FIGURE 5.3 A Microstrainer. (Courtesy of Envirex, Inc.)

that may cause violations of the discharge permits of the plant. Microstrainers have also been used to reduce the suspended solids content of wastewaters treated by biological treatment. Openings of microstrainers are very small. They vary from 20 to 60 fim and the cloth is available in stainless steel or polyester construction.

5.1.1 Head Losses in Screens and Bar Racks

Referring to b of Figure 5.2, apply the Bernoulli equation, reproduced below, between points 1 and 2.

where P, V, and h are the pressure, velocity, and elevation head at indicated points; g is the acceleration due to gravity. V1 is called the approach velocity; the channel in which this velocity is occurring is called the approach channel. To avoid sedimentation in the approach channel, the velocity of flow at this point should be maintained at the self-cleansing velocity. Self-cleansing velocities are in the neighborhood of 0.76 m/s.

Remember from fluid mechanics that the Bernoulli equation is an equation for frictionless flow along a streamline. The flow through the screen is similar to the flow through an orifice, and it is standard in the derivation of the flow through an orifice to assume that the flow is frictionless by applying the Bernoulli equation. To consider the friction that obviously is present, an orifice coefficient is simply prefix to the derived equation.

Both points 1 and 2 are at atmospheric, so the two pressure terms can be canceled out. Considering this information and rearranging the equation produces

From the equation of continuity, V1 may be solved in terms V2, cross-sectional area of clear opening at point 2 (A2), and cross-sectional area at point 1 (Aj). V1 is then V = A2V2/A1. This expression may be substituted for V1 in the previous equation, whereupon, V2 can be solved. The value of V2 thus solved, along with A2, permit the discharge Q through the screen openings to be solved. This is

1 A2 Ai

2gAh i - 2A222222 Ai

Recognizing that the Bernoulli equation was the one applied, a coefficient of discharge must now be prefixed into Equation (5.3). Calling this coefficient Cd,

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