Endogenous Respiration and
Stability of Aerated Dairy
Waste Sludge*
SAM   R.   HOOVER,   LENORE   JASEWICZ   AND   NANDOR   PORGES
Eastern Regional Research Laboratoryf
Philadelphia, Pennsylvania
In previous work, we determined the rate and extent of conversion
of milk solids to bacterial cells by the mixed culture of microorganisms
which becomes dominant during aeration of milk waste (1, 3, 4).
Chemical equations for the synthesis of cell tissue were derived from
these data. From these equations and separate determinations of the
rate of respiration, the rate at which oxygen is taken from the solution
was calculated to be 0.075 pound of oxygen/pound milk solids/hour/
500 p.p.m. bacterial cells (0.4 p.p.m. 02/min.) during the period of
rapid growth of cells.
The further digestion of these cells by their own (endogenous)
respiration has a definite practical importance. If this respiration
proceeds at a great enough rate, the cells can oxidize their own tissue
(autodigestion of sludge) rapidly enough to keep the system in balance,
i.e., no sludge will accumulate. If the autodigestion is not sufficient,
sludge will accumulate, and disposal is necessary. The rate of endogenous respiration is of further interest because of the proposed partial-
treatment process described previously, in which the cells would be
discharged to a stream daily (2). The consequent drain on the oxygen
supply of the stream would be directly related to this rate of endogenous
respiration.
The experimental techniques used have been described previously
(1, 6, 7, 9). An active culture containing about 500 p.p.m. bacterial
solids was maintained by continuous feeding of 1,000 p.p.m. of skim
milk solids into an aeration tank. The detention time was 20 hours.
The rate of respiration was measured in a Warburg manometric
apparatus (1) or by evolution of CO2 (8). The latter method was
especially valuable for experiments continued for several days.
* Report of a study made under the Research and Marketing Act of 1946.
f One of the laboratories of the Bureau of Agricultural and Industrial Chemistry,
Agricultural Research Administration, U. S. Department of Agriculture.
541