SEWAGE TREATMENT — KEEFER 379 



the under side of the plates and the bottom of each container. These 

 containers are placed in one or more rows in the bottom of the aeration 

 tanks adjacent and parallel to one of the tank walls. From the diil'user 

 containers air pipes extend from the aeration tanks to air blowers or 

 compressors. 



When a tank similar to that shown in figure 7 is in operation, the 

 flow of the sewage through the tank and the upward force of the air 

 adjacent to one of the tank walls imparts a spiral motion to the mixed 

 liquor. This motion results in new sewage surfaces coming in con- 

 tact with the air at the tank surface, permitting the dissolving of 

 additional oxygen. For aeration tanks to be effective the mixed liquor 

 should contain an appreciable quantity of dissolved oxygen at all 

 times. Usually from 2 to 4 parts per million is adequate. The amount 

 of suspended solids in the mixed liquor resulting from the addition 

 of activated sludge is generally kept between 1,000 and 3,000 parts per 

 million. Maintaining higher percentages of activated sludge in the 

 mixed liquor will result in a greater purification of the sewage but 

 more air will be required. The amount of air used varies as a rule 

 from 0.5 to 1.5 cubic feet per gallon of sewage treated. 



After the mixed liquor discharges from the aeration tanks, the acti- 

 vated sludge must be removed promptly. Conventional sedimentation 

 tanks, either circular, square, or rectangular, with adequate sludge- 

 removal equipment are usually provided. The sludge must be with- 

 drawn in a fresh condition, as most of it is returned to the aeration 

 tanks for a continuation of the treatment process. 



The effluent produced by an activated-sludge plant is generally some- 

 what better than that from a trickling-filter plant with the 5-day bio- 

 chemical oxygen demand and the suspended solids averaging from 10 

 to 25 parts per million. The process has been adopted very widely in 

 this country and abroad for treating the sewage from both large and 

 small cities. A few of the notable installations are in New York, 

 Chicago, and Los Angeles. 



LAND TREATMENT OF SEWAGE 



The application of sewage to the land and the use of the land for 

 agricultural purposes is one of the oldest methods of sewage treatment, 

 dating back to the middle of the sixteenth century. The sewage thus 

 serves to fertilize and irrigate the soil. A sewage farm must be pro- 

 vided with the necessary pipes and ditches, and the land must be 

 graded to prevent the accumulation of sewage in stagnant pools. Be- 

 fore applying sewage to the land, it is desirable to remove a consider- 

 able portion of the suspended solids, which normally tend to clog the 

 soil. The disadvantages of the process are that large areas of land 

 are needed, odors are liable to be produced, and the proper treatment 



