Welcome to Secondary Sedimentation...

Floculation/Secondary Sedimentation

• Floculation...

Following the dispersal of the coagulants into the water and the completion of the destabilization reactions, Van der waals forces will act to join the colloids if sufficient collisions of the destabilized colloids can occur. The purpose of the flocculation step is to create a gentle agitation to encourage collisions of the colloids to form larger floc particles. In the flocculation step, much less power dissipation is required than in the rapid mix step. If rapid mixing continues too long, or if the flocculation is too violent, the floc will shear apart as it forms, and no adequate floc formation will occur.

As such, flocculation is generally accomplished by gentle agitation, often using large horizontal or vertical slow moving paddles in vessels much larger than a vessel containing a mechanical mixer. Mean velocity gradients of 20 to 70/ second are usually provided for adequate flocculation. Detention time in the flocculation step is much longer in than in the rapid mix step, usually ranges from 30 to 45 minutes.

• Secondary Sedimentation...

In the sedimentation step, water containing the large floc particles which have formed in the flocculator flows to a much larger vessel, and the velocity of water flow is slowed down in order to allow the floc particles to settle. Sedimentation basins are usually built to allow a minimum of two to four hours of detention time. Sedimentation basins can be rectangular, square, or circular. Usually the basins are sloped on the bottom to allow the sludge to collect at one point to facilitate removal. Older basins are usually not equipped with continuous sludge removal devices. These basins are taken out of service periodically, and the sludge is removed with a fire hose. Newer basins may be equipped with automatic sludge removal devices which periodically blow sludge from the bottom of the basin. These require very little downtime.

Flocculation and sedimentation facilities should be built at least in duplicate so that when the facilities are removed from service for desludging or maintenance, the plant may still operate.

The settled water is collected at the effluent of the sedimentation basin by collecting weirs or submerged openings. Settled water is always collected at the top of the basin to avoid collecting any of the settled floc particles. Generally, the weirs or openings have a large amount of surface area, which helps to keep the velocity of the water as low as possible as the water leaves the sedimentation basin. This helps to prevent the breakup of any residual floc particles which have not settled. Regardless of the quality of the source water, an effective coagulation, flocculation, and sedimentation process should produce water which generally does not exceed an average turbidity of 5 NTU.

Navigate Belmont WTA's Site: