Chemical Properties of
Digested Sludge and Their Useful
Application in Wastes Treatment
R. H. L. HOWE, Sanitary Engineer
S. M. PARADISO,  Assistant Manager of Engineering and Maintenance
L. H. WOLLENWEBER, Manager of Engineering and Maintenance
Tippecanoe Laboratories
Eli Lilly and Company
Lafayette, Indiana
Digested sludge is a well-known name to wastes treatment people, and it
needs no introduction. Historically, it is the end product of an ancient sewage and waste treatment practice. In China, digested sludge from deep
"sewage wells" has been widely utilized by farmers for hundreds of years as
a soil nutrient applied in liquid form to rice fields when fertilizers were not
available. In modern sewage treatment practice, well-digested sludge has
been de-watered, dried, and used as a soil conditioner. The analytical value
of digested sludge as a soil improver has been extensively investigated by
many, and it is now well recognized (1). However, little has been done on
the investigation of the utilization of digested sewage sludge other than as a
soil improver.
To date, only a few reports on the particular application of digested
sludge to sewage or wastes treatment have been published or known to the
public.
In 1949, Mortenssen (2) reported that he had treated raw sewage with
digested sludge before settling and found that the digested sludge was able
"to produce a more concentrated sludge    and a supernatant liquid
substantially free  of suspended  solids and low in soluble organic material
content."
In 1953, Howe and Hughes detected the presence of Vitamin B12 in digested sludge and analytical results indicated a concentration of 1. 67 ug B12
per gm of digested sludge solids by the Boxer-Rickards Method (3).
Recently, Fall and Kraus (4) reported the improvement of suspended
solids removal from sewage by using the anaerobic contact process in which
the sewage is passed through a digested sludge blanket.
The few reports mentioned above merely illustrate that digested sludge
possesses properties other than that known as a soil conditioner.
It is the objective of this paper to describe briefly some chemical properties of digested sludge based on research and application data collected
since 1956 in relation to chemical wastes treatment.
CHEMICAL PROPERTIES OF DIGESTED SLUDGE
Digested sludge is a blackish,   amorphous, nonplastic material which is
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