Dewatering Paper Mill Sludges by
Vacuum Filtration
ALAN M. LINDSEY, Pollution Abatement Supervisor
West Virginia Pulp and Paper Company
Covington, Virginia
INTRODUCTION
The disposal of solids settled out in pulp and paper mill primary and secondary treatment systems is of major interest. Present methods for disposal include
lagooning, filtration and centrifugation. In the case of filtration and centrifuga-
tion, ultimate disposal is accomplished by trucking to a land disposal area or by
incineration. The Covington mill used lagooning from 1955 when the waste treatment plant went into operation until 1965 when a dewatering system utilizing filters and trucking to a land disposal site began operation. By 1959, the overflow
from the lagoon began to contain significant quantities of BOD and had to be
treated in the waste treatment plant. During the summer, the lagoon overflow
created a BOD load on the plant of up to 30, 000 lbs/day and a hydraulic load of
2,000,000 gpd. By 1963, it became apparent that the lagoon would be full in
three years and that a new lagoon would involve the expenditure of significant
capital and would not solve the necessity to treat the lagoon overflow. In 1964,
extensive dewatering studies were performed to evaluate filters, centrifuges, and
screw presses. For Covington, filters and trucking to a land disposal area seemed
to have an advantage over other methods.
DESIGN OF SYSTEM
Sludge consisting of primary and waste activated sludges is pumped from two
100-ft diameter primary clarifiers on alternating seven-min cycles into a 50-ft
diameter thickener. The influent sol ids concentration averages 2. 5 per cent and the
thickener underflow averages 3. 3 percent. The pilot data indicated a mass loading
of 25 lbs/sq ft/day should be used to give a six per cent solids underflow. To
obtain a mass loading of 25 lbs/sq ft/day would have required construction of a 90-
ft diameter thickener. A 50-ft diameter tank which was idle and adjacent to the
proposed filter building was converted to a thickener. With a mass loading of 80
lbs/sq ft/day based on a 80-ton/day sludge load, the underflow averaged 3.3 per
cent for 1967 or just slightly less than predicted. The thickening mechanism was
manufactured by Rex Chain Belt and is designed for a torque of 50, 000 ft-lbs With
a 70, 000 ft-lbs motor cut off. The rake arms can be manually raised a distance
of 36 ins.   The rake mechanism is equipped with a torque meter.
Sludge is removed from the thickener by centrifugal pumps equipped with
variable drives. The pumps are capable of 500 gpm. The filters use coil
springs. Each filter is 11 ft in diameter by 16 ft long and has an area of 575 sq
ft for a total filter area of 1150 sq ft. The drum faces and coil springs are 304
stainless steel. The vat is equipped with an agitator. Coil springs are cleaned by
showers mounted just before grooved return rolls. Presently, 170 gpm is required
for continuous cleaning of springs on both drums. Significant differences between
this filter and other filters are the external pipings for filtrate removal and the
permanent-type filter media.   The dram speed can be varied from 1. 5 to 11 mins
- 438 -