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Digester Gaslifter Mixing
at Aurora
Walter A. Sperry, Superintendent
Aurora Sanitary District
Aurora, Illinois
On October 4th, 1957, the Aurora Sanitary District placed in operation, and for test, the first unit of a device for applying effective mixing
to digestors. The purpose of this paper is to describe the apparatus and
report some of the test results obtained.
The Aurora plant had just completed (June 1956) two new and
identical primary digesters. They were 50 ft square with a side wall
depth of 16 ft and a capacity of 40,000 cu ft or 300,000 gallons each.
They had flat fixed covers (8 in total rise) supported with four internal columns. The gas domes were 4 ft square and 4 ft 6 in high. The
floor had a slope of 1 to 6 from a center sludge pocket, the center depth
to the floor being 20 ft. They were heated by Walker Process "Heatx"
equipment with 250 gpm circulating pumps. Primary sludge enters the
digester from a tipping bucket meter about 8 ft out from the west wall
and 5 ft below the digester roof. Since both digesters are fed with equal
amounts of primary sludge, tests of the apparatus against an unmixed
control could easily be made.
The new gas recirculation apparatus consisted of three gas lift
pumps operated by a Roots-Connersville compressor formerly used to
preaerate raw sewage. This compressor was slowed to the proper capacity, and was equipped with an excess pressure relief, low pressure
by-pass regulator, and excess temperature controller.
The three Gaslifter pump tubes were 16 in in diameter and 14 ft
long. The upper ends of these tubes were welded into the bottom of a
common discharge head 36 in in diameter and with a 12 in side wall.
The pump tubes and discharge head were supported from an agle-iron
cage welded to the underside of the heavy gas dome coverplate. Thus
the whole assembly could be lowered into position in the digester with
a crane, and the gasketed coverplate bolted home. It was unnecessary
therefore, to disturb the digester. When in position, the pump assembly
hung centrally suspended from the gas dome cover. The overflow edge
of the upper discharge head was 2.5 ft below the roof of the digester.
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