Step Screen

Self-cleaning mechanized screen with moving steps that lift and convey screenings, providing reliable screening with minimal maintenance.

Overview

Step screens feature a series of interlocking steps that move upward and backward, creating a self-cleaning action that continuously removes screenings from the wastewater flow. The step mechanism is highly effective at handling variable loads and fibrous materials without blinding.

When to Use

• Municipal headworks with variable flows
• Combined sewer systems (CSO/storm flows)
• Industrial wastewater with stringy materials
• Locations requiring high reliability
• Pumping station protection
• Applications where manual raking is impractical

Key Characteristics

• Excellent self-cleaning action
• No spray wash required (screenings lift dry)
• Can handle high solids slugs
• Minimal operator intervention
• Low headloss maintenance
• Robust mechanical design

Operating Modes

Basic Mode

• Fixed removal efficiency by bar spacing
• Simplified headloss model
• Constant step speed

Advanced Mode

• Dynamic particle capture modeling
• Variable step speed control
• Headloss prediction with fouling factor
• Power optimization based on load

Mathematical Model

Governing Equation - Step Screen Headloss

$$h_L = K \times \frac{v^2}{2g} \times \left(\frac{w}{b}\right)^{1.33} \times f_{clean}$$

Where:

• $h_L$ = headloss (m)
• $K$ = loss coefficient (1.5-2.5 for step screens)
• $v$ = approach velocity (m/s)
• $g$ = 9.81 m/s²
• $w$ = step width (m)
• $b$ = step spacing (clear opening) (m)
• $f_{clean}$ = cleaning effectiveness factor (0.9-1.0 for step screens)

Capture Efficiency

$$\eta = 1 - \exp\left(-\frac{L_{screen}}{v \times b} \times k_{capture}\right)$$

Where:

• $\eta$ = capture efficiency (fraction)
• $L_{screen}$ = length of screen in flow direction (m)
• $k_{capture}$ = capture rate constant (depends on particle type)

Screenings Lift Rate

$$Q_s = v_{step} \times W_{step} \times h_{layer} \times n_{steps}$$

Where:

• $Q_s$ = volumetric screenings removal rate (m³/h)
• $v_{step}$ = step velocity (m/min)
• $W_{step}$ = step width (m)
• $h_{layer}$ = thickness of screenings layer on step (m)
• $n_{steps}$ = number of steps per minute emerging from water

Power Requirement

$$P = \frac{F \times v_{step}}{60 \times \eta_{mech}}$$

Where:

• $P$ = power (kW)
• $F$ = total force (N), includes hydraulic drag + screenings weight
• $v_{step}$ = step speed (m/min)
• $\eta_{mech}$ = mechanical efficiency (0.7-0.8)

Design Parameters

Input Parameters

Parameter	Symbol	Unit	Range	Default	Description
Bar spacing	b	mm	3-25	6	Clear opening between steps
Screen width	W	m	0.5-3.0	1.5	Channel width
Screen angle	θ	degrees	50-75	60	Inclination from horizontal
Step speed	v_step	m/min	1-6	3	Chain velocity
Approach velocity	v	m/s	0.6-1.0	0.8	Upstream velocity
Screen length	L	m	2.0-5.0	3.5	Active screening length

Output Parameters

Parameter	Unit	Description
TSS removal	%	Suspended solids captured
Headloss (clean)	m	Pressure drop when clean
Headloss (operating)	m	Typical running headloss
Screenings mass	kg/d	Dry solids removed
Screenings volume	m³/d	Wet volume lifted
Power consumption	kW	Electrical demand

Configuration Guide

Step-by-Step Setup

1. Select Bar Spacing

   • 3-6mm: Fine screening, high TSS removal
   • 6-10mm: Standard municipal application
   • 10-25mm: Coarse screening, heavy debris

2. Determine Screen Dimensions

   • Width: From channel width and flow
   • Length: Longer = more residence, better capture
   • Angle: 60° typical for best balance

