Preliminary Treatment Sampling And Testing

During normal operation of grit removal systems (with the exception of the screening and shredding processes), the plant operator is responsible for sampling and testing as shown in Table 12.1.

12.10.1 Other Preliminary Treatment Process Control Calculations

The desired velocity in sewers in approximately 2 fps at peak flow, because this velocity normally prevents solids from settling from the lines; however, when the flow reaches the grit channel, the velocity should decrease to about 1 fps to permit the heavy inorganic solids to settle. In the example calculations that follow, we describe how the velocity of the flow in a channel can be determined by the float-and-stopwatch method and by channel dimensions.

■ Example 12.10. Velocity by Float-and-Stopwatch

,, , .. ,, , Distance traveled (ft) Velocity (fps) = (12.9)

Time required (s)

TABLE 12.1 SAMPLING AND TESTING GRIT REMOVAL SYSTEMS

Process

Location

Test

Frequency

Grit removal (velocity)

Influent

Suspended solids (fixed)

Variable

Channel

Depth of grit

Variable

Grit

Total solids (fixed)

Variable

Effluent

Suspended solids (fixed)

Variable

Grit removal (aerated)

Influent

Suspended solids (fixed)

Variable

Channel

Dissolved oxygen

Variable

Grit

Total solids (fixed)

Variable

Effluent

Suspended solids (fixed)

Variable

Chemical addition

Influent

Jar test

Variable

Preaeration

Influent

Dissolved oxygen

Variable

Effluent

Dissolved oxygen

Variable

Equalization

Effluent

Dissolved oxygen

Variable

Problem: A float takes 30 s to travel 37 ft in a grit channel. What is the velocity of the flow in the channel?

Solution:

37 ft

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