Comminutors

Comminutors are mechanical cutting devices installed in raw wastewater streams to reduce large solids and debris into smaller, more manageable particles typically 6-19mm in size. The equipment features rotating cutting blades that shear material against fixed cutter bars as wastewater flows through the unit at velocities of 2-4 ft/sec. Modern comminutors can handle flows from 0.5 to 50 MGD with solids reduction efficiencies of 85-95% for materials larger than 25mm. However, comminutors require frequent maintenance due to blade wear and are susceptible to jamming from stringy materials like rags and plastics, making them less favorable than bar screens in many contemporary municipal applications.

• Headworks Screening: Comminutors serve as primary screening devices in smaller plants (0.5-5 MGD), installed after bar screens to macerate rags, plastics, and organic solids into 1/4" to 3/8" particles. They connect between the influent channel and grit removal systems, eliminating downstream clogging issues in pumps and fine screens.
• Pump Station Protection: Installed upstream of raw sewage pumps, particularly in lift stations serving residential areas with high rag content. Units like JWC Environmental's Muffin Monster or Sulzer's XRipper protect chopper pumps and reduce maintenance calls by pre-shredding debris to 6mm particles before pumping.
• Bypass/Emergency Lines: Comminutors provide backup screening when primary fine screens are offline for maintenance. Typical installations include 6-12" bypass lines around mechanical bar screens, allowing continued operation while maintaining downstream equipment protection in 2-15 MGD facilities.
• Return Sludge Lines: Smaller comminutors (2-4" pipe sizes) macerate debris in waste activated sludge returns, preventing clogging in sludge pumps and thickeners while maintaining biological process integrity.

Daily Operations: Operators monitor motor amperage and operating hours through SCADA systems, checking for abnormal current spikes indicating jams or excessive debris loading. Visual inspection of discharge quality ensures proper cutting action. Automated systems require minimal adjustment, though manual bypass activation may be needed during peak debris events or maintenance periods.

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Maintenance: Monthly lubrication of gear reducers and bearing assemblies using food-grade lubricants. Quarterly cutter inspection requires confined space entry with full PPE including respiratory protection. Cutter replacement typically every 12-18 months depending on debris characteristics, requiring millwright skills for proper alignment. Annual motor and control system inspection by qualified electricians ensures reliable operation.

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Troubleshooting: High amperage draw indicates cutter dulling or jam conditions - reversing rotation usually clears soft jams. Excessive vibration suggests bearing wear or cutter imbalance requiring immediate shutdown. Reduced cutting efficiency manifests as larger particles in effluent, indicating tooth wear. Typical service life ranges 15-20 years for mechanical components, with cutters requiring replacement every 1-2 years in high-debris applications.

• Cutting Chamber: Cast iron or stainless steel housing contains rotating cutting mechanism. Chamber sizes range from 4" to 24" diameter for municipal applications, with 316SS construction preferred for corrosive environments. Selection based on peak flow capacity and debris loading.
• Cutting Mechanism: Rotating cutter bar with hardened steel teeth operates against fixed cutting combs. Cutter speeds typically 35-70 RPM with reversible rotation for jam clearing. Tooth spacing determines particle size (1/4" to 1/2" typical).
• Drive System: TEFC motors (1-15 HP) with gear reducers providing high torque at low speeds. Include torque monitoring and automatic reverse capability. Variable frequency drives increasingly common for energy optimization and jam protection.
• Flow Control: Integral bypass dampers or external gates direct flow through cutting chamber. Pneumatic or electric actuators with position feedback ensure proper flow distribution during normal and cleaning cycles.
• Monitoring Systems: Torque sensors, vibration monitors, and motor current analyzers detect jams and wear. Modern units include PLC controls with remote monitoring capabilities and predictive maintenance alerts.

