remove color from water more rapidly and completely and make it possible to obtain by filtration a more brilliant water than do iron salts.