Step Screens

Step screens are mechanically-cleaned fine screening devices that remove debris from wastewater influent using a series of stepped, rotating cleats or flights that lift solids up and out of the flow channel. The system operates continuously with cleats moving upward along the screen face to transport captured material to a discharge point at the top. Typical installations achieve 85-95% removal efficiency for solids larger than the bar spacing (typically 6-25mm in municipal applications). The primary trade-off is higher maintenance complexity and power consumption compared to static bar screens, requiring regular mechanical maintenance and creating potential single points of failure in critical screening applications.

• Primary Headworks Screening: Step screens serve as the primary coarse screening at plant influent, typically handling 2-50 MGD flows with 6-15mm bar spacing. They're selected for high debris loads and consistent removal efficiency, connecting upstream from grit chambers and downstream to fine screens or primary clarifiers. Municipal plants favor them for reduced bypass frequency compared to static screens.

• Bypass Channel Protection: Installed in emergency overflow channels to protect downstream processes during peak wet weather events. These units operate intermittently but must handle 150-300% design flows with heavy debris loads. The self-cleaning action prevents channel blockage during critical storm events.

• Pump Station Upgrades: Retrofitted into existing pump stations where static bar racks cause frequent clogging. Step screens fit into confined spaces while providing automated cleaning, reducing operator callouts and protecting downstream pumps from ragging and damage.

Daily Operations: Operators monitor runtime hours, debris discharge volumes, and differential head across the screen. Most units operate on timer control (15-30 minute intervals) or differential level switches. Visual inspection for proper chain tracking and debris removal efficiency. Modern units include remote monitoring capabilities for runtime tracking and alarm conditions.

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Maintenance: Monthly lubrication of drive components and quarterly chain tension adjustments. Annual gear reducer oil changes and drive component inspection. Requires confined space entry procedures for below-grade maintenance. Operators need basic mechanical skills for chain adjustments and minor repairs. Critical safety requirement: lockout/tagout procedures and fall protection for elevated work.

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Troubleshooting: Chain derailment typically indicates worn sprockets or improper tensioning - warning signs include unusual noise or visible chain slack. Drive motor overload suggests excessive debris loading or mechanical binding. Expected service life: 15-20 years for structural components, 8-12 years for drive systems, 5-8 years for chains depending on debris characteristics and maintenance quality.

• Drive Assembly: Includes gear motor (typically 2-5 HP), drive sprockets, and control panel. Motors are TEFC-rated for continuous outdoor duty. Selection based on screen width (4-12 feet typical) and debris loading expectations.

• Step Chain System: Stainless steel 316 chain with integrated cleaning steps, sized for specific screen dimensions. Chain pitch ranges 6-12 inches depending on application. Critical for reliable debris transport and screen cleaning effectiveness.

• Screen Frame Structure: Hot-dip galvanized or stainless steel construction housing the rotating mechanism. Frame depth varies 8-20 feet based on channel geometry. Must accommodate upstream stop gate and downstream debris handling systems.

• Debris Discharge Chute: Integrated collection system directing screenings to containers or conveyors. Includes wash water spray system for cleaning. Sizing matches anticipated debris volumes (typically 1-5 cubic yards per day per MGD).

• Flow Parameters: Design flow rate: 0.5-50 MGD (0.35-35 cfs), Peak flow capacity: 2.5-3.0 × average daily flow, Minimum flow velocity: 2.0 ft/s (prevents solids settlement), Maximum approach velocity: 3.5 ft/s (prevents hydraulic overload)

• Physical Specifications: Bar spacing: 6mm (1/4"), 10mm (3/8"), or 15mm (5/8") standard, Channel width: 2-12 feet (matches existing channel geometry), Screen rake angle: 35-45 degrees (optimizes solids discharge), Installation depth: 8-25 feet below grade

• Loading Criteria: Hydraulic loading: 10-30 gpm/ft² screen area, Solids loading: 0.5-2.0 ft³/MG (varies by collection system age), Head loss (clean): 0.1-0.3 feet, Head loss (fouled): 1.0-2.0 feet maximum

• Performance Standards: Removal efficiency: 15-30% TSS, 5-15% BOD, Availability: >95% (includes redundancy requirements), Power consumption: 0.5-2.0 kW per screen unit, Screenings production: 0.5-3.0 ft³/MG typical municipal range

• Bar Spacing Selection (6mm vs 10mm vs 15mm)? Threshold: >15 years collection system age typically requires 6mm spacing due to higher debris loads. Consequence: Oversized spacing allows damaging debris through, undersized creates excessive head loss and maintenance. Need: Collection system assessment, downstream equipment vulnerability analysis, historical debris characterization.

• Single vs Dual Screen Configuration? Threshold: >10 MGD plants require redundancy per most state regulations. Plants with critical downstream processes need backup even at smaller capacities. Consequence: Single screen failure shuts down plant or bypasses raw sewage. Need: Regulatory requirements, downstream process criticality, bypass capacity availability.

• Manual vs Automatic Operation Mode? Threshold: Facilities with 24/7 staffing can consider manual operation for <2 MGD plants. Automatic required for unmanned operations or >5 MGD. Consequence: Manual operation requires constant staffing, automatic reduces labor but increases electrical/control complexity. Need: Staffing patterns, maintenance capabilities, power reliability assessment.

• Channel Modifications vs Retrofit Constraints? Threshold: >6 feet channel width allows most standard units, <4 feet requires custom fabrication (30-50% cost premium). Consequence: Inadequate hydraulics cause bypassing or structural failure. Need: Existing channel survey, structural capacity evaluation, construction access limitations.

• Primary: Division 46 23 61 - Sewage Screening Equipment

• Secondary: Division 40 32 00 - Mechanical Screening Equipment (for integrated headworks applications)

• Standard specification covers step screen mechanisms, drives, controls, and ancillary equipment

• Material/Equipment Verification: Verify 316SS construction for all wetted parts, Confirm rake material (typically hardened steel or polyurethane), Check motor IP ratings (minimum IP65)

• Installation Requirements: Crane access for 8,000-15,000 lb units, Concrete anchor bolt templates ±1/4" tolerance, 480V/3-phase power with VFD integration

• Field Challenges: Channel modifications often required for retrofits, Screenings conveyor alignment critical, Access platforms must accommodate maintenance

• Coordination Issues: 16-20 week lead times typical, Early electrical coordination for control integration

• Headworks International - BioScreen step screens, widely used in 1-50 MGD plants with 6mm standard spacing

• Huber Technology - RakeMax and Multi-Rake systems, strong in larger facilities 10-100 MGD

• Lakeside Equipment - Raptor step screens, popular for retrofit applications

• JWC Environmental - Monster step screens, focusing on smaller municipal plants 0.5-10 MGD with robust construction

• Bar screens with mechanical rakes cost 30-40% less but require more frequent maintenance and produce wetter screenings. Preferred for budget-constrained projects under 5 MGD.

• Rotating drum screens offer excellent fine screening (1-3mm) but cost 50% more and suit facilities prioritizing downstream equipment protection.

• Static wedge wire screens work well for smaller flows under 2 MGD with minimal maintenance requirements.

Establish direct relationships with manufacturer field service teams - they provide invaluable troubleshooting support and spare parts guidance. Budget 15-20% above base equipment cost for installation accessories (platforms, conveyors, wash systems). Consider standardizing on one manufacturer across multiple plants to leverage spare parts inventory and operator training. Request factory testing for critical applications over 25 MGD.