The Effect of
Temperature on High-Rate Digestion
of Activated Sludge
J. F. MALINA, JR.
Assistant Professor of Civil Engineering
University of Texas
Austin, Texas
The high-rate digestion of decomposable solids is of primary concern because of the large volume of activated sludge resulting from the treatment of
waste waters. Information dealing with the effect of temperature on high-rate
digestion processes currently employed in the treatment of waste waters is
almost nonexistent. It is generally accepted that careful control of the temperature is imperative for efficient operation of the anaerobic digestion
process.
The information included in this paper presents an evaluation of the effect of temperatures greater than 30 C on the high-rate digestion of activated
sludge.
LITERATURE REVIEW
The processes employed in primary treatment of waste water are designed
in part to separate a portion of the suspended particulate materials from colloidal and dissolved matter. In secondary treatment processes the colloidal
and dissolved substances are converted microbially into suspended material
which can be separated by gravity sedimentation. The solids separated in
these processes require additional treatment, and in many treatment works
are partially decomposed by an anaerobic microbial population in a sludge
digestion tank.
In current practice the anaerobic fermentation of the sewage solids has
been carried out in what is generally referred to as either conventional sludge
digestion tanks or high-rate sludge digestion tanks. The conventional tank is
one in which the contents are unmixed, and the loading to these digesters was
about 0.05 lbs of volatile material per cu ft of capacity per day. The detention time was usually in excess of 30 days. However, within the past decade
high-rate digestion systems have been developed. The results of studies conducted in the laboratory in pilot units and in field installations indicate that
loadings of volatile material up to 0.35 lbs of volatile material per cu ft of
digester capacity per day could be digested in less than 15 days at temperatures near 30 c.
Laboratory and pilot investigations by Fuhrman (3), Morgan (10), Sawyer,
(11), and Torpey (14) indicate that up to seven times the conventional loading to the digester are successfully treated in about one-third the conventional
detention time.   Table I summarizes these early studies.
Successful plant-scale high-rate digestion units have been installed employing both gas circulation and draft-tube mixers. The results obtained at
these Field installations confirm the results of the laboratory and pilot-plant
studies discussed above.
- 232 -