les or tees should be provided at both the inlet and outlet. The inlet device slows the incoming water and minimizes disturbance of the settled sludge. Baffles are preferred since tees are more likely to become blocked. The outlet baffle or tee serves to restrict the pas- sage of floating material to the drain field. According to the Manual of Septic Tank Practice, the bottom of the inside surface of the inlet pipe (invert) should be at least 3 in. above the liquid (outlet) level to prevent backflow. The baffle bottom or inlet tee should extend 6 to 12 in. below the liquid surface, and the top should extend to within 1 to 2 in. of the inside surface of the top of the tank. The space at the top is necessary to allow for the passage of gases. If baffles are used, a provision for deflecting gases rising from the sludge should be made (Figure 4.2). Often solids rise with the gases. They could block the inlet or outlet and could be carried into the drain field. The bottom of the inside surface of the outlet pipe is, of course, at the level of the liquid surface. The Manual of Septic Tank Practice suggests that the bottom of the outlet tee or baffle be extended to a distance below the surface of the liquid to a depth equal to 40% of the total liquid depth. The top of the tee or baffle device should also provide space for the passage of gases. The center partition also must provide for the passage of gases, this commonly being achieved by a 2-in. by 8-in. horizontal slot at the top of the partition. Liquid flow from one compartment to the other is often through an ell fitting in the partition. The bottom of this ell should be at about half of the liquid depth. Some manufac- turers mold a slot in the center wall. A slot may reduce the potential for the passage to become blocked, and therefore may be preferable to an ell fitting. The liquid level itself should account for about 80% of the total WASTE PROCESS Figure 4.2 Gas baffles. depth of the tank. The 20% of the volume allowed above the liquid level provides space for 