288 



Philadelphia sludges would usually satisfy U.S.D.A. and N.Y. 

 State requirements (although copper concentrations are margi- 

 nal), while New York City's sludges do not, or would only 

 extremely rarely, qualify to be disposed on land. 



5. Is there a significant difference in the sewer systems of 

 these two cities which may affect the relative cleanliness 

 of their sludges? 



A detailed comparative study of the sewer system of Philadelphia 

 and New York City would be required to fully assess whether or 

 not any significant differences in the two systems exist which 

 would affect the relative "cleanliness" of the sludges. The two 

 ^systems are generally similar with respect to their age, and 

 both are combined systems in which rainfall runoff from the 

 streets is combined with domestic and industrial sewage in a 

 sewer system leading to the treatment plants. However, many 

 -other factors can affect sludge quality, including: the charac- 

 teristics of the local water supply (particularly its acidity) 

 and soils, the materials comprising the sewer and piping sys- 

 tems, the nature and quantity of industrial wastes entering the 

 system, and the types of treatment processes used. It is inter- 

 esting to note that Philadelphia estimates that approximately 

 10% of its inflow comes from industrial sources, while New York 

 City estimates that only approximately 4% of the influent to its 

 twelve plants is industrial. This fact when combined with evi- 

 dence that concentrations of at least some heavy metals may be 

 moderately higher in New York's sewage sludges than in 



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