3. Set Step Speed

   • Variable speed based on:
     • Headloss differential sensor
     • Flow rate
     • Timer (backup mode)
   • Typical: 2-4 m/min during cleaning cycle

4. Configure Control System

   • Level differential sensor (primary)
   • Flow-proportional operation (optional)
   • Manual override capability
   • High-level bypass alarm

5. Design Screenings Discharge

   • Chute from top of screen
   • Direct to container or conveyor
   • Optional washing zone (if required)
   • Compaction unit (reduces volume 60-70%)

Typical Design Values

Application	Spacing	Angle	Approach V	TSS Removal	Notes
Municipal fine	6mm	60°	0.8 m/s	60-70%	Standard configuration
Municipal coarse	15mm	65°	0.7 m/s	40-50%	Pre-fine screen
Combined sewer	10mm	60°	1.0 m/s	50-60%	High storm flows
Industrial (fibers)	3mm	55°	0.6 m/s	70-80%	Slow approach for fibers

Hydraulic Capacity

$$Q_{max} = v \times W \times d \times 3600$$

Where:

• $Q_{max}$ = maximum capacity (m³/h)
• $d$ = water depth at screen (m)
• Typical v×d product: 0.3-0.6 m²/s per meter width

Performance Criteria

Removal Efficiencies

Bar Spacing	TSS Removal	BOD Removal	Typical Application
3mm	70-80%	25-35%	Pre-MBR, high removal
6mm	60-70%	20-28%	Standard municipal
10mm	50-60%	15-22%	Pre-grit, general screening
15mm	40-50%	12-18%	Coarse screening
25mm	30-40%	8-12%	Heavy debris removal

UK Design Standards

Parameter	Value	Standard
Approach velocity	0.6-1.0 m/s	MWH
Maximum velocity	<1.2 m/s	UK industry practice
Headloss (clean)	50-150mm	UK industry practice
Headloss (max operating)	300-500mm	UK operator
Screen angle	55-70°	Typical UK practice

Mechanical Design

Step Mechanism

The step screen consists of:

1. Moving steps: Interlocking plates on continuous chain
2. Guide rails: Support and align steps during cycle
3. Drive system: Motorized chain drive
4. Control panel: PLC-based automation

Operating Cycle

1. Screening phase: Steps stationary or slow-moving, solids accumulate
2. Cleaning phase: Steps accelerate upward, lifting screenings
3. Discharge: Screenings tip off at top into chute
4. Return: Steps descend on backside (out of flow)

Advantages Over Bar Screens

Feature	Step Screen	Bar Screen
Self-cleaning	Mechanical lifting	Rake or spray wash
Screenings dryness	Excellent (drained)	Variable
Blinding resistance	Very high	Medium
Power consumption	Moderate	Low (static) to high (spray)
Maintenance	Moderate (chain, drive)	Low to high
Footprint	Larger (inclined)	Compact (vertical possible)

Troubleshooting

Issue	Cause	Solution
Excessive headloss	Steps not moving	Check drive, emergency stop, power
	Screenings buildup	Increase step speed/frequency
	Debris jamming steps	Inspect, remove obstruction
	Upstream bypass closed	Check penstock gates
Steps jamming	Foreign object	Stop, remove object, inspect chain
	Bent step or guide	Replace damaged components
	Alignment issue	Re-align guide rails
Poor TSS removal	Spacing too wide	Install finer unit or add second stage
	High velocity forcing through	Reduce flow, check channel depth
	Steps moving too fast	Slow step speed
Screenings not discharging	Step angle too shallow	Check alignment, may need adjustment
	Wet, sticky screenings	Add washing/scraping at discharge
Chain/drive problems	Worn chain	Inspect, replace per schedule
	Motor overload	Check for jamming, reduce speed
	Sprocket wear	Replace sprockets, check alignment

Operation & Maintenance

Daily Checks

• Visual inspection of step movement
• Check for unusual noise or vibration
• Monitor headloss differential
• Verify screenings discharge
• Observe for debris accumulation

