Page 158 -

CLARIFICATION 7.21

A more controllable feed design includes using a distribution well inside a large feed
well that is about 3% to 4% of the basin area. This distribution well has multiple ports
hooded with adjustable biased gates. The gates balance tangential feed discharges by im-
posing about a 4-in. (10-cm) head loss through the ports. This type of discharge causes
the homogenized mass within the large feed well to rotate around the vertical axis at about
2 ft/s (61 cm/s). The well-distributed, fine-scale turbulence within and below the feed well
encourages floc aggregation, and the overall slow rotation ensures that flow from the bot-
tom of the skirt into the hindered sludge mass moving radially across the floor has equal
displacement vectors.
Density and displacement currents for circular basins are much the same as for long,
rectangular basins. The vector system is influenced by well-flocculated influent mass sink-
ing to the bottom adjacent to the feed well area, typically in the center one-third of the
basin (about 10% of the total basin area). The vector system shows displacement radially
along the bottom in the blanket zone and upwelling next to the peripheral wall. Clarified
water generally flows across the surface toward the effluent.

Outlet Design. Clarified water collection must be uniform around the perimeter of the
basin. This is accomplished by a circular trough around the perimeter with V-notch weirs
or with submerged orifices. Some designs use a double-sided weir trough mounted in-
board along at least 15% of the tank radius. This has the advantage of reducing wall flow
disturbances and drawing overflow from a more widely distributed region to offset the
effects of bottom density currents running up the peripheral wall. Inboard weir troughs
also partially break up wind current stirring. Troughs should have small-diameter holes
in the bottom to reduce buoyant uplift forces when they are empty.
Some designers prefer orifice troughs to overflow weir troughs because less floc breakup
occurs. Others point out that the velocity gradient in a weir trough is no greater than in an
orifice trough. Weir troughs are far easier to adjust for equal linear overflow, but if not
properly adjusted, they have greater variation in flow than improperly adjusted orifices.
Submerged orifice troughs reduce passage of floating trash to the filters and permit varia-
tion in basin water depths during operation. This capability is useful for balancing differ-
ences in plant inflow and discharge rates, such as when multiple filter washing occurs.
Regulatory agencies sometimes stipulate that weir rates should not exceed around
20,000 gpd per linear foot (248,000 L/m) of weir. Flood (1961) found that weir overflow
rates several times this value could be used if the weirs were well distributed over a sub-
stantial portion of the surface. Placing a double-sided weir trough 1 ft (0.3 m) away from
the peripheral wall satisfies the regulatory requirements, but still draws overflow from a
narrow band of surface immediately in the path of the upwelling peripheral flow.

Sludge Removal Sludge is removed from circular basins using circular collection equip-
ment powered by a center turntable drive and plows that move sludge into a center sludge
hopper.
Turntable Drives. The tried and true, relatively trouble-free drive for both bridge-
supported and pier-supported circular collectors is the sealed-turntable drive with the gear
and pinion running in oil. Properly lubricated and with automatic condensate overflow,
these drives operate for years without major repair. Typical turntable drives rotate on re-
newable bearing strips, and the gear is split so that the ball bearings and strips can be re-
placed without dismantling the remainder of the equipment.
These drives are protected by an indicator and overload circuit breaker device actu-
ated by the thrust of the primary worm gear driving the pinion and turntable gear. The
indicator senses the torque load exerted on the collection arms by the sludge and turns
off power if the load exceeds a preset limit.
Sludge Hopper and Bottom Slopes. Because a circular sludge hopper surrounding a
central pier holds the greatest volume, it is preferable to the older-style offset hopper de-