Wastewater Screening Design Calculator

Headloss equals 1/0.7 times opening velocity squared minus approach velocity squared over 2g

Solution

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How It Works

Screening is the first step in a wastewater treatment plant, catching rags, sticks, plastics, and other large debris before they damage pumps and clog pipes. Bar racks use spaced parallel bars (typically 25–50 mm openings), while fine screens have openings as small as 0.5–6 mm. The headloss across each type is the key design parameter for channel sizing.

Bar rack headloss depends on the velocity difference through and upstream of the bars. Fine screen headloss uses the flow rate, open area, and a discharge coefficient.

Example Problem

A bar rack has an approach velocity of 0.6 m/s, a bar-opening velocity of 0.9 m/s, and g = 9.81 m/s². What is the headloss?

  1. hL = (1/0.7) × (0.9² − 0.6²) / (2 × 9.81)
  2. hL = 1.429 × (0.81 − 0.36) / 19.62
  3. hL = 1.429 × 0.0229 = 0.033 m (33 mm)

Clean bar rack headloss is typically 10–40 mm; it increases as debris accumulates.

Frequently Asked Questions

What is the purpose of screening in wastewater treatment?

Screening removes large solids -- rags, plastics, sticks, and debris -- that could damage pumps, clog pipes, or interfere with downstream biological and chemical treatment processes. It is the first unit operation in virtually every treatment plant.

What is the difference between coarse and fine screens?

Coarse screens (bar racks) have bar spacings of 25–50 mm and catch large debris. Fine screens have openings of 0.5–6 mm and remove smaller solids, sometimes replacing primary clarifiers in compact treatment plants.

How much headloss do bar racks cause?

Clean bar racks typically produce 10–40 mm of headloss. As screenings accumulate, headloss rises and can exceed 150 mm, triggering automatic rake cleaning. Maximum allowable headloss is set during design, usually at 150–300 mm.

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