The change that takes place in the sewage while it remains in the tank is first that of being liquefied and then disintegrated by bacterial action. That such a change does take place is evidenced by the residue that is found in the tank in the process of cleaning. This is a black granular substance, composed mostly of humus and commonly known as sludge. The amount of accumulated sludge is relatively small, and the operation of cleaning is not necessary more than twice a year and is not the disagreeable task one might suppose. =The Septic Tank With a Sand-bed Filter.=--In places where the use of the septic tank alone is not possible, it sometimes happens that the natural conditions are such as will permit the effluent to be drained directly into the soil. With such a condition, the effluent goes into a filter bed composed of gravel or other loose material, where it undergoes still further bacterial action and if the process is complete, the water which comes from the filter bed is clear and odorless. Under good conditions it is clear sparkling water and contains but a small amount of impurities. Septic tanks are made in many forms but that illustrated in Figs. 151 and 152 is commonly used. In Fig. 151 the tank is shown in position to receive the sewage from the house drain, where it is to undergo the first treatment and then to be conducted to a filter bed made of porous tile, set in loose soil. The tank is shown in detail in Fig. 152. It is a cemented brick cistern with an opening to the surface that contains a double cover as a protection during cold weather. A brick partition divides the tank into spaces _G_ and _H_, that contain volumes that are to each other as 1 to 2. The tank is of such size as will hold a volume of sewage equal to 24 hours’ use; that is, it is expected that any portion of sewage will remain in the tank for that length of time. The sewage enters at _A_, in such a way as will give the least disturbance of the liquid of the tank. An opening _C_ allows the liquid to pass from _H_ into _G_, where any additional sewage entering _H_ will displace an equal amount in _G_, which will pass out at _B_ to the filter bed. The partition _D_ is high enough so that the scum that forms on the surface will not pass directly into the space _G_. The entrance and exit pipes are made of vitrified sewer tile with the openings below the surface. [Illustration: FIG. 151.--Sectional view of a septic tank, connected with a sand-bed filter; for the disposal of sewage from a residence.]