Owing to the rather complicated hydraulic features and the somewhat difficult engineering principles involved in the construction and operation of sprinkling or trickling filters, it is not deemed advisable to attempt to describe them in sufficient detail to furnish directions for their construction. The design of a sprinkling-filter system for small as well as for large installations should always be entrusted to an engineer conversant with this line of sanitary engineering. It is believed, however, that the description of such filters given above will aid those who are about to install sewage-disposal plants in the selection of the type of plant best suited to their particular needs and conforming to the conditions peculiar to their situation. Summarizing the foregoing descriptions and directions with reference to sewage filters, it may be stated that where natural facilities for disposing of sewage by simpler methods do not exist, the construction of a sewage filter of some one of the above described types offers a solution of every problem thus encountered. It is well to repeat that sub-surface irrigation, where feasible, should be adopted, and this will be the method indicated in a large majority of cases where small disposal plants are to be constructed. The principal point to be remembered in connection with sewage filters is that their construction is but a good beginning, and that their proper operation is very necessary to the success of the undertaking. They constitute, with the developed bacteria in the filter, a rather sensitive mechanism capable of efficient work if properly handled, but each filter unit must be carefully operated and must be regularly given extended periods of rest for the restoration of the void or open-space capacity of the filter, and to provide for the necessary aëration of the filtering material. The peculiar action which takes place in a filter in the reduction of sewage, and which is not even yet fully known, is best evidenced by the fact that sewage filters, especially of the coarse-grained type, do not attain their highest efficiency until after several weeks or months of operation. With a knowledge of these points, it will be seen that there should be little divergence from accepted standards in the construction and operation of sewage filters if they are to prove satisfactory when installed. CHAPTER VI BROAD IRRIGATION