382 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



the material decomposes and eventually loses its objectionable odor. 

 Furthermore, the digested material can be more readily dewatered 

 than the raw sludge. The first digesters were open masonry tanks, in 

 which the sludge was stored for several months before being removed 

 for further treatment or disposal. The first two large sludge-digester 

 installations were put in service about 1911 in Baltimore, Md.. and 

 Birmingham, England. 



In the 10- or 15-year period following World War I, as a result of 

 considerable research, many of the factors affecting sludge digestion 

 were determined. These included the effect of temperature, the pH 

 value of the sludge, and the percentage of well-digested sludge used 

 for seeding purposes. As a result of this work many improvements 

 were incorporated in the design of digesters. These improvements 

 consisted of providing digesters with rigid or floating covers to collect 

 the sludge gas and prevent the escape of heat from the sludge, facili- 

 ties for heating the sludge, and equipment for mixing the tank 

 contents. 



When sludge is maintained at a temperature ranging from 85° to 

 100° F., digestion can be accomplished in about 30 days. During the 

 past two or three years, by following certain procedures, satisfactory 

 digestion in about 10 days has been reported by some investigators. 



SLUDGE-GAS COLLECTION AND UTILIZATION 



When sewage sludge digests, it produces considerable volumes of 

 gas. As this gas contains from 60 to 75 percent of methane with a net 

 heat value of from 540 to 675 B.t.u. per cubic foot, the usual procedure 

 is to collect and utilize it. The quantity of gas produced, which de- 

 pends upon the amount of organic matter in the sludge, averages about 

 1 cubic foot per day per capita served by the sewage plant. The gas 

 is used for many purposes, the chief of which is to maintain a suitable 

 temperature in digesters so that the decomposition of the sludge will 

 continue at a rapid rate. The gas is also used for generating power, 

 heating buildings, incinerating sewage screenings, and drying and 

 incinerating sewage sludge. In a few instances the gas is sold to 

 municipal gasworks for general use. 



Several methods of heating digesters have been perfected, one of 

 which consists of using the sludge gas to fire boilers and produce 

 steam or hot water. The hot water is pumped through pipe coils sus- 

 pended in the digester. Another method of heating involves the use 

 of a gas-fired heater, which contains a series of pipe coils. Sludge 

 from the digester is pumped through these coils back again into the 

 digester. 



Sludge gas is extensively used as a fuel in internal-combustion 

 engines for power production. About 17 cubic feet of gas with a heat 

 content of 600 B.t.u. per cubic foot will produce one horsepower-hour. 



