Appearance
Coarse Screen
Mechanical bar screen with 25-75mm spacing for removal of large solids, rags, and debris from raw wastewater.
Overview
Coarse screens are the first line of defense in wastewater treatment, protecting downstream equipment from large objects that could cause blockages, damage pumps, or interfere with treatment processes. They consist of parallel bars or rods arranged to intercept floating and suspended materials.
When to Use
- Headworks of all municipal WWTPs
- Pumping stations (before pumps)
- Combined sewer overflows (CSO)
- Industrial wastewater with debris
- Upstream of fine screens
Key Characteristics
- Robust construction for heavy debris
- Self-cleaning or manually raked
- Low headloss when clean
- Protects downstream equipment
- Screenings disposal requirement
Operating Modes
Basic Mode
- Fixed removal efficiency (default 95% of >25mm particles)
- Simplified headloss calculation
- Constant bar spacing
Advanced Mode
- Kirschmer equation for headloss
- Particle size distribution tracking
- Clogging effects
- Cleaning cycle optimization
Mathematical Model
Governing Equation - Kirschmer Headloss
$$h_L = \beta \times \left(\frac{w}{b}\right)^{4/3} \times \frac{v^2}{2g} \times \sin\theta$$
Where:
- $h_L$ = headloss through clean screen (m)
- $\beta$ = bar shape factor (dimensionless)
- $w$ = bar width facing flow (m)
- $b$ = clear spacing between bars (m)
- $v$ = approach velocity (m/s)
- $g$ = gravitational acceleration (9.81 m/s²)
- $\theta$ = screen angle from horizontal (degrees)
Bar Shape Factor (β)
| Bar Shape | β Value |
|---|---|
| Sharp-edged rectangular | 2.42 |
| Rectangular with semicircular upstream | 1.83 |
| Rectangular with semicircular both sides | 1.67 |
| Circular bars | 1.79 |
| Tear-drop optimized | 0.76 |
Capture Efficiency
$$\eta = 1 - \exp\left(-\frac{d_p}{b}\right)$$
Where:
- $\eta$ = capture efficiency (fraction)
- $d_p$ = particle diameter (m)
- $b$ = bar spacing (m)
Screenings Volume
$$V_s = Q \times C_s \times \eta \times \rho_s$$
Where:
- $V_s$ = screenings volume (m³/d)
- $Q$ = flow rate (m³/d)
- $C_s$ = suspended solids concentration (kg/m³)
- $\rho_s$ = screenings bulk density (kg/m³, typically 800-1000)
Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Clear spacing | b | mm | 25-75 | 50 | Distance between bars |
| Bar width | w | mm | 10-30 | 12 | Thickness of bars |
| Screen angle | θ | degrees | 45-90 | 75 | Angle from horizontal |
| Channel width | W | m | 0.5-3.0 | 1.2 | Width of screen channel |
| Approach velocity | v | m/s | 0.6-1.0 | 0.8 | Velocity upstream |
| Bar shape | - | - | See table | Rectangular | Cross-section type |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Headloss (clean) | m | Pressure drop through clean screen |
| Headloss (clogged) | m | Maximum headloss before cleaning |
| Screenings removed | kg/d | Mass of captured material |
| Screenings volume | m³/d | Volume for disposal |
| Capture efficiency | % | Fraction of particles >spacing removed |
| Channel velocity | m/s | Velocity through openings |
Configuration Guide
Step-by-Step Setup
Determine Bar Spacing
- Municipal: 50mm typical
- Heavy debris: 75mm
- Pre-fine screen: 25-40mm
Calculate Channel Dimensions
- Width from peak flow and velocity limits
- Depth to maintain submergence
- Allow for headloss buildup
Select Bar Configuration
- Bar shape (rectangular most common)
- Bar width 10-12mm
- Angle 60-80° for self-cleaning
Configure Cleaning System
- Mechanical rake (automatic)
- Cleaning frequency from headloss sensor
- Manual backup capability
Plan Screenings Handling
- Conveyor/compactor
- Washing unit
- Container/skip storage
Typical Design Values
| Application | Spacing | Approach Velocity | Max Headloss | Notes |
|---|---|---|---|---|
| Municipal headworks | 50mm | 0.8 m/s | 0.5m | Most common |
| Combined sewer | 75mm | 0.6 m/s | 0.3m | High debris |
| Pre-fine screen | 25-40mm | 0.7 m/s | 0.4m | Two-stage system |
| Pumping station | 50mm | 0.9 m/s | 0.6m | Pump protection |
Channel Velocity Limits
$$v_{channel} = v \times \frac{W}{W - n \times w}$$
Where:
- $v_{channel}$ = velocity through bars (m/s)
- $n$ = number of bars
- Limit: $v_{channel}$ < 0.9 m/s (prevents forcing debris through)
Cleaning Frequency
$$t_{clean} = \frac{h_{max}}{K \times Q \times SS}$$
Typical K = 0.001-0.005 depending on wastewater characteristics
Performance Criteria
UK Design Standards
| Parameter | Value | Standard |
|---|---|---|
| Peak flow approach velocity | 0.6-1.0 m/s | MWH |
| DWF approach velocity | 0.4-0.7 m/s | UK industry practice |
| Max headloss (clean) | 150mm | UK industry practice |
| Max headloss (before clean) | 600mm | UK operator |
| Channel through-bar velocity | <0.9 m/s | All |
Screenings Production
| Source | Screenings Yield |
|---|---|
| Municipal (50mm) | 5-20 L/1000m³ |
| Combined sewer | 20-60 L/1000m³ |
| Industrial | 10-100 L/1000m³ |
Installation
Upstream Requirements
- Approach channel length: >6× channel width
- Benching to concentrate flow
- Handrailing and access platform
Downstream Requirements
- Recovery channel to restore velocity
- Scour protection below screen
- Bypass channel with penstock gate
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| Excessive headloss | Rake malfunction | Inspect/repair rake mechanism |
| High debris load | Increase cleaning frequency | |
| Blinding (fats/fibers) | Add screen washing | |
| Debris pass-through | Bent/broken bars | Replace damaged bars |
| Excessive velocity | Reduce flow or widen channel | |
| Odor problems | Screenings accumulation | More frequent removal |
| Poor washing | Install/improve washing system | |
| Mechanical failure | Overload | Check for obstructions |
| Worn components | Scheduled maintenance |
Operation & Maintenance
Daily Checks
- Visual inspection for debris buildup
- Check rake operation
- Monitor headloss differential
- Inspect screenings removal
Weekly Maintenance
- Lubricate rake mechanism
- Check drive chains/belts
- Clean spray wash nozzles
- Inspect screenings conveyor
Monthly Tasks
- Measure actual headloss vs. calculated
- Verify cleaning cycle timing
- Check structural condition
- Review screenings disposal records
Advanced Features
Headloss Monitoring
- Differential pressure sensors
- Automatic cleaning trigger
- Alarm on excessive buildup
Optimized Cleaning
- Variable rake speed
- Flow-proportional cleaning
- Energy optimization
Screenings Treatment
- Compaction (reduce volume 3:1)
- Washing (reduce organics 30-50%)
- Dewatering to >30% dry solids
Related Units
- Fine-Screen - Downstream finer screening
- Grit-Chamber - Next treatment step
- Comminutor - Alternative shredding approach
- Pumps - Protection from debris damage
References
- WRc (1998). Sewage Treatment in Hot Climates. John Wiley & Sons.
- Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed. McGraw-Hill.
- MWH (2005). Water Treatment Principles and Design, 2nd Ed.
- Tchobanoglous, G. et al. (2003). Wastewater Engineering: Treatment and Reuse, 4th Ed.
- Kirschmer, O. (1926). Untersuchungen über den Gefällsverlust an Rechen. Hydraulisches Institut der TH München.