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7.22 CHAPTER SEVEN

sign. A pair of heavy stirrups reach down from the arms of the circular scraper to move
dense sludge around the hopper to the outlet, to prevent buildup of anchor sludge and grit.
The sludge drawoff pipe should never be less than 6 in. (15 cm) in diameter, and it should
be designed so that a rotor rodder, or "go-devil," can be placed into the line from outside
the basin in case of clogging. In lime softening plants, this line should be given a short
purge of clear water after each blowdown cycle to flush out residual slurry.
The slope of the basin bottom is important, especially when there is heavy or sticky
sludge. Plow blades keep the bottom free of adhesions, literally plowing extremely dense
sludge and grit to the center hopper. Otherwise, the thixotropic sludge flows along the
bottom to replace the blown-down underflow. As the dense sludge approaches the basin
center, the plow blade spacing reduces, and the shorter radius results in reducing tangen-
tial blade velocities.
In large basins, a second set of arms is typically employed to cover the center half
(25% of the basin area) because the blade movement at this point is extremely slow. Deep
blades formed into spiral sections bridge the main and auxiliary arms to push the crowded
sludge into the hopper.
In basins larger than about 80 ft (24 m) in diameter, it is advisable to use a double
bottom slope. The double bottom slope is essential for basins more than 125 ft (38 m) in
diameter. The two-slope design gives steeper slopes (greater hydraulic gradient) at the
sludge hopper without excessive basin depth. The greater center depth dissipates scour-
ing currents and is needed because of the concentration of influent energy in this rela-
tively small region.

Square Basins
Basins larger than 30 ft (9 m) square are typically equipped with circular sludge collec-
tion systems with comer sweeps, always in pairs, to clean sludge from comers. Comer
sweeps eliminate the need for larger comer fillets, which are generally unacceptable ex-
cept in very small basins. Comer sweeps should be avoided in basins larger than 100 ft
(30 m) square because of structural problems and wear associated with large cantilevered
comer sweep units.
Hydraulic problems usually occur with larger square basins. Radial density and dis-
placement currents impinge on the peripheral walls at various angles, drift toward the cor-
ners, meet, and may cause a rising "comer floc" phenomenon that often contributes tur-
bidity to effluent.
Where inboard weir troughs are used in square tanks, they are designed to cut across
the comer to avoid rising corner floc. Where radial troughs are used (as is typical with
upflow basins), they are always arranged to straddle the corner for the same reason. It is
best not to run a single peripheral weir around the walls of a square basin because of the
comer floc effect. Conventional square basins are not generally recommended.

HIGH-RA TE CLARIFICATION
High-rate clarification refers to all processes that can be loaded at higher rate than is typ-
ically used in designing conventional clarifiers. The principal types of units currently be-
ing used are
• Tube settlers
• Plate settlers
• Solids contact units