It is evident that some of the matter that should remain in the septic tank and be removed as sludge will be carried into the anaerobic filter. This will, of course, form an insoluble deposit that will accumulate and in the course of time the filter will become clogged. It should be expected that such a filter will ultimately need renewing, for this reason the top is made of a slab of reinforced concrete that may be raised to allow the removal and refilling of the filter material. The automatic siphon discharges the water from the chamber _S_, whenever it fills. The discharged water from the siphon is conducted into a drain tile, placed in a ditch filled with gravel or other loose material, which serves as an additional filter and in which the water undergoes a still further purification. This filter ditch is constructed as indicated in longitudinal section. The water from the siphon enters the tile _C_ and seeping through the filling is drained away in the tile shown at _D_. The tiles are not set close together, but the joints are left open and covered by pieces of broken tile as shown in _H_. This is to prevent the filter material from entering the tile and thus stopping the ready flow of the water. The filter ditch of the plant will be constructed according to the contour of the ground and will follow the natural drainage. The course of the ditch--if it is desired to use one--will accommodate itself to the character of the ground. The final discharge of the water will be determined by the natural drainage. That a plant of this kind will work perfectly when new is is beyond a doubt but that it will continue indefinitely to give perfect satisfaction is not reasonable to expect. The septic tank will require cleaning, probably once a year. The anaerobic filter will require renewing at intervals, depending on the amount of sewage the filter is required to take care of and the rate at which the plant is worked, probably once in 4 or 5 years. If the septic tank is of insufficient size to readily digest the sewage, the accumulation of sludge in the anaerobic filter will be greater than should occur. It would be only reasonable to suppose that the siphon will sometimes refuse to discharge. Even though it is an automatic siphon, circumstances may cause it at times to fail to act. For this reason the manhole is placed so that the siphon may be inspected and repaired, should it be necessary. It must not be supposed that once such a plant is in place that all of the work is over. The success of a good sewage-disposal plant of this kind demands eternal vigilance. [Illustration: FIG. 155.--Septic tank with a settling basin and windmill pump.]