The F.B. Leopold Company Inc.

Methods of Clarification

There are two fundamental methods of clarification, one distinguished by the use of gravity, the other distinguished by the use of buoyancy flotation.

SEDIMENTATION

Sedimentation employs the force of gravity to cause solids particles to fall through the water under the influence of gravity to the bottom of the tank where it is collected and removed. Particles whose density is greater than the density of the water will settle out. The greater the density of the particle, the faster it will settle out. The speed at which the particles fall through the water is called "settling velocity."

As the accompanying illustration shows, the settling of a particle has both horizontal and vertical dynamics in a horizontal-flow rectangular tank. The objective is to increase VV to maximize the removal of solids particles from water. This is done by chemical coagulation, a process in which a polymer is introduced to the water for binding small particles into larger aggregates, increasing their density and their settling velocity.

Conventional Sedimentation

Most sedimentation clarifiers used in water treatment are horizontal-flow. Horizontal-flow clarifiers may be either rectangular or circular. In water treatment, rectangular sedimentation clarifiers are the most common. Most sedimentation clarifiers are continuously cleaned with mechanical equipment.

Conventional sedimentation clarifiers are long and rectangular for stable flow characteristics and sedimentation performance. The influent flow is uniformly distributed across the end of the clarifier and travels across the length of the clarifier. The Effluent overflows into long weirs or into cross-troughs. Floating sludge collectors or submerged sludge collectors are used to remove sludge from the bottom of the clarifier.

High-Rate Sedimentation

High-rate sedimentation is accomplished with smaller clarifiers employing inclined plates or tubes. These clarifiers can work with shorter detention times and higher surface-loading rates.

Inclined-Plate Sedimentation

Inclined-plate sedimentation clarifiers consist of a series of plates inclined 55° from horizontal. Flow enters at the side, near the bottom of each plate, and exits at the top. Solids that settle on the plates slide by gravity to the bottom of the clarifier. The Effluent is collected by a header system. Because the floor of the clarifier is not available from the surface of the water, sludge is removed by submerged sludge collectors

Tube Sedimentation

Tube sedimentation clarifiers employ a system of inclined tubes. The influent flows up through the tubes and the sludge moves down the tubes by gravity to the bottom of the clarifier. Several possible configurations of tube shapes and sizes are used. The tubes occupy two-thirds to three-fourths of the clarifier. The open area at the entrance of the clarifier provides for removal of readily settleable floc by conventional sedimentation. Again, because the floor of the clarifier is not available from the surface of the water, sludge is removed by submerged sludge collectors.

FLOTATION

Buoyancy flotation involves floating the solid particles to the surface of the water where they are removed as floating sludge, usually by mechanical skimming. This is accomplished by the attachment of bubbles to the particle surfaces. This causes the apparent density of the bubble-solid agglomerates to be less than that of the water. The efficiency of clarification by flotation is improved by chemical coagulation. The creation of larger agglomerates of solids particles, or floc, provides more surface area for the attachment of bubbles.

Dissolved-air flotation (DAF) is a method whereby bubbles are produced by the reduction of pressure in a water stream saturated with air. The air is dissolved in water under pressure according to Henry's Law of Dissolution. Releasing the pressure back to STP (normal conditions or standard temperature and pressure) via a special device creates millions of "microbubbles" approximately 30 to 100 microns in diameter. These tiny bubbles surround slow-settling particles and float them to the surface for removal.

Dissolved Air Flotation (DAF) is a particularly capable technology for removing filter-clogging and taste- and odor-causing algae and other low-density particles that cannot be removed by gravity sedimentation. A typical low-turbidity raw water particle count is 20 to 30 million per gallon. The presence of large numbers of bubbles increases the chance of bubbles attaching themselves in very dilute floc suspensions typical in water treatment. At an 8 percent recycle flow at a 70-psig saturation pressure, the number of 40-micron-diameter bubbles created at 40°F is 681 million per gallon. At 68°F the number of bubbles per gallon is 530 million.

A typical DAF process train consists of chemical coagulation, flocculation, air injection, and flotation.