Screens

Physical removal of large objects and debris from raw wastewater

Overview

Screens are the first treatment unit in WWTPs, removing large solids (rags, wood, plastics, grit) that could damage pumps or clog downstream processes. Types include bar screens (coarse, 25-50mm), fine screens (3-10mm), and micro-screens (<1mm). Screenings are typically 5-40 L/ML treated, requiring compaction and disposal.

Theory and Equations

Basic Mode

$$Q_{screenings} = Q \times f_{screen}$$

Where:

• Q = flow rate (ML/d)
• f_screen = specific screenings production (L/ML)
  • Coarse bar screens: 5-15 L/ML
  • Fine screens: 10-30 L/ML
  • Micro-screens: 20-40 L/ML

Advanced Mode

Headloss Through Clean Screen:

$$h_L = \frac{v^2}{2g} \times C \times \left(\frac{w}{b}\right)^{4/3} \times \sin\theta$$

Where:

• v = approach velocity (m/s)
• C = bar shape coefficient (2.42 sharp, 1.79 round, 1.67 teardrop)
• w = bar width (m)
• b = clear spacing (m)
• θ = screen angle from horizontal

Headloss with Clogging:

$$h_L = h_{L,clean} \times (1 + f_{clog})$$

Where f_clog = 1.5-3.0 (50-200% increase)

Model Parameters

Parameter	Coarse Bar	Fine Screen	Micro-Screen	Units
Bar/Opening Size	25-50	3-10	0.5-3	mm
Approach Velocity	0.6-1.2	0.6-0.9	0.3-0.6	m/s
Through Velocity	<1.0	<1.2	<0.8	m/s
Headloss (clean)	150-300	200-400	300-600	mm
Screenings	5-15	10-30	20-40	L/ML

Design Criteria

Type	Clear Opening	Application	Cleaning
Coarse Bar	25-50 mm	Preliminary	Manual/mechanical
Medium Bar	10-25 mm	Preliminary	Mechanical
Fine Screen	3-10 mm	Primary bypass	Mechanical
Micro-Screen	0.5-3 mm	Enhanced primary	Continuous

Worked Example

Given: Fine screen design

• Peak flow: 500 L/s
• Bar width: 10 mm
• Clear spacing: 6 mm
• Screen angle: 60° from horizontal

Calculate: Number of bars and headloss

1. Approach area (assume 1.2m wide × 1.0m deep): 1.2 m²
2. Approach velocity: 0.5 / 1.2 = 0.42 m/s
3. Headloss: Use equation with C = 2.42, calculate ~250mm
4. Number of clear spaces: 1.2m / 0.016m = 75 openings

References

1. Metcalf & Eddy (2014). Wastewater Engineering, 5th Edition
2. WEF (2018). Design of Water Resource Recovery Facilities, MOP 8

Change Log

Date	Version	Changes
2025-12-30	1.0	Initial documentation