bottom of hopper-bottom tanks. The roof of the tank may be covered with earth, but access openings should extend at least to the ground surface. Although ells or tees may be used at inlet and outlet connections, straight connections are better for rodding. Instead of ells, wooden baffles, located approximately 18 inches from the ends of the tank and extending 18 inches below and 12 inches above the flow line, are provided. Elevations should permit free flow into and out of the tank. The bottom of the inlet sewer should be at least 3 inches above the water level in the tank. The inlet and outlet connections should be sufficiently buried or otherwise protected to prevent damage by traffic or frost.

When a tank will discharge into a leaching field greater than 500 feet in length, a dosing tank and siphon should be incorporated into the system (fig. 10-3). The rush of sewage that occurs when the siphon discharges results in better distribution throughout the leaching field. While the dosing tank is refilling, the resultant resting period is favorable to maintaining aerobic conditions in the receiving soil. The dosing tank should have a capacity about 60 to 75 percent of the interior capacity of the leaching pipe to be dosed at one time and should automatically dose once in 3 to 4 hours. Double the amount of dosing siphons for each additional 500 feet of leaching tile or pipe.

Although properly designed septic tanks require little operating attention, they must be inspected periodically. The frequency is determined by the size of the tank and the population load. The minimum frequency should be once every 2 months at periods of high flow. The inspection should assure that the inlet and outlet are free from clogging, that the depth of scum and sludge accumulation is not excessive, and that the effluent passing to subsurface disposal is relatively free from suspended solids. A high concentration of suspended solids in the effluent quickly clogs subsurface disposal facilities. Sludge and scum accumulation should not exceed one-fourth the tank capacity. It should not be assumed that septic tanks liquefy all solids, that they never need cleaning, and that the effluent is pure and free from germs. Perhaps 40 to 60 percent of the suspended solids are retained and the rest are discharged in the effluent.

Separating sludge and scum from the liquid in septic tanks is difficult. In small tanks these wastes are customarily mixed, and the entire contents are removed when the tanks are cleaned. The

Figure 10-3.-Septic tank with dosing siphon.