Volumetric Dry Feeders

Volumetric dry feeders deliver precise quantities of dry chemicals (alum, lime, polymer, chlorine compounds) by measuring and dispensing pre-calibrated volumes of material into treatment processes. These mechanical systems use rotating screws, belt conveyors, or vibratory trays to meter chemicals at rates typically ranging from 5-5,000 pounds per hour, depending on plant size and chemical demand. Feed accuracy generally achieves ±2-5% of setpoint under steady operating conditions. The primary limitation is sensitivity to material bridging, moisture absorption, and particle size variations that can significantly affect volumetric consistency and dosing accuracy.

• Lime/Sodium Bicarbonate pH Adjustment: Volumetric feeders dose powdered lime or sodium bicarbonate for finished water pH adjustment at 1-50 MGD plants. Selected for consistent feed rates with granular materials that bridge in gravimetric systems. Fed downstream of clearwells, upstream of distribution pumps, with feed rate typically 5-200 lbs/hr based on alkalinity demand

• Activated Carbon for Taste/Odor: Powdered activated carbon (PAC) feeding at rapid mix or flocculation basins for seasonal taste/odor events. Volumetric feeders handle the light, fluffy PAC (bulk density 15-25 lbs/ft³) better than belt feeders. Feed rates range 1-15 mg/L, requiring feeders sized 10-150 lbs/hr capacity

• Polymer Conditioning: Dry polymer feeding for biosolids dewatering at 2-20 MGD plants with centrifuges or belt filter presses. Volumetric feeders provide steady polymer dissolution rates of 0.5-5 lbs/hr, feeding into makeup water upstream of polymer maturation tanks

• Coagulant Aid: Bentonite clay or dry alum feeding at conventional treatment plants during low-turbidity periods. Volumetric feeders handle these fine powders reliably at 5-100 lbs/hr, feeding into rapid mix chambers alongside primary coagulants

Daily Operations: Operators monitor hopper levels via sight glass or level transmitters, checking for bridging or rat-holing of chemicals. Feed rate adjustments made through HMI based on process demands - pH readings for lime systems, residual chlorine for PAC applications. Calibration verification performed weekly using calibration tube, comparing actual delivery to controller setpoint.

​

Maintenance: Monthly lubrication of drive bearings and gear reducers using food-grade grease. Quarterly auger wear inspection and dust collector filter replacement. Annual drive belt tensioning and motor megger testing. Requires confined space entry procedures for hopper cleaning. Operators need respiratory protection (P100

• Hopper Assembly: Stainless steel or carbon steel construction, 5-50 ft³ capacity for municipal applications. Includes level sensors, access ports, and dust collection connections. Sizing based on 8-24 hour chemical storage requirements and bulk density of specific chemicals

• Auger/Screw Conveyor: Variable-speed helical screw (304 SS or mild steel with coating) sized 2-8 inches diameter. Drives chemical from hopper bottom to discharge. Speed control via VFD provides 10:1 turndown ratio, critical for seasonal demand variations

• Drive Assembly: Gear reducer and motor combination (0.5-3 HP typical) with torque overload protection. Enclosed TEFC motors prevent dust infiltration. Gear ratios 50:1 to 1800:1 provide precise low-speed operation for accurate volumetric delivery

• Calibration Tube: Clear graduated cylinder (1-5 gallon capacity) with isolation valves for field calibration verification. Allows operators to verify actual feed rates against controller setpoints without interrupting chemical feed

• Dust Control System: Baghouse filter or cartridge dust collector with 50-500 CFM capacity. Prevents chemical dust escape during filling operations, critical for operator safety and regulatory compliance

• Feed Rates: 0.5-2,000 lbs/hr typical range for municipal applications. Small plants (0.5-5 MGD) typically require 5-50 lbs/hr for polymer feeding, 10-200 lbs/hr for lime. Medium plants (5-25 MGD) need 50-500 lbs/hr polymer, 200-1,000 lbs/hr lime. Large facilities (25-50 MGD) require 200-2,000 lbs/hr capacity

• Accuracy Requirements: ±2% for polymer feeding, ±5% acceptable for lime/soda ash applications. Turndown ratios of 20:1 minimum, with 40:1 preferred for variable demand applications

• Bulk Density Considerations: Design for 20-60 lbs/ft³ range covering typical chemicals (polymer 25-35 lbs/ft³, lime 50-60 lbs/ft³, soda ash 35-45 lbs/ft³)

• Hopper Capacity: Size for 8-24 hour storage at maximum feed rate. Minimum 2 ft³ for small feeders, up to 50 ft³ for large installations

• Environmental Conditions: IP65 enclosures minimum for outdoor installations. Operating temperature range -10°F to 120°F. Humidity resistance essential for chemical storage areas

• Power Requirements: 115V single-phase for units under 100 lbs/hr, 230V for larger capacity feeders with variable frequency drives

• What feed rate range and accuracy is required? Plants must define minimum/maximum chemical demand based on seasonal loading variations and future capacity. Undersized feeders limit treatment flexibility, while oversized units sacrifice accuracy at low feed rates. Engineers need 5-year demand projections and peak hour loading data to properly size equipment

• Which auger/agitation system matches the chemical properties? Free-flowing chemicals like soda ash work with standard augers, while bridging-prone polymers require live-bottom hoppers or vibrating agitators. Wrong selection causes feed interruptions and maintenance issues. Requires bulk density testing and angle of repose data for specific chemical brands

• What level of automation and control integration is needed? Basic units offer manual rate adjustment, while advanced systems provide 4-20mA control, loss-in-weight feedback, and SCADA integration. Higher automation costs $5,000-15,000 more but enables precise dosing control. Decision depends on operator staffing levels and process criticality

• How will chemical delivery and storage be handled? Bulk delivery requires pneumatic conveying compatibility, while bag systems need dust collection. Storage capacity affects delivery frequency and chemical costs. Engineers must coordinate feeder inlet design with storage system and building constraints

• Division 40-48: Process Integration

• Primary: 46 13 16 - Chemical Feed Dry Feeders

• Secondary: 46 05 00 - Common Work Results for Water and Wastewater Equipment (for installation requirements and testing)

• Material/Equipment Verification: Verify chemical compatibility with wetted materials (316SS vs. carbon steel), Confirm motor ratings and explosion-proof classifications, Check calibration certificates and accuracy specifications

• Installation Requirements: Adequate clearance for maintenance access and hopper removal, Proper electrical classification for chemical storage areas, Foundation requirements for vibration isolation

• Field Challenges: Moisture infiltration causing bridging in hoppers, Dust control during filling operations, Access limitations in retrofit applications

• Coordination Issues: Integration with existing SCADA systems, Sequencing with upstream bulk storage equipment, Lead times typically 8-12 weeks for standard units, 16-20 