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5.2.4 Flocculent Settling

Particles settling in a water column may have affinity toward each other and coalesce to form flocs or aggregates. These larger flocs will now have more weight and settle faster overtaking the smaller ones, thereby, coalescing and growing still further into much larger aggregates. The small particle that starts at the surface will end up as a large particle when it hits the bottom. The velocity of the floc will therefore not be terminal, but changes as the size changes. Because the particles form into flocs, this type of settling is called flocculent settling or type 2 settling.

Because the velocity is terminal in the case of type 1 settling, only one sampling port was provided in performing the settling test. In an attempt to capture the changing velocity in type 2 settling, oftentimes multiple sampling ports are provided. The ports closer to the top of the column will capture the slowly moving particles, especially at the end of the settling test.

For convenience, reproduce the next equation.

0 Vo

As shown in this equation, the fractional removal R is a function of the settling velocity vp. The question is that if the settling is flocculent, what would be the value of the vp? In discrete settling, the velocity is terminal and since the velocity is terminal, the velocity substituted into the equation is the terminal settling velocity. In the case of flocculent settling, would the velocity to be substituted also be terminal?

In the derivation of Equation (5.37), however, nothing required that the velocity be terminal. If the settling is discrete, then it just happens that the velocity obtained in the settling test approximates a terminal settling velocity, and this is the velocity that is substituted into the equation. If the settling is flocculent, however, the same formula of vp = Zp 2t is still the one used to obtain the velocity. Since removal does not require that the velocity be terminal but simply that removal is proportional to velocity, this velocity of flocculent settling can be substituted in Equation (5.37) to calculate the fractional removal, and it follows that the same formula and, thus, method can be used both in discrete settling as well as in flocculent settling.

Each of the ports in the flocculent settling test will have a corresponding Zp. During the test each of these Zp's will accordingly have corresponding times t and thus, will produce corresponding average velocities. These velocities and times form arrays that correspond to each other, including a corresponding array of concentration. In other words, in the flocculent settling test more test data are obtained. The method of calculating the efficiency of removal, however, is the same as in discrete settling and this is Equation (5.37).

Example 5.8 Assume Anne Arundel County wants to expand its softening plant. A sample from their existing softening tank is prepared and a settling column test is performed. The initial solids concentration in the column is 250 mg/L. The results are as follows:

Sampling Time (min)

ZP /Zo