Mechanical Bar Screen

Overview

A mechanical bar screen is the first primary treatment stage in wastewater plants, removing large solids (rags, plastics, wood, grit) >5 mm from raw sewage. The rotating rake mechanism periodically clears the vertical or inclined bar rack, conveying screenings to a discharge chute for disposal.

Bar screens protect downstream equipment (pumps, treatment tanks) from blockage and abrasion, while producing a quantifiable screenings waste stream (typically 10–80 L per 1000 m³ of influent). Automatic control via differential pressure sensing triggers rake operation only when blockage occurs, minimizing energy consumption.

Screenings Characteristics

Typical municipal wastewater screenings composition (% by volume):

• Rags, cloth, paper: 40–50%
• Plastics, bags, film: 20–30%
• Hair, fibers: 10–15%
• Food waste, debris: 5–10%
• Grit, sand: 5–10%

Mass loading:

• Small plants: 10–20 L/1000 m³ (poor sewer maintenance)
• Average plants: 30–50 L/1000 m³ (typical)
• Combined sewer systems: 50–80 L/1000 m³ (stormwater inflow)

Moisture content: 80–90% water; oven-dry solids = 10–20% of wet mass.

Hydraulics

Head Loss Calculation:

The blockage-free bar rack induces minimal head loss (0.05–0.15 m), calculated as:

h_L = (v²/2g) × [(α × A_bar) / A_open]

Where:

• v = approach velocity = Q / A_total
• α = drag coefficient (~1.0 for cylinder)
• A_bar = projected area of bars = n × d × L (n = bar count, d = diameter, L = length)
• A_open = free flow area between bars

For a 2 m wide × 1 m high bar rack with 10 mm bars at 20 mm spacing and 1 m³/min flow:

• v = 1 m³/min / (2 m × 1 m) = 0.5 m/min = 0.0083 m/s
• A_total = 2 × 1 = 2 m²
• A_bar = 10 bars × 0.01 m × 2 m = 0.2 m²
• A_open = 2 - 0.2 = 1.8 m²
• h_L = (0.0083² / 19.6) × (1.0 × 0.2 / 1.8) = negligible

As blockage accumulates, head loss increases. At 50% blockage:

• h_L ≈ 0.3–0.5 m
• Differential pressure across rack: ΔP = ρgh = 1000 × 9.81 × 0.4 / 10,000 ≈ 0.39 bar

The controller triggers rake at 0.15–0.25 bar setpoint (50–100% blockage level).

Raking Mechanism Operation

The drive system rotates the rake arm at 0.5–5 rpm. Typical cycle:

1. Contact Phase: Rake descends, teeth engage trapped solids and pull them upward and into the Discharge Chute
2. Travel Speed: 0.1–0.3 m/min (slow to allow water draining from screenings)
3. Dwell Time: 5–10 seconds at top position
4. Return: Rapid descent for next cycle

Cycle duration is programmable: 5–30 minutes. Demand-based control (pressure sensing) overrides time and initiates immediate rake operation if blockage setpoint is reached.

Torque Considerations:

At blockage onset, the rake encounters maximum resistance. The torque limiter provides slip-clutch protection, allowing the motor to stall or slip rather than break the rake arm.

Typical stall torque: 500–2000 N·m depending on motor power and rake size.

Differential Pressure Control

The differential pressure transmitter measures the pressure drop across the bar rack:

ΔP_transducer = P_upstream - P_downstream

A differential pressure switch (0.1–0.3 bar setpoint) is wired to the PLC. Two control modes:

Mode 1: Timed Intervals

• Rake cycles every 5–15 minutes regardless of blockage
• Simple but potentially wasteful (energy for unnecessary raking)
• Used in small plants with consistent screenings load

Mode 2: Demand-Based (Pressure Sensing)

• Rake activates only when ΔP exceeds setpoint (typically 0.2 bar)
• PLC monitors transmitter signal (4–20 mA)
• Improves efficiency; rake runs only when blockage warrants it
• Suitable for variable-load plants (combined sewers, seasonal variation)

Screenings Discharge

The Discharge Chute conveys wet screenings gravity or vibration-assisted. Two scenarios:

Gravity Discharge (Simple):

• Chute slope: 45–60°
• Water drains back to channel via perforations
• Manual dumpster or conveyor at bottom
• Suitable for small plants; labor-intensive

Vibrating Chute (Mechanical):

• Electromagnetic vibrator oscillates chute at 10–60 Hz
• Fluidizes wet screenings, accelerating discharge
• Reduces blockages and bridging
• Typical for larger plants; adds 0.5–2 kW power consumption

Wash Water System:

The Flushing System applies wash water during/after rake operation:

• Flow: 1–5 m³/h at 2–5 bar
• Purpose: Clean trapped silt/sand from screenings, reducing odor and volume
• Wash water is recirculated to channel (nutrients recovered)

Screening Quality vs. Plant Size

Operational Issues

Bridging: Large rags or plastic bags jam between bars before rake reaches them. Prevention:

• Undersized bar spacing (5 mm instead of 10 mm)
• Increased rake frequency (every 5 min)
• Staggered bar arrangement

Grit Accumulation: Sand and gravel settle in the channel below screenings. If not removed:

• Increases solids load to downstream treatment
• Abrades pump impellers and compressor valves
• Necessitates a Grit Classifier for grit recovery

Screenings Odor: Decomposition in wet screenings produces H₂S and mercaptans. Mitigation:

• Frequent rake cycles (minimize dwell time in channel)
• Wash water flushing (oxidizes some sulfur compounds)
• Screenings dewatering/composting off-site

Design Considerations

Flow Distribution: Uneven flow across the bar rack causes local blockages. Design requires:

• Smooth transition inlet (no sharp bends)
• Approach channel width ≥ rack width
• Uniform approach velocity 0.3–0.5 m/s (prevents grit settling before bar rack)

Bypass Capacity: During peak flows (combined sewer overflow), flow exceeding bar-screen capacity must bypass without damaging the equipment. Typical:

• Movable stoplogs or sectional gates
• Emergency weir overflow to bypass channel
• Sized for 2 × design flow

Replacement Interval: Bar racks are subject to corrosion and abrasion:

• Stainless steel (304/316): 10–15 years typical life
• Carbon steel with paint: 5–8 years (high corrosion risk)
• Stainless-preferred given long-term maintenance cost

Standards and Guidelines

• ASME B29.1: Roller Chain and Sprockets
• ISO 11571: Bar screen design and testing
• WPCF MOP 8: Preliminary Treatment
• EPA Design Manual: Preliminary Treatment