• Flow Rate Capacity: 0.1-25 MGD per unit for municipal applications, with peak flow factors of 2.5-4.0 times average daily flow. Standard channel widths range from 6-48 inches.
• Cutting Screen Opening: Typically 3/8" to 3/4" (9.5-19mm) for municipal applications. Smaller openings (3/8") provide better protection but require more maintenance. Larger openings (3/4") reduce maintenance but allow more debris passage.
• Approach Velocity: 2-4 ft/sec optimal range. Minimum 1.5 ft/sec to prevent grit settling upstream; maximum 6 ft/sec to avoid excessive turbulence and bypass.
• Head Loss: Clean screen: 0.1-0.3 feet typical. Maximum allowable: 1.0-2.0 feet before bypass or alarm activation. Varies significantly with debris loading and screen opening size.
• Power Requirements: 1-15 HP motors typical for municipal installations. Larger units (>10 MGD) may require 20-30 HP. Include variable frequency drives for energy optimization.
• Channel Dimensions: Minimum 18" width for maintenance access. Standard depths 3-8 feet. Provide 2:1 approach transition length minimum for uniform flow distribution.
• Debris Handling: 2-20 cubic feet per million gallons typical for municipal wastewater, varying significantly with collection system age and infiltration/inflow levels.

• 1. What peak flow capacity is required? Must handle 2.5-4.0x average flow without bypass. Undersized units cause frequent bypassing and downstream equipment damage. Oversized units waste energy and capital. Need accurate flow projections including wet weather infiltration data and future growth projections.
• 2. What screen opening size balances protection vs. maintenance? 3/8" openings protect downstream equipment better but require 2-3x more cleaning cycles. 3/4" openings reduce maintenance but may allow damaging debris through. Decision impacts downstream pump station design, maintenance staffing requirements, and lifecycle costs.
• 3. Should design include bypass capability? Fixed bypass weirs activate at 1-2 feet head loss but allow untreated flow during maintenance. Mechanical bypass systems cost 15-25% more but provide operational flexibility. Decision affects regulatory compliance during maintenance periods and emergency operations.
• 4. What level of automation is appropriate? Basic timer-controlled units cost 20-30% less but may not optimize cleaning cycles. Level-controlled systems adjust to actual loading but require reliable instrumentation. Affects long-term operational efficiency and staffing requirements for smaller facilities.

• Division 46 - Water and Wastewater Equipment
• Section 46 05 33 - Comminutors
• Primary specification section covering mechanical wastewater screening equipment. May also reference Section 46 05 36 for related screening equipment coordination.

• Material/Equipment Verification:
• Verify 316SS construction for all wetted parts
• Confirm motor IP68 rating and Class I, Division 1 certification
• Check cutter configuration matches design solids loading
• Installation Requirements:
• Crane access for maintenance (units weigh 2,000-8,000 lbs)
• Electrical conduit routing for submersible connections
• Channel modifications may require bypass pumping
• Field Challenges:
• Alignment critical - misalignment causes premature wear
• Concrete anchor bolt placement tolerance ±1/4"
• Coordination Issues:
• 16-20 week lead times typical
• Coordinate with electrical for VFD integration

• JWC Environmental - Muffin Monster series (2000, 3000, 15000 models) dominates municipal installations with over 40,000 units installed
• Franklin Miller - Taskmaster TM series grinders popular for smaller plants (0.5-5 MGD)
• Sulzer - XRipper comminutors for larger facilities, particularly popular in Canada
• Huber Technology - RakeMax screening/comminution systems gaining market share in retrofit applications where space is constrained

• Bar screens with downstream grinders - Lower capital cost ($15-25K vs $35-50K for comminutors) but higher O&M. Preferred for plants <2 MGD.
• Drum screens - Better for high-debris applications, 30-40% higher cost but superior screenings handling.
• Fine screens (3-6mm) - Emerging preference for plants with downstream membrane systems, removing rather than cutting solids. Cost similar to comminutors but eliminates rags entirely.

Establish direct relationships with manufacturer service technicians early - they provide invaluable troubleshooting support and spare parts guidance. Many plants negotiate service contracts during procurement for 10-15% cost savings versus emergency calls. Consider purchasing spare cutting assemblies during initial order (typically 20% discount). Monitor power consumption trends - gradual increases often indicate dull cutters before visible performance degradation occurs, allowing planned maintenance versus emergency repairs.