Civil Engineering Reference
In-Depth Information
greater capacity per unit volume than other equipment available at the time. The sizable
reduction in modular unit tankage greatly increased the capacity attainable in a single
truck-transportable unit. For example, using tube settlers and mixed-media filters, a
single factory-assembled truck-transportable package unit can be built with a capacity
of 1 mgd. Following conventional design criteria, a plant of the same physical dimen-
sions would produce less than 0.25 mgd.
A flow diagram for a conventional package plant is shown in Figure 25-1.
USFilter Waterboy and Aquarius
USFilter manufactures two versions: ''Water Boy'' with a treatment capacity range of
10 to 100 gpm (Fig. 25-2), and the ''Aquarius'' (Fig. 25-3), which generally consists
of dual units with a combined capacity of 200 to 1,400 gpm. As illustrated in the flow
diagrams, raw water is either pumped or flows by gravity to the treatment plant. The
influent flow is adjusted to the desired rate; the control system is designed to start and
stop the treatment plant according to a clearwell level that reflects system demand.
The coagulant and disinfectant chemicals are added at the influent control valve. A
polyelectrolyte coagulant aid is applied as the water enters the flash mix chamber.
After the treatment chemicals are added and mixed, the water is introduced into a
mechanical flocculator designed to form a quick-settling floc. Flocculation detention
time can vary from 10 minutes in small units to 20 minutes in larger units. The
flocculated water is then distributed through a bank of tube settlers, which consist of
many 1-inch-deep, 39-inch-long split-hexagonal-shaped passageways that provide an
overflow rate, related to available settling surface area, of less than 150 gpd / ft
2
. This
overflow rate, together with a settling depth of only 1 inch, results in effective removal
of flocculated turbidity with a detention time of less than 15 minutes.
After passing through the tube settlers, the clarified water flows to a gravity mixed-
media filter. Type A and B modular units utilize a 30-inch-deep mixed-media filter
bed consisting of 18 inches of 1.0 to 1.1 mm effective size anthracite coal, 9 inches
of 0.35 to 0.45 mm effective size silica sand, and 3 inches of 0.20 to 0.30 mm effective
size garnet sand, designed so that there is a uniform gradation from coarse to fine in
the direction of filtration. In all units, the design filtration rate is 5 gpm / ft
2
.
The filters are designed to operate at a constant flow rate. Rate control is accom-
plished with a low-head filter effluent transfer pump discharging through a float-
operated valve. With this means of filter rate control, once the plant raw water flow
is established, there is no change in the filter rate throughout the entire filtration cycle,
provided that the inflow to the plant remains the same. If there is a slight change in
Raw Water
Treated
Water
Rapid
Mix
Floccu-
lation
Sedimentation
Filtration
Fig. 25-1.
Flow diagram of a conventional package plant