Weekly Maintenance

• Lubricate chain and bearings (per manufacturer)
• Inspect steps for damage or wear
• Check drive motor operation
• Clean level sensors
• Verify control system operation

Monthly Tasks

• Detailed mechanical inspection
• Check chain tension
• Inspect guide rails for wear
• Measure headloss vs. design
• Review power consumption trends
• Test backup/manual modes

Quarterly Tasks

• Drain channel, inspect submerged components
• Check step alignment and leveling
• Inspect drive sprockets and gearbox
• Lubricate gearbox (if required)
• Test emergency stop systems

Annual Tasks

• Major overhaul and inspection
• Chain replacement assessment (every 3-5 years typical)
• Drive motor and gearbox service
• Structural inspection (supports, channel)
• Efficiency testing
• Update O&M manual

Advanced Features

Variable Speed Control

• Headloss-based automatic speed adjustment
• Flow-proportional operation
• Energy optimization during low loads

Intelligent Control

• Predictive cleaning based on solids load
• Remote monitoring and diagnostics
• Alarm system for faults
• Data logging for optimization

Screenings Treatment

• Integrated washing zone at discharge
• Compaction chute design
• Odor control enclosure
• Automated container handling

Worked Example

Given

• Flow: DWF 600 m³/h, PWF 1,200 m³/h
• Influent TSS: 320 mg/L
• Bar spacing: 6mm
• Target: 65% TSS removal

Design

Step 1: Select Screen Width

• Channel width available: 1.8m
• Use W = 1.6m (allowing for walls and support)

Step 2: Determine Approach Velocity and Depth

• Choose v = 0.8 m/s (within 0.6-1.0 range)
• At PWF: Q = 1,200/3600 = 0.333 m³/s $$d = \frac{Q}{v \times W} = \frac{0.333}{0.8 \times 1.6} = 0.26 \text{ m}$$

Round to d = 0.3m minimum water depth ✓

Step 3: Select Screen Angle and Length

• Angle: θ = 60° (standard)
• Screen length: L = 3.0m (sufficient for 6mm spacing)

Step 4: Estimate Headloss

• Step width w ≈ 0.01m, spacing b = 0.006m $$h_L = 2.0 \times \frac{0.8^2}{2 \times 9.81} \times \left(\frac{0.01}{0.006}\right)^{1.33} \times 0.95 = 0.13 \text{ m}$$

Clean headloss ≈ 130mm ✓ (acceptable)

Step 5: Screenings Production

• TSS removed: 600 × 320 × 0.001 × 0.65 = 125 kg/h = 3,000 kg/d
• Wet screenings (factor 1.5 for step screens, well-drained): 4.5 m³/d

Step 6: Power Estimate

• Typical for this size: 2-4 kW
• Choose 3 kW motor with VFD for variable speed

Comparison with Other Screening Technologies

Technology	Capital Cost	O&M Cost	Reliability	Screenings Quality	Footprint
Step Screen	High	Medium	Excellent	Excellent (dry)	Large
Bar Screen + Rake	Medium	Medium	Good	Good	Medium
Drum Screen	High	Medium	Excellent	Very good	Small
Static Fine Screen	Medium	Low	Good	Variable	Medium

Best Applications for Step Screens:

• Municipal plants: >10,000 m³/d
• Combined sewer systems
• Industrial sites with fibrous materials
• Where reliability is critical
• Where drier screenings are valuable (easier handling/disposal)

Related Units

• Coarse-Screen - Upstream protection
• Fine-Screen - Alternative technology
• Drum-Screen - Alternative compact design
• Grit-Chamber - Downstream treatment
• Comminutor - Alternative (grinding vs. removal)

References

1. Huber Technology. Step Screen ROTAMAT® RoK 4: Technical Manual.
2. Lakeside Equipment. Raptor Step Screen: Design and Operation Guide.
3. Wamgroup. Step Screen Technology: Application Guide.
4. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8.
5. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed.
6. AquaSentry. Step Screen Performance in Municipal Applications. Technical Bulletin.