Among the earliest plants for the removal of iron were the filters constructed at Amsterdam and The Hague in Holland. At Amsterdam the water is derived from open canals in the dunes draining a large area. The water has its origin in the rain-water falling upon the sand. The sand is very fine and contains organic matter in sufficient amount so that the ground-water is impregnated with iron. In flowing to a central point in the open canals the water becomes aerated and the iron oxidized. There are also algæ growths in the water which perhaps aid the process. Sand filters of ordinary construction are used, and remove both the iron and the algæ, and the rate of filtration is not higher than is usually used in the treatment of river-waters, although it could probably be largely increased without detriment to the supply. The works at The Hague are very similar to those at Amsterdam, but covered collectors are used to supplement the open canals. Both of these plants were built before much was known about iron in ground-waters and the means for its removal, but they have performed their work with uniformly satisfactory results. In the more recent German works various aerating devices are employed, and filters similar in general construction to ordinary sand filters, but with larger connections suited to very high rates of filtration, are employed. The plant at Asbury Park was the first of importance constructed in America. The water is raised from wells from 400 to 1100 feet deep by compressed air by a Pohle lift. It is delivered into a square masonry receiving-basin holding some hours’ supply. The aeration of the water by this means is very complete. It is afterwards pumped through Continental pressure filters direct into the service-pipes. The reservoir for the aerated water was not a part of the original plant, but was added afterwards to facilitate operation, and to give more complete aeration before filtration.