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CLARIFICATION 7.25

SLUDGE

HYDR(NIYCL ~1
BALLASTED FLOCS
TO HYDROCYCLONE

CO/

RAW
WATER

FIGURE 7.14 ACTIFLO process for water treatment. (Courtesy of Kruger Inc.)

7.14. In this process, a microsand is added after chemical coagulation, but before floccu-
lation, to act as a nucleus for floc formation. The sand adds weight to the floc, thus in-
creasing its settling velocity and allowing for tube clarifier loading rates of up to 40 gpm/ft 2
(100 m/h), although more typically in the 15 to 20 gpm/ft 2 (23 to 50 m/h) range. The ef-
fect is to provide a substantially reduced settling area compared to tube settlers without the
sand. The microsand also improves the speed at which floc formation takes place, which
further reduces space requirements. Figure 7.14 shows the sequence of coagulation, mi-
crosand addition, maturation (flocculation), and clarification. Sludge is typically collected
with a circular scraper and is pumped to a hydroclone for reclamation of the microsand.
The solids side stream is very dilute and may require additional treatment before sending
to thickeners or to a sewer system. If the stream is disposed to a lagoon, the dilute nature
of the solids would not be a problem. The side stream also may be as high as 4% to 5%
of the plant flow, so additional treatment is also recommended to reclaim the water.

Plate Settlers
Plate settlers were developed to improve the efficiency of conventional rectangular set-
tling basins by taking advantage of the theory that settling depends on the settling area
rather than detention time. Plate settlers date back to an English patent in 1886 (Purac)
and were used primarily in the mining and mineral industries to separate heavy particles
from slurries. In the late 1950s they were developed in Europe for treating drinking water.

Theory. Based on sedimentation theory, shallow basins provide the same settling as a
deep basin. However, horizontal shallow basins are subject to the scouring action of the
flow-through velocity, which lowers removal efficiency. Also, if horizontal shallow basins
are stacked to reduce plant area, there is no easy method of sludge removal. For these
reasons, plate settlers are designed to be vertically inclined, similar to tube settlers, to al-