IS indeed strange that so many operators fail to appreciate the vast increase in plant efficiency and the reduction in operating problems which result from a conscientious and thorough study of the all-important problem of preparing a water for filtration. A contrast of the condition of the Richmond filters some years ago full of mud balls and cracks -with that since preparation methods have been perfected is adequate to convince anyone of the value of this phase of plant operation. With present methods, a clean, sharp sand is maintained with no attention but the standard washing at from 100to 300-hour intervals; and the improvement in the quality of the finished water is striking. Finding the best chemical and mechanical preparatory treatments for any particular water is not always an easy task, but once the proper method is found, the plant is assured that its filters will bear a minimum burden of purification, acting, for the most part, as mere strainers and polishers of the supply. Before proceeding with a discussion of the various types of treatment used in preparing a water for filtration, it will be well, here, to define several of the terms as they are used in this article. Pre-chlorination means applying chlorine before coagulation. Super-chlorination means such a heavy pre-chlorination that a controllable method of reducing the residual becomes necessary. De-chlorination means the addition of some chemical or treatment for the purpose of reducing the chlorine residual to a desired value. Break-point chlorination means using that amount of chlorine which just satisfies the chlorine demand of the water. Some prefer