Closer Look At The Mechanical Clarification Process And The Chemistry Of Clarification

So by now it should be clear that what the process of clarification is all about is removing suspended solids from water. Important concepts that we have eluded to, but maybe not spelled out so clearly up to now are:

1. Stable solids suspensions in water- The mechanisms involved in keeping solids suspended in water,

2. Chemical treatments -How organic polymers and inorganic coagulants work to counteract solids stabilization mechanisms and enhance removal of solids from water, and

3. The function of clarification unit operations- How these units work and how chemical treatment enhances their performance.

A term that we should get into our vocabulary is "subsidence". This term essentially means settling. While a degree of clarification can be accomplished by subsidence, most industrial processes require better quality water than can be obtained from settling only. Most of the suspended matter in water would settle, given enough time, but in most cases the amount of time required would not be practical. As we have shown from our derivations of expressions describing the classical theory of sedimentation, settling characteristics depend upon the :

1. Weight of the particle,

2. Shape of the particle,

3. Size of the particle, and

4. Viscosity and/or factional resistance of the water, which is a function of temperature.

The settling rates of various size particles at 50° F (10° C) is illustrated in Table 1.

Table 1. Some Settling Rates for Different Particles (assumed spherical) and Sizes

Particle Diameter (mm)

Particle Type

Time to Settle One Foot

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