Trash Rakes

Trash rakes are mechanical screening devices that remove large debris from raw water intakes and headworks channels in municipal treatment plants. These automated systems use steel tines or bars to lift accumulated debris (leaves, branches, plastic, rags) from bar screens, typically operating on timed cycles or differential head loss triggers. Modern trash rakes can handle debris loads up to 50 cubic yards per day in plants processing 10-50 MGD, with rake speeds ranging from 3-15 feet per minute. The primary trade-off is higher capital and maintenance costs compared to manual cleaning, though automated systems reduce labor requirements and improve safety in remote or hazardous locations.

• Raw Water Intake Structures (0.5-10 MGD): Trash rakes protect downstream pumps and screens by removing large debris like branches, bottles, and rags at the intake channel. Positioned upstream of traveling water screens or fine screens, they handle the heaviest debris load. Selected for their ability to operate continuously in variable flow conditions while requiring minimal operator intervention.

• Headworks/Influent Channels (1-50 MGD): Primary application upstream of preliminary treatment equipment including grit chambers and primary clarifiers. Removes debris that could damage pumps, clog pipes, or interfere with biological processes. Typically installed in approach channels with 2-6 feet of water depth, handling flows from 0.5-25 MGD per unit.

• Pump Station Wet Wells: Protects sewage pumps from clogging and mechanical damage by removing large solids before they reach impellers. Critical in lift stations serving combined sewer systems where storm flows carry significant debris loads.

Daily Operations: Operators visually inspect debris accumulation levels and discharge areas during routine rounds. Monitor cycle counters to track cleaning frequency - normal operation ranges 2-10 cycles per day depending on season and upstream conditions. Adjust cleaning cycles based on weather events and seasonal debris patterns.

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Maintenance: Weekly lubrication of drive components and monthly inspection of tine condition and alignment. Quarterly replacement of brush cleaning systems and annual drive motor servicing. Requires confined space entry procedures for below-grade installations. Basic mechanical skills sufficient for routine maintenance, with manufacturer service recommended for major repairs.

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Troubleshooting: Common failures include tine jamming from oversized debris (wire, plastic sheeting) and drive motor overload from excessive debris loading. Warning signs include increased cycle frequency, unusual noise, or visible tine damage. Typical service life 15-20 years for mechanical components, 8-12 years for drive systems with proper maintenance.

• Rake Mechanism: Steel tines spaced 1-6 inches apart depending on application, with hydraulic or electric drive systems. Tine spacing selection based on downstream equipment protection requirements - closer spacing (1-2") for fine screening protection, wider (4-6") for pump protection.

• Drive System: Electric motors (1-5 HP typical) with gear reducers providing 10-50 ft/min rake speed. Hydraulic drives used in remote locations or where explosion-proof requirements exist. Variable speed capability allows adjustment for debris loading conditions.

• Cleaning Mechanism: Debris discharge systems including rotary brushes, spray wash, or gravity discharge. Brush systems require 40-60 PSI wash water supply. Discharge conveyor capacity ranges 1-10 cubic feet per hour for typical municipal applications.

• Control System: Level sensors trigger automatic operation when debris accumulation reaches preset depths (typically 6-12 inches). Manual override capability and cycle counters for maintenance scheduling included in modern installations.

• Flow Parameters: Channel velocity: 2-4 ft/s (optimal debris transport without excessive head loss), Peak flow capacity: 1.5-3.0x average daily flow, Head loss through clean rake: 0.1-0.3 ft maximum, Channel width: 2-20 ft (standard modular widths: 2, 4, 6, 8, 10, 12, 16, 20 ft)

• Physical Specifications: Bar spacing: 0.5-6 inches (fine: 0.5-1", medium: 1-3", coarse: 3-6"), Rake angle: 60-85° from horizontal (75° typical for balanced cleaning efficiency), Channel depth: 4-15 ft below normal water level, Debris loading: 0.5-8.0 ft³/MG (varies by collection system type and season)

• Operational Parameters: Cleaning cycle: Time-based (5-60 minutes) or differential head-based (0.05-0.15 ft), Rake speed: 10-30 ft/min during cleaning cycle, Motor horsepower: 0.5-15 HP depending on size and debris loading, Availability requirement: 95-99% (redundancy typically required for plants >5 MGD)

• Bar Spacing Selection (0.5" vs 1" vs 3" vs 6"): Finer spacing (0.5-1") captures more debris but increases cleaning frequency and power requirements. Coarser spacing (3-6") reduces O&M but allows larger debris downstream. Need influent characterization study and downstream equipment vulnerability assessment. Wrong choice leads to either excessive maintenance costs or downstream equipment damage.

• Manual vs Mechanical Operation Threshold: Plants <2 MGD may justify manual rakes with 3-6" spacing for cost savings ($15K vs $150K). Above 2 MGD or with high debris loading (>2 ft³/MG), mechanical rakes become cost-effective. Consider staffing levels, safety requirements, and peak flow handling capability.

• Redundancy Requirements (Single vs Dual Units): Plants >5 MGD typically require redundant rakes or bypass capability. Single rake failure can force plant shutdown. Evaluate criticality, available bypass options, maintenance access, and capital cost impact ($200K-400K for second unit).

• Control Strategy (Timer vs Differential Head): Timer control simpler but may over/under-clean. Differential head sensors ($5K-10K additional) optimize cleaning cycles but require calibration and maintenance. Consider debris variability, operator sophistication, and energy costs.

• Primary: 46 13 11 - Water Clarification Equipment - Covers mechanical bar screens and trash rakes for water treatment applications

• Secondary: 44 21 00 - Water Utility Transmission and Distribution - For intake and raw water screening applications

• Material/Equipment Verification: Verify 316SS construction for all wetted components, Confirm rake tooth spacing matches design debris size, Check motor horsepower and electrical specifications

• Installation Requirements: Requires precise channel dimensions and embedment details, 6-12 week lead times typical for custom channel widths, Crane access needed for installation and maintenance

• Field Challenges: Channel concrete tolerances critical for proper sealing, Electrical connections often in wet environments, Access platforms frequently added as change orders

• Coordination Issues: Early coordination with structural engineer for embedments, Electrical rough-in timing with concrete placement

• Hydro-Dyne Engineering - Model HydroRake series, widely used in 1-50 MGD plants with proven municipal track record

• Huber Technology - RakeMax and RoRake systems, strong presence in larger facilities (10-100 MGD)

• JWC Environmental - Monster series, popular for smaller plants (0.5-10 MGD)

• Lakeside Equipment - Raptor rake systems, established municipal supplier with extensive service network

• Fine screens (1-6mm) - Higher capital cost but reduced downstream maintenance, preferred for plants with limited grit removal

• Coarse bar screens (25-50mm) - Lower cost option for smaller plants with manual cleaning acceptable

• Drum screens - Better for high debris loads but require more headroom

• Trash rakes typically cost 30-50% more than static bar screens but 40-60% less than fine screens

Establish maintenance relationships early - most manufacturers offer 24/7 emergency service contracts worth the investment. Budget 15-20% contingency for channel modifications during installation, as existing concrete rarely matches original drawings perfectly. Consider dual units on critical applications; single-point failures create major operational headaches. Operators prefer manual override capabilities and local control panels over fully automated systems.