SEWAGE TREATMENT — KEEFER 383 



Gas engines usually operate electric generators, centrifugal sewage 

 pumps and blowers. Gas engines can be obtained that use either sludge 

 gas or oil as fuel. If there should be insufficient gas, there need be 

 no interruption in operation, since oil can be used. The advantages 

 of using sludge gas as fuel for gas engines is that considerable power 

 can be developed and at the same time the water used to cool the engine 

 can be pumped through pipe coils in sludge digesters for heating pur- 

 poses. Many sewage plants have been provided with gas engines. 

 Typical examples are the plants serving Miami, Fla., Toledo, Ohio, 

 and Peoria, 111. 



SLUDGE DEWATEKING 



As the sludge drawn from digesters contains from 90 to 95 percent 

 water, it is desirable to reduce its volume by decreasing its water con- 

 tent. The two most widely adopted methods of accomplishing this is 

 by sludge- drying beds or vacuum filters. 



Sludge-drying beds. — These are level beds of porous material, situ- 

 ated out-of-doors and consisting usually of sand, superimposed on a 

 layer of gravel. Underdrains with open joints are provided under the 

 gravel at regular intervals. Sludge beds are generally divided by 

 means of wood planks or thin concrete partitions into compartments 

 to facilitate operating procedures. The wet sludge flows to the beds 

 through pipes or open channels. The necessary sludge bed area, which 

 depends upon climatic conditions, amounts to about one square foot 

 per person served by the sewage works. Under favorable atmospheric 

 conditions well-digested sludge can be dried in about one or two weeks. 

 When its moisture content has been reduced to about 70 percent or less, 

 the sludge can be removed and another application can be made. 



At many sewage-treatment plants (pi. 3, fig. 2) the sludge-drying 

 beds are covered with greenhouses. The advantages of this type of 

 construction are that a somewhat smaller drying area is needed, the 

 problem of odors is less acute, and some sludge can be dried in winter. 



The use of drying beds for dewatering sludge has several dis- 

 advantages. In the first place unpleasant odors will result if the 

 sludge is not well digested. Moreover it is impossible to dry sludge 

 when the weather is very cold. Extra volume, therefore, must be pro- 

 vided for the storage of the sludge in tanks during the winter. 



Vacuum filters. — Vacuum filters (pi. 4, fig. 1) have been used for 

 a number of years in this country to dewater both raw and digested 

 sludge. The moisture content of wet sludge, which will vary from 

 about 90 to 99 percent, can be reduced to approximately 65 to 83 per- 

 cent by filtration. The amount of water removed will depend upon a 

 number of factors such as the type and characteristics of the sludge, 

 the rotating speed of the filter drum, and the kind and amount of 

 coagulant used. 



