Course: BFM 203 Water Treatment and Control

Lecture

What is Flocculation

What is Flocculation?

Flocculation is the slow stirring process that causes small, coagulated particles to gather into larger, settleable particles. The process provides contact between particles to promote their gathering into floc for each of removal by sedimentation or filtration. Flocculation helps create floc of good size,density, and toughness. Gentle stirring is the key to good floc formation. Reduction in the amount of stirring motion between the first and last floc basic can be used to create a larger, tougher floc.The best floc size is between 0.1 mm to 3.0 mm depending on the type of removal processes used.

Large settling basins (see photo, top left) are used to gather floc. However, smaller plate or tube settlers can be substituted for the large basin if floc is sufficiently formed and adheres to the plates or tubes. Floc particles are visible in the bottom left photo.

Floc formation is controlled by the rate at which collisions occur between particles and by the effectiveness of these collisions in promoting attachment between particles.

- Operators must take into account a number of process considerations during flocculation:
- Detention time, basin shape, and stirring intensity.
- Insufficient mixing results in poor floc formation.
- Excessive mixing results in tearing the floc apart after it has clumped together.
- Minimum detention time is determined by treatment type and plant design.
- Basins are shaped to prevent short circuiting.
- Stirring intensity can be increased by mechanical means.

Note that basin shape and insufficient mixing may allow floc to settle in the floc tank rather than the settling tank. The most important consideration is selection of the proper type and amount of coagulant chemicals.

Flow and basin parameters must be monitored to ensure consistent conditions exist to create a good floc throughout the process. Jar tests should be run at least daily and more often when the quality of the raw water changes.

What is a jar test? This is a laboratory procedure that simulates a water treatment plant's coagulation/flocculation units with differing chemical does and also energy of rapid mix, energy of slow mix, and settling time. The purpose of the jar test is to estimate the minimum or ideal coagulant dose required to achieve certain water quality goals. Jar testing is used when comparing new chemicals to currently used coagulants and coagulant aids.

The jar test commonly uses six jars. Predetermined amounts of chemicals are added to each jar. The contents are stirred. Testers then observe settling. The chemical dosage that provides the best results should be used in the treatment process. Floc quality and loading should be considered when evaluating jar tests. The jar tester to the right has six connected stirrers so the same conditions are created in each jar.