Review Of Equipment Options

Although we have discussed the major hardware, it is still worthwhile reviewing these in relation to the major classes of clarifier processes. The major categories of this process are:

• conventional

• solids-contact

• sludge-blanket

Conventional clarification is the simplest form of the process. It relies on the use of a large tank or horizontal basin for sedimentation of flocculated solids. Figure 21 provides a sketch of the basic configuration. The basin normally contains separate chambers for rapid mix and settling. The first two steps critical in achieving good clarification. An initial period of turbulent mixing is needed for contact between the coagulant ans suspended solids. This is followed by a period of gentle stirring which helps to increase particle collisions and floe size. Retention times are typically between 3 and 5 minutes, 15 to 30 minutes for flocculation, and 4 to 6 hours for settling. Coagulants are added to the wastewater in the rapid mix chamber, or sometimes immediately upstream. The water passes through the mix chambers and enters the settling basin. Refer again to Figure 21, which is a classical large-tank clarifier. The water passes out to the circumference, while the flocculated particles settle to the bottom. Accumulated sludge are scraped into a sludge collection basin for removal and disposal (sometimes post processing, as discussed in Chapter 10). The clean water flows over a weir and is held in a tank , which is referred to as a clearwell. A rectangular version of a conventional clarifier is illustrated in Figure 22. This unit is referred to as a horizontal basin clarifier.

Drive

Drive

Figure 21. Sketch of a large tank or circular clarifier.
Figure 22. Sketch of a horizontal basin clarifier.

It is often advantageous to employ a zone of high solids contact to achieve a better quality effluent. This is accomplished in an upflow clarifier, so called because the water flows upward through the clarifier as the solids settle to the bottom. Most upflow clarifiers are either solids-contact or sludge-blanket type clarifiers, which differ somewhat in theory of operation. Cross-sections of these two types of units are illustrated in Figures 23 and 24. Both units have an inverted cone within the clarifier. Inside the cone is a zone of rapid mixing and a zone of high solids concentration. The coagulant is added either in the rapid mix zone or somewhere upstream of the clarifier.

Agitator

Agitator

Figure 23. Sketch of a solids-contact clarifier.

Agitator

Agitator

Figure 24. Sketch of a sludge-blanket clarifier.

In the solids-contact clarifier, raw water is drawn into the primary mixing zone, where initial coagulation and flocculation take place. The secondary mixing zone is used to produce a large number of particle collisions so that smaller particles are entrained in the larger floe. Water passes out of the inverted cone into the settling zone, where solids settle to the bottom and clarified water flows over the weir. Solids are drawn back into the primary mixing zone, causing recirculation of the large floe. The concentration of solids in the mixing zones is controlled by occasional or continuous blowdown of sludge.

Some Application Pointers for Clarifiers l. 2.

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