Milk Wastes in Sewage Sludge-
Digestion Tanks
David Backmeyer, Plant Superintendent
Marion Sanitation Department
Marion,  Indiana
At the Third Industrial Waste Conference held at Purdue Uni¬
versity in May, 1947, this writer discussed the problems resulting from
the overloading of an activated-sludge treatment plant by the dis¬
charge of cheese whey to the city sewerage system. At that time, men¬
tion was made of the fact that the cheese whey could be successfully
reduced in the plant sludge-digestion units, provided it was added
directly to the tanks and not mixed with the sewage flow entering the
plant. The material in this paper deals principally with a report of
sludge-digester operation during a fifteen-month period when consider¬
able quantities of whey were disposed of in the sludge digesters that
were normally handling sewage sludge solids and ground-garbage solids.
Two other brief discussions are presented—one which reviews an
experimental test in which whole whey was digested in small test
drums, and one which describes the digestion, on a plant-scale basis at
the sewage-treatment plant, of a shock load of whole condensed milk.
The sludge-digestion tanks at the Marion sewage-treatment plant
are two in number, identical in design and equipment. Both tanks
have Pacific flush-tank-type floating covers and the necessary gas col¬
lection equipment for proper removal of the gas generated during
digestion. The actual working capacity of each tank is about 70,000
cubic feet, providing for 5.2 cubic feet per capita (1940 population)
on the basis of total capacity. The normal load handled by the tanks
consists of primary sewage sludge mixed with waste activated sludge,
plus the garbage from the city, which is ground at the plant and
added directly to the tanks.
The structure and general arrangement of the sludge digesters is
shown in Figure 1. The digestion system is rather heavily overloaded
during the summer months when garbage collections are the greatest,
the result being that no supernatant of good quality can be withdrawn
from the system during these periods. This lack of capacity is further
evidenced by an examination of the so-called digested sludge taken from
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