For the filtration of these waters after coagulation either sand or mechanical filters can be employed. As the principal work in this case is done by the coagulant, the kind of filtration employed is of less consequence than where filtration alone is relied upon, and the cheapest form of filter will naturally be employed. Under present conditions mechanical filters will usually be cheaper than sand filters for use in this way; but where waters, in addition to the mud, carry bacteria in such large numbers as to make high bacterial efficiency a matter of importance, sand filters may be selected, as the bacterial efficiency obtained with them is not dependent upon the use of coagulant; and is therefore less subject to interruptions from the failure to apply coagulant in the right proportion. Mechanical filters have also been used for the treatment of comparatively clear waters where bacterial efficiency was the principal object of filtration. For this purpose the efficiencies obtained with them are usually inferior to those obtained with sand filters, while the cost of coagulants is so great as to make their use often more expensive than that of sand filters. In the case of many streams which are comparatively clear for a part of the year, but occasionally are quite turbid, the use of sand filters has this advantage, that the use of coagulants can be stopped and the cost of operation reduced whenever the water is clear enough to allow of satisfactory treatment by them; and that coagulant can be employed on those days when otherwise insufficient clarification would be obtained. In this case the high bacterial efficiency is secured at all times, while the cost of coagulant is saved during the greater part of the time. In such cases, also, the preliminary process of sedimentation and storage should be developed as far as possible. The application of other processes of filtration to special problems are not sufficiently well understood to allow general discussion, and must be taken up separately with reference to the requirements of each special situation. COST OF FILTRATION. The cost of filtration of water depends upon the character of the raw water, upon the nature of the plant employed, upon its size, and upon the skill and economy of manipulation. These conditions affect the cost to such an extent as to make any accurate general estimate quite impossible. Nevertheless a little consideration of the subject, although not leading to exact results, may be helpful as furnishing a rough idea of the probable cost before estimates for local conditions are made.