Inclinedsurface Clarifiers

Inclined-surface basins, also known as a high-rate settler, use inclined trays to divide the depth into shallower sections. Thus, the depth of all particles (and therefore the settling time) is significantly reduced. Wastewater treatment plants frequently use this concept to upgrade the existing overloaded primary and secondary clarifiers. Part B

EFFLUENT BAFFLE ADJUSTABLE

EFFLUENT BAFFLE ADJUSTABLE

FIG. 7.17.2 Parts of a rectangular basin. (Courtesy of the FMC Corp., Material Handling Systems Division)

Sludge

Circular-solids-contact clarifier

Directio of flow

Sludge

Directio of flow

To sludge collection

Counter-current flow in tubes

To sludge collection

Circular-solids-contact clarifier

Counter-current flow in tubes nfluent

Sludge

Effluent

Parallel-inclined-plates in a circular clarifier

Sludge

Effluent

Tube settlers in a rectangular clarifier

Parallel-inclined-plates in a circular clarifier

Tube settlers in a rectangular clarifier

FIG. 7.17.4 Types of clarifiers. A. Circular-solids-contact clarifier. B. Parallel inclined plates in a circular clarifier. C. Tube settlers in a rectangular clarifier. D. Counter-current flow in tubes.

in Figure 7.17.4 shows the operating principles of inclined surface clarifiers.

Inclined-surface clarifiers provide a large surface area, reducing clarifier size. No wind effect exists, and the flow is laminar. Many overloaded, horizontal-flow clarifiers are upgraded with this concept. The major drawbacks of the inclined-surface clarifiers include:

• Long periods of sludge deposits on the inner walls can cause septic conditions.

• The effluent quality can deteriorate when sludge deposits slough off.

• Clogging of the inner tubes and channels can occur.

• Serious short-circuiting can occur when the influent is warmer than the basin temperature.

Two design variations to the inclined-surface clarifiers are tube settlers and parallel-plate separators.

Tube Settlers

In these clarifiers, the inclined trays are constructed with thin-wall tubes. These tubes are circular, square, hexagonal, or any other geometric shape and are installed in an inclined position within the basin. The tubes are about 2 ft long and are produced in modules of about 750 tubes. The incoming flow enters these tubes and flows upward. Solids settle on the inside of the tube and slide down into a hopper.

The most popular commercially available tube settler is the steeply inclined tube settler. The angle of inclination is steep enough so that the sludge flows in a countercurrent direction from the suspension flow passing upward through the tube. Thus, solids drop to the bottom of the clarifier and are removed by conventional sludge removal mechanisms.

Test results for alum-coagulated sludge indicate that solids remain deposited in the tubes until the angle of inclination increases to 60° or more from the horizontal.

Parallel Plate Separators

Parallel-plate separators have parallel trays covering the entire tank. The operational principles for these separators are the same as those for the tube settlers.

Other Inclined-Surface Separators

Another design of shallow-depth sedimentation uses lamella plates (see Figure 7.17.5), which are installed parallel at a 45° angle. In this design, water and sludge flow in the same direction. The clarified water is returned to the top of the unit by small tubes.

Flow distribution orifices Discharge flumes

Feed box

Overflow box

Flocculation tank

Overflow (effluent)

Feed (influent)

Flocculation tank

Overflow box

Feed (influent)

Flash mix tank

Coagulant aid

Overflow (effluent)

Picket fence sludge thickener

Underflow (sludge)

FIG. 7.17.5 Lamella plates. (Courtesy of Parkson Corp.)

Flash mix tank

Coagulant aid

Picket fence sludge thickener

Underflow (sludge)

FIG. 7.17.5 Lamella plates. (Courtesy of Parkson Corp.)

Effluent

Effluent nfluent

Slotted distribution Dead Space

A. Good design B. Effect of density flow or thermal stratification

A. Good design B. Effect of density flow or thermal stratification

Wind-driven circulation cell

C. Effect of thermal stratification

D. Effect of wind in formation of circulation cell

FIG. 7.17.6 Flow patterns in rectangular sedimentation tanks.

Dead space

Wind-driven circulation cell

C. Effect of thermal stratification

D. Effect of wind in formation of circulation cell

FIG. 7.17.6 Flow patterns in rectangular sedimentation tanks.

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