## 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)

 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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