Method and apparatus for dewatering sludge

Sludge to be dewatered is deposited on drainage slabs which slope toward a trough to which water draining from the deposited sludge is permitted to flow. Supported at the open top of the trough is a perforate member, and the trough is initially filled with a support water immersing the perforate member, the support water allowing sludge to flow over the perforate member, but minimizing the tendency of such sludge to settle through the perforate member. In the practice of the process disclosed, the support water is permitted to drain from its trough at approximately the rate new water drains to the trough from the deposited sludge.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method and apparatus for dewatering sludges and 
slurries, such as sewage sludges and paper mill slurries. 
2. Prior Art 
It is known to dewater sludges by placing the sludge on perforate plates or 
screens which may be inclined and which are supported above drainage 
plates, which may also be inclined. Examples of such dewatering apparatus 
appear in U.S. Pat. Nos. 1,537,818 and 596,540. It has also been observed 
that sludge can be dewatered and sun-dried by merely spreading the sludge 
upon a sloped slab such as a concrete slab. Thus, it has been known to 
separate two such slabs, which are sloped oppositely to form a trough, by 
a sand filter to which the water drained from the sludge is permitted to 
flow. Other and different types of techniques for dewatering sludge and 
the like appear in the following U.S. Pat. Nos.: 1,673,728, 2,259,688, 
2,928,548, 1,940,952, 2,491,912, 3,662,896. 
British Pat. No. 21,355 illustrates a dewatering technique applied to the 
withdrawal of wort from a mash tun. 
U.S. Pat. No. 3,216,569 illustrates a sludge dewatering apparatus which has 
received attention recently in a number of U.S. sewage plants. In the 
technique described in this last named patent, sewage sludge is deposited 
onto a perforate screen or plate which is immersed into water, 
characterized as support water, such water supported in a chamber located 
below the perforate plate. After the sludge has been accumulated above the 
perforate plate to a desired depth, the support water located in the 
chamber underlying the perforate plate is slowly removed at a controlled 
rate. The slow water removal avoids a discharge of the sludge being 
dewatered through the perforations in the perforate plate. Consequently, a 
large fraction of the water normally present in the sludge is withdrawn 
from the sludge through the perforate plate with a minimum of escape of 
sludge. The sludge lying immediately upon the perforate member thus 
functions as a filter, which minimizes a downflow of sludge through the 
perforate member. 
SUMMARY OF THE INVENTION 
While the dewatering method disclosed in U.S. Pat. No. 3,216,569 is 
encountering an increasing acceptance in small sewage installations, 
particularly those serving populations not exceeding 5,000 to 6,000 
people, and in some cases installations serving industries whose sludge 
dewatering demands are not excessive, the technique disclosed in U.S. Pat. 
No. 3,216,569 has not been applied to larger sewage facilities due to the 
expense in producing and maintaining the perforate plate means upon which 
the sludge is deposited and, more importantly, the expenses involved in 
removing the dewatered sludge from the perforate member upon which the 
sludge rests. Thus when the daily sludge accumulation is small and can be 
removed manually with simple devices such as shovels, the process of U.S. 
Pat. No. 3,216,569 is applicable. For serving larger population centers 
wherein sludge removal requires mechanized equipment which is easily 
damaging to the perforate member upon which the sludge rests, it has 
heretofore been more feasible to utilize other sludge dewatering means 
such as vacuum dewatering plants and/or micro-strainers, which are found 
more economical to operate in the service of large population centers. 
The present invention bridges the gap between the process of U.S. Pat. No. 
3,216,569, which can serve only small population centers, and the vacuum 
dewatering and like processes, which are economically feasible only when 
serving very large population centers, by dramatically increasing the 
amounts of sludge that can be processed in accordance with the dewatering 
technique as described in U.S. Pat. No. 3,216,569. 
In the present invention, an elongate water trough is constructed between 
elongate concrete slabs which flank the water trough and which slope 
downwardly toward the water trough. The trough is covered with a perforate 
means adequately supported to permit vehicles to travel thereover. In 
operation, a support water charged to the trough overflows the trough to 
submerge the perforate means. Sludge is then distributed upon the concrete 
slabs and flows downwardly onto the perforate means. The support water is 
then slowly withdrawn so as to permit water draining down the concrete 
surfaces toward the water trough to replace the support water being slowly 
withdrawn.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 schematically illustrates two elongate slabs 10 and 12, which may be 
concrete slabs. The slabs 10 and 12 extend parallel to one another on the 
opposite sides of a trough 14, which is preferably of sheet metal 
construction. As appears in FIG. 5, the trough has generally vertical side 
walls 13 and 15 and a base 17. Also as appears in FIG. 1, the trough has 
end walls 19 and 21. 
As appears in FIG. 2, the slabs 10 and 12 slope upwardly from the opposite 
sides of the trough 14. Staggered along the length of the trough 14 are 
two rows of vertically disposed support plates 20, which are a structural 
steel affixed to the base 17 as by welding, not shown. Also extending 
lengthwise of each of the side walls 13 and 15 are angle bars 16, which 
are welded or otherwise affixed to the side walls 13 and 15. 
A wedgewire screen 18 which comprises parallel wire elements, each having 
loops 22 encircling support rods 26 at spaced intervals along the length 
of the wires, rests upon the angle bars 16 and upon the staggered support 
plates 20. As appears in FIG. 3, the top surfaces 23 of the support plates 
20 and the horizontal flanges 25 of the angle bars 16 are generally 
coplanar so as to support the screen 18 in a generally horizontal 
position. As appears in FIG. 5, the top surface of the screen is generally 
coplanar with the top edges of the sides 13 and 15 of the trough 14. Also 
as appears in FIG. 5, the concrete slabs 10 and 12 have upper surfaces 
which are generally flush with the upper edges of the walls 13 and 15. 
Affixed to one end of the trough 14, as by welding not shown, is an 
effluent box 28. The abutting wall surfaces of the trough 14 and the 
effluent box have aligned apertures therein which receive a pipe 30. 
Connected to the pipe 30 within the effluent box 28 is a flow control 
valve 32 subject to the regulation of a control member 34. 
Also located in the effluent box is a sump pump 36 to which is connected a 
discharge tube 38. It is contemplated in the practice of the present 
invention that the discharge tube 38 will be connected by means, not 
shown, to return water removed from the effluent box 28 to a sewage works 
being served by the apparatus herein disclosed. 
In a preferred embodiment, the concrete slabs 10 and 12 are each 
twenty-four feet (7.32 meters) wide and one hundred thirty feet (39.62 
meters) long. The upper surfaces of such slabs each slope downwardly 
toward the trough 14 at an approximately two degree angle. Preferably, the 
trough 14 and also the slabs 10 and 12 slope downwardly, an inch or two 
over their entire length, toward the effluent box 28 so that any water 
run-off will be toward the effluent box. 
The trough 14 is constructed from one-quarter inch (0.635 centimeter) 
carbon steel, the trough being two feet (0.61 meter) wide and one hundred 
thirty feet (39.62 meters) long (outer dimensions). The trough 14 is also 
four inches (10.16 centimeters) deep (outer dimension), and the horizontal 
surfaces 25 of the angle bars 16 are located approximately three-quarters 
of an inch (1.91 centimeters) below the upper edges of the side walls 13 
and 15. The support plates 20 are each approximately one foot (0.3048 
meter) long, one-quarter inch (0.635 centimeter) thick and extend upwardly 
approximately three and one-quarter inches (8.26 centimeters) to engage 
the undersides of the loops 22. The support plates are arranged in two 
parallel rows, one spaced about eight inches (20.32 centimeter) from one 
side wall of the trough 14, and the other spaced about eight inches (20.32 
centimeters) from the opposite side wall for the trough 14. The support 
plates in one of such rows are staggered with respect to the support 
plates in the other of such rows; and the support plates in each row are 
spaced approximately one foot (0.3048 meter) apart. The effluent box 28 is 
approximately two feet (0.61 meter) square (outside dimension), and the 
valve 32 is a two-inch (5.08-centimeters) gate valve. 
In the operation of such a unit, the trough 14 is filled to overflowing 
with water from a suitable hose, not shown, so as to submerge the screen 
18. Accordingly, the edges of the slabs 10 and 12 adjacent the trough 14 
may be also partially submerged with water. At this filling time, the 
valve 32 is closed. 
The slabs 10 and 12 are of an adequate width and are preferably adequately 
reinforced to permit small dump trucks or the like, not shown, to travel 
along the length thereof. The dump trucks are operated to deposit on the 
slabs 10 and 12 a layer of sludge which is approximately six to eight 
inches (15.24 to 20.32 centimeters) deep. While the dump trucks are 
operated so as not to deposit sludge directly onto the screen 18, the 
downward slope of the slabs 10 and 12 toward the screen 18 will cause 
sludge to flow onto the screen 18, producing a sludge layer over the 
screen which is supported by underlying water 40 as schematically 
illustrated in FIG. 5. Due to the presence of the support water in the 
trough 14, there will be only a nominal tendency of the sludge to settle 
through the screen 18. 
As shown in FIG. 4, the resulting sludge layer 42 which has been spread 
over the slabs 10 and 12 is spaced inwardly from the edges of the slabs to 
minimize a run-off of water, particularly at the ends of the slabs. 
After deposition of the sludge onto the slabs 10 and 12, the valve 32 is 
opened slowly to permit a controllably slow discharge of water from the 
trough 14 to the effluent box 28. As water enters the effluent box, the 
sump pump 36 is energized to pump the water accumulating in the effluent 
box to whatever works is being served. 
In this phase of operation, it is important that the discharge of water 
from the trough 14 does not exceed the rate at which water drains from the 
sludge toward the trough 14. Thus it is not desired at this stage of 
operation that the level of support water remaining in the trough 14 can 
drop below the level of the screen 18. With experience, a proper setting 
for the valve 32 can be determined which will enable the selection of a 
proper diameter for the pipe 30 and the use of a simple on-off mechanism 
which permits the pipe 30 to be either entirely open or entirely closed. 
Over a period of time, such as twelve to twenty-four hours, all of the 
water capable of freely draining from the sludge will have left the sludge 
and entered the trough 14. The consistency of the sludge is then 
sufficiently pastey and self-supporting that the sludge will not collapse 
through the screen 18 and all support water can be permitted to drain from 
the trough 14. 
The sludge can now be immediately removed or alternatively permitted to 
further dewater by evaporation through the action of the air and sunlight. 
The dewatered sludge is conveniently removed with the aid of front end 
loader trucks, not shown, which are used to scrape the sludge off the 
slabs 10 and 12. Preferably, after the slabs 10 and 12 have been scraped 
free of sludge, plastic blades are attached to the scrapers, not shown, to 
protect the screen 18, and the sludge remaining on the screen is scraped 
off the screen. 
The screen 18 and the slabs 10 and 12 are then thoroughly hosed down, and 
any further sludge thereby removed from the surfaces being hosed is washed 
through the trough 14 and the pipe 30 to the effluent box 28, where such 
remaining sludge can be collected and returned to the works being served. 
While the preferred embodiment is described as covered with a sludge layer 
six to eight inches (15.24 to 20.32 centimeters) thick, it is to be 
understood that such thickness is variable over a wide range, depending 
upon the nature of the sludge being processed and the daily processing 
demands placed upon the dewatering system. Those skilled in the art will 
appreciate, however, that it is desirable to minimize as much as possible 
the thickness of the deposited sludge since, as the thickness increases, 
the overlying sludge tends to compact the underlying sludge with the 
result that water from the upper layers drains less freely through the 
lower layers of sludge. 
One of the particular advantages of the present invention is that a large 
volume of sludge is processed over a large area with the result that the 
compaction of underlying sludge by overlying sludge can be minimized. 
Other advantages residing in the dewatering equipment disclosed in the 
present application are that the screen 18 is easily washed and is 
reusable an indefinite number of times. Further, there is no carry-over of 
sludge from one use of the equipment to the next, such as inevitably 
occurs in the use of sand filters, which in time become clogged or blinded 
with sludge retained in the sand from prior uses and thus require 
backwashing or the like. 
Although the preferred embodiment of this invention has been described, it 
will be understood that various changes may be made within the scope of 
the appended claims.