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CLARIFICATION 7.11

Carriage-Type Collectors. Oscillating-bridge collectors have become more popular
during the past several decades. These top-of-wall running units span the width of one or
more long, rectangular basins. One type cleans thick sludge by means of a single trans-
verse vertical blade about 24 to 30 in. (0.61 to 0.76 m) deep that conveys dense sludge
into a cross hopper. Another type uses a transverse suction header that discharges into a
longitudinal trough along one side of the basin.
The transverse blade has an adjustable angle of attack and slides on flat bottom rails
or sometimes on a heavy neoprene squeegee riding on the floor. The blade automatically
adjusts to varying floor slopes and can be raised above the water level for maintenance.
It pushes dense sludge at about 6 ft/min (1.8 m/min) toward the cross hopper on the clean-
ing run, and then it is hoisted about 3 ft (0.9 m) and travels at double speed back to the
influent end to repeat.
Carriage units generally run on double-flanged iron wheels along heavy steel rails
mounted on the long walls of the clarifier. Rubber tires running on top of concrete walls
have been used occasionally in western Europe, but their use is discouraged in colder cli-
mates because of problems with snow and ice buildup. The units are traction-driven with
automatic compensation to prevent "crabbing" or, preferably, driven on cog rails located
adjacent to steel rails on either side of the basin.
Power to drive and hoist motors is typically supplied through a flexible power cable
reeled in and paid out by a cable reel. Motor-driven cable reels work best because sim-
ple spring motor reels often overstress the cable when the carriage is at the far end. An-
other problem is that they commonly do not have enough reserve force to reel in the ca-
ble at the near end, and they may run over and sever the loose, kinked cable. Several
cleverly designed power reels have been developed, either using a backstay cable to power
the reel as it retrieves the cable or synchronizing the cable reel to carriage travel and em-
ploying a spring motor slave reel core to compensate for minor variations in cable length.
Feed rails are sometimes used for power feed, but they are vulnerable to vandalism
and sometimes burn out or carbonize because of the slow speed of the brushes. Another
form of power supply is an overhead power cable festooned and sliding back and forth
on a taut steel carrier cable running a few feet above the surface for the length of the
basin. This type of power system is unsightly and generally suitable only for short basins.
Cross Collectors/Cross Hoppers. A cross hopper is a trench, typically 3 or 4 ft wide
by 2 or 4 ft deep (1 or 1.2 m wide by 0.6 or 1.2 m deep), running the width of one or
more longitudinal sections of the sedimentation basin. Dense sludge falls into this cross
trench and is scraped at about 2 ft/min (0.6 m/min) by chain-driven flights 8 in. (20 cm)
deep, spaced 5 ft (1.5 m) on centers. These scraper flights deposit dense sludge into a
deeper accumulating hopper at the end of the cross trench. The underflow is withdrawn
hydraulically or by pumping from the hopper. Figure 7.6 shows one type of cross-
collector arrangement.
A helicoid screw is sometimes used in the cross trench in place of chain-driven flights.
The screw turns slowly, paced to have a theoretical capacity of 4 times the volume of
sludge to be actually moved, to minimize bearing wear. The bottom of the cross trench
is filleted to accommodate the outside diameter of the screw. In screw cross collector ap-
plications, instead of propelling dense sludge all the way to one end, the flights of the he-
licoid screw may be opposed so that dense sludge is carried only one-half the trench length
to the center point where the accumulating hopper is placed.
Some designers like to use the traditional steep-sided (60 ° ) hopper to remove sludge
underflow, as illustrated in Figure 7.7. For basins greater than 10 ft (3 m) wide, more than
one hopper must be used to keep the hopper depth within reason. Multiple hoppers can
be more expensive than cross hoppers and not as satisfactory in operation; if they are
joined with a manifold to a common pipe, only one hopper will be served. Sludge must