Scum control system for peripheral feed channel of sedimentation tank

A peripheral feed sewage treatment final clarifier having a channel with lower distribution ports is provided with a vertically adjustable riser which is placed in the port where the scum tends to collect in the channel. The riser is set vertically so that diurnal variations in the elevation of the flow in the channel raises and lowers the collected scum past the upper lip of the riser at which times the scum is removed.

BACKGROUND OF THE INVENTION 
The typical wastewater treatment plant includes a final clarifier in which 
the suspended solids from the prior treatment step is allowed to settle 
out. These solids generally have very low settling rates which makes the 
hydraulic efficiency of the tank quite important. Until about 1958, a 
final clarifier was invariably of the center feed type; since then the 
advantages of peripheral feed as disclosed in U.S. Pat. Nos. 2,961,099 and 
2,961,100 have been demonstrated. 
In such tanks, the peripheral feed channel or channels are of diminishing 
cross-section and can be provided with a series of relatively large ports 
which are relatively free of clogging. Generally, all of such ports may be 
of the same size and spaced to provide the equal distribution of the feed 
around the tank. Where especially large ports are provided, a lower 
distribution baffle as in U.S. Pat. No. 3,228,530 may also be provided. 
In recent years the waste in some treatment plants has included grease 
solids which are not always entirely removed by the prior treatment step; 
this has made it necessary to provide for skimming of the tank and the 
peripheral feed channel. 
U.S. Pat. No. 3,136,724 shows manually operable gates for the selective 
removal and disposal of the scum which has collected at the ends of the 
peripheral feed channels. The object of the present invention is to 
provide dependable and readily adjustable means for removing the 
accumulated scum from the feed channel but which does not require manual 
attention or an automatic timing and operating mechanism. The invention is 
based upon the regular, daily variation in the flow through the usual 
sewage treatment plant and the observation that as the flow increases, 
somewhat more grease passes through the plant untreated and the 
accumulation continues to about the time the flow rate diminishes towards 
its low day- or night-time rate. 
BRIEF SUMMARY OF THE INVENTION 
At critical flows, scum tends to collect in the terminal portions of the 
open peripheral feed channel of the final clarifier in an activated sewage 
treatment system. The channel has a series of lower ports of uniform size; 
a vertically adjustable riser is placed in the selected port where such 
scum tends to collect and provides for scum withdrawal as the diurnal 
fluctuations in the elevation of the flow passes a critical elevation 
which is about one quarter of an inch above the lip of the riser. The scum 
is not directly removed from the tank but is rejoined with the flow into 
the tank and is removed by the surface skimming apparatus within the tank. 
At high flow rates the riser allows the normal flow through the port; at 
lower flow rates the port is closed without any adverse effect.

DESCRIPTION OF THE PREFERRED EMBODIMENT SHOWN IN THE DRAWINGS 
The sedimentation tank 11 or clarifier shown in the drawings is of concrete 
construction and includes the floor 12 and circular sidewall 13. Floor 12 
slopes downwardly to the central sludge receiving sump 14 to which the 
sludge withdrawal pipe 15 is connected. The feed channels and effluent 
trough or launder as shown are also of concrete construction and integral 
with sidewall 13. 
The upper part of sidewall 13 and the concentric inner wall 19 define the 
sides of the left and right feed channels 22 and 23. Said feed channels 
extend in opposite directions around the tank 11 from the inlet or feed 
box 24 and their floors 25 and 26, respectively, slope upwardly from feed 
box 24 to their adjoining ends. Each floor 25 and 26 is provided with a 
series of ports 27 and 28 through which the feed passes into the upper end 
of a relatively large annular distribution chamber which extends fully 
around tank 11. As shown, there is also a first port 29 at the feed box 24 
and an opposite port 30 at the adjoining shallow ends of channels 22 and 
23. In practice, a minimum depth of the feed channels 22 and 23 may be 
required to prevent vortexing, as will be mentioned again. For that 
purpose the terminal portions of channel floors 25 and 26 may be flat 
while their major portions slope upwardly from feed box 24. 
In the construction of tank 11, the ports 27-30 are defined by steel liners 
such as liner 31 shown in FIG. 4 and which are left in place. The 
distribution chamber 33 referred to is defined by the undersides of 
channel floors 25 and 26, the annular skirt 34 extending downwardly 
therefrom, and the tank sidewall 13. 
The feed passes below skirt 34 to enter the tank and flows toward the 
center over the tank floor 12. The clarified flow then moves upwardly and 
toward the peripheral overflow launder 35 which is formed by the inner 
wall 19 of channels 22 and 23, the floor 36 projecting therefrom and the 
weirwall 37. The effluent pipe 38 from launder 35 extends through skirt 34 
and sidewall 13 of tank 11. 
Various means for moving the settled sludge to sump 14 may be provided. In 
the drawings an eductor 41 is shown; it is supported and moved over floor 
12 by the drive column 42 and the upper drive mechanism 43 which may be 
mounted on a central pier, not shown, or supported by a bridge, not shown. 
As shown in FIGS. 1 and 2, drive column 42 also supports a rigid skimming 
blade 45 which extends from the column to the pivoted blade 46 which is 
adapted to operate up and over the scum receiving trough 47 shown only in 
FIG. 2. The operation of skimming blades 45 and 46 and the scum trough 47 
is entirely conventional. For reference purposes, U.S. Pat. No. 3,770,132 
shows such apparatus in another type of settling tank. 
The upper extension arm 50 is supported from blade 45 and extends over the 
channels 22 and 23 as the arm is rotated by the column. Arm 50 carries the 
flexible pusher or blade 51 which extends downwardly between sidewall 13 
and inner wall 19. Such a pusher is shown in FIG. 5 of U.S. Pat. No. 
3,717,257. Its operation in connection with the present invention will be 
described hereinafter. 
In the operation of tank 11, the feed from pipe 50 enters the feed box 24 
and flows into the two channels 22 and 23. It is important to note (a) 
that the elevation of the surface of the liquid in the tank varies only 
from a fraction of an inch above the weir of wall 36 at low flows and for 
example only up to two inches above the weir at the maximum, or design 
flow of the tank and (b) that the headloss through the ports 27 - 30 is 
with reference to whatever that elevation is. In a typical installation, 
for example here, the headloss through ports 27 - 30 varies from 1/4 inch 
(6.35 mm) at low flows to 7 inches (17.78 cm). On the other hand, the 
headloss required for the flow to the last ports at the ends of the 
channels may be considered nominal. In the typical installation, the ports 
27 - 30 may be of a 4 inch (10.16 cm) diameter and the minimum depth of 
channels 22 and 23 would be about 4 inches so that vortexing is precluded. 
In a typical municipal sewage treatment plant, at low flows the grease 
content of the raw sewage entering the sewage treatment plant is 
relatively low; as the flow increases due to laundry and industrial 
activity, the grease content increases and at a near to maximum flow, some 
of the grease passes through the preceding treatment step and is carried 
into the tank. The amount of such grease is usually such as to require 
regular or daily skimming of the tank as with the apparatus described. In 
general, most of the grease is carried through ports 27 - 30 but the 
fraction which rises to the top of the flows in channels 22 and 23 is 
carried toward port 30 where it collects until it extends over some number 
of the ports and must eventually be removed. To a degree, the flexible 
blade 51 pushes the scum downwardly so that it is passed into the tank, 
and particularly through the ports 28 of channel 23 in which the blade 
moves against the flow in the channel. 
According to the present invention, the vertically adjustable pipe or riser 
61 may be set in port 30 or the last port 28a of channel 23 where by 
observation it is determined that scum tends to collect between passes of 
blade 51. As shown in FIGS. 2-4, the riser 61 is set in the second last 
port 28b of channel 23. The upper lip of riser 61 also is set at an 
elevation intermediate the minimum and maximum elevations of the flow in 
the channels and more particularly as will be described. At low flows when 
the elevation is below the lip of riser 61, there is of course, no flow 
through port 28b but this is of no general consequence as to the hydraulic 
efficiency of the tank. 
As the flow through the plant increases during the day, at some point, the 
channel flow reaches and slowly passes the lip of riser 61. During that 
period, the flow through port 28b is drawn from the surface of the flow 
and would include whatever grease is present. During the day as the flow 
rate continues to increase and decrease again, the port 28b is essentially 
operative as it would be without the riser pipe. That is, the surface of 
the feed in the channels is well above riser 61 and the flow through riser 
61 and port 28b is drawn from the body of the liquid. It is during this 
period that the scum begins to collect in greatest quantity. As the 
elevation in channel 23 approaches again the lip of riser 61, this grease 
flows into the riser and into tank 11 from where it is thereafter removed, 
as described. 
The riser 61 may comprise merely a length of pipe as shown which fits 
within the selected port. As shown in FIG. 4, port 28b is defined by the 
steel liner which is set in the concrete of floor 23 and the lower end of 
riser 61 fits therein and is held upright thereby. The collar 63 on riser 
61 includes the set screw 64 which allows the collar to be secured on the 
riser where desired. When set in place, collar 63 rests on floor 26 to 
support the riser vertically. 
As mentioned according to the present invention, the vertically adjustable 
pipe or riser 61 may be set in any of various ports. Ordinarily, riser 61 
would be set in port 30 which is the last port of the two channels. In 
some tanks, a single feed channel is provided, as shown for example in 
FIG. 8 of U.S. Pat. No. 3,717,257. The riser would then be set in the last 
port of such a channel, assuming here that the pusher or blade 51, if 
provided, pushes in the direction of the flow. 
Assuming, with reference to FIG. 4 disclosing the present invention, that 
blade 51 moves in a clockwise direction, blade 51 pushes the collecting 
scum from channel 22 some distance into the terminal portion of channel 23 
and against the flow in channel 23. Thus, the scum is here most likely to 
collect over port 28a so that riser 61 is best placed in port 28b as shown 
so that the scum will be pushed toward the riser. In other instances, 
riser 61 could be placed in port 28a or in port 28c which is the third 
last port of channel 23. 
The profile of the terminal portions of channels 22 and 23 are shown in 
FIG. 3. The horizontal upper and lower broken lines respectively indicate 
the levels of the flow in the channels at maximum and minimum daily flow 
rates. As described, the riser 61 is vertically adjustable so that its 
upper lip can be selectively positioned between such upper and lower 
levels. 
As shown, the riser 61 is placed in the second last port of channel 23 but 
could be placed in another port wherever the grease or scum regularly 
collects. The broken lines from the ports 27, 28 and 30 indicate only 
diagrammatically the distribution of the flow from the ports to the lower 
edge of the skirt 34 of tank 11 where the flow then enters the tank. 
Various embodiments of the invention may be employed within the scope of 
the following claims.