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7,36 CHAPTER SEVEN

Inlet Design

Flow to the basin is controlled through the inlet distribution system. Flow from the vac-
uum chamber typically enters a conduit serving two halves of the basin. From this con-
duit, laterals extend to either side to provide uniform flow across the bottom of the basin.
Laterals are spaced at about 3.5 ft (1.1 m) on center. Orifices in these laterals further serve
to equally distribute flow across the basin. Orifices are designed to provide a certain en-
trance velocity based on the vacuum chamber hydraulic head, to ensure even flow distri-
bution and create an energy level to enhance flocculation in the mixing zone as flow en-
ters the basin. Baffles above the inlet laterals further ensure uniform flow to the blanket.

Outlet Design. Clarified water is collected at the clarifier surface in uniformly spaced
laterals with submerged orifices. The lateral spacing is about 6.5 ft (2 m). Orifices are de-
signed to induce enough head loss to aid in maintaining uniform upflow velocities in the
blanket. Laterals discharge to an effluent channel located above the sludge concentrators.
Submerged orifices are used because flow through them is not as affected by the pulsing
action of the water surface as it would be over a weir.

Sludge Removal. As the sludge blanket builds in volume, it rises above the level of an
overflow weir to sludge concentrators, which are hoppers located between the two halves
of the basin. Sludge flows into concentrators, where it is allowed to partially thicken. It
is periodically drawn off through timer-controlled valves, usually by gravity, to a sump
from which it may be pumped or flow by gravity to sludge handling facilities. No me-
chanical equipment is used, which is one of the attractive features of this type of clarifier.

DISSOLVED AIR FLOTATION

In flotation, the effects of gravity settling are offset by the buoyant forces of small air
bubbles. These air bubbles are introduced to the flocculated water, where they attach to
floc particles and then float to the surface. Flotation is typically sized at loading rates up
to 10 times that for conventional treatment. Higher rates may be possible on high-quality
warm water.
Dissolved air flotation (DAF) is an effective alternative to sedimentation or other clar-
ification processes. Modern DAF technology was first patented in 1924 by Peterson and
Sveen for fiber separation in the pulp and paper industry (Kollajtis, 1991). The process
was first used for drinking water treatment in Sweden in 1960 and has been widely used
in Scandinavia and the United Kingdom for more than 30 years.
Previous uses of the process in the United States have been to thicken waste-activated
sludge in biological wastewater treatment, for fiber separation in the pulp and paper in-
dustry, and for mineral separation in the mining industry. Only recently has this process
gained interest for drinking water treatment in North America. It is especially applicable
when treating for algae, color, and low-turbidity water. The first use in the United States
was at New Castle, New York, in a 7.5 mgd (28 ML per day) plant that began operation
in 1993. A typical DAF unit is shown in Figure 7.21.

Theory and Operation
Effective gravity settling of particles requires that they be destabilized, coagulated, and
flocculated by using metal salts, polymers, or both. The same is true for DAF. In gravity
settling the flocculation process must be designed to create large, heavy floc that settles