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7.28 CHAPTER SEVEN

Outlet Design. Proprietary designs also provide for collection of clarified water in dif-
ferent ways. One design uses the upper half of the side inlet channel to collect clarified
water laterally at the top of each plate section. Another design places a collection laun-
der along the top of the plates with a control orifice located above each plate to induce
enough head loss to ensure good flow distribution.
Other designs have uniformly spaced launders with weirs or submerged orifices that
collect the flow along the entire surface. Launders should be spaced on the order of 6 ft
(t.8 m). Submerged orifices should be designed to create sufficient head loss to ensure
good flow distribution. An orifice velocity of 1.5 to 2.5 ft/s (46 to 76 cm/s) will gener-
ally be adequate.
Solids Removal. Settled solids that slide down the plates collect uniformly at the bot-
tom of the basin, and they must be periodically removed. Typical removal equipment
would be chain-and-flight collectors or bottom track units. Circular equipment with drive
units above the plates has also been used.

SOLIDS CONTACT/SLURRY REClRCULATION UNITS
Solids contact units, slurry recirculation, or sludge blanket types are designed to provide
more efficient flocculation and greater opportunity for particle contact within the blanket,
which also acts partially as a filter. The hydraulic design also provides for more uniform
flow and is less subject to short-circuiting. For these reasons, solids contact units can han-
dle 3 to 4 times the hydraulic loading of conventional basins. However, density currents
created by differences in water temperature between the incoming flow or direct sunlight
on the basin surface can be disruptive, by causing the blanket to "boil" with resulting ris-
ing of the floc particles into the effluent. In the most severe situations, the entire sludge
blanket can be lost.
Solids contact units combine flocculation and sedimentation functions into a single
basin. Some units operate with chemical feed directly to the inlet pipe, but a separate rapid
mixer may provide better coagulation for turbidity or color removal applications. A large
volume of previously settled solids is recirculated to the mixing zone to act as nuclei to
form additional floc and to make more complete use of coagulation chemicals. Recircu-
lation rates vary with the application, and may be up to 12 times influent flow for soft-
ening and up to 8 times influent flow for turbidity removal.
Slurry recirculation units were developed primarily for softening applications, where
they have been extensively employed for many years. They have also been used in ap-
plications for turbidity or color removal.

Operation and Design Criteria
The basic theory of solids contact units is that contact of newly formed coagulation par-
ticles with previously formed floc enhances floc formation, creates greater opportunity
for particles to make contact, allows for larger floc development, and allows higher load-
ing rates. Providing this recirculation within a single basin, compared with pumping set-
tled solids to a flocculator in a conventional plant, reduces equipment requirements and
lowers facilities costs. In addition, recirculating settling floc within the basin is less de-
structive to the floc than recirculating after it has settled and had time to thicken and fur-
ther agglomerate. A typical slurry recirculation unit is shown in Figure 7.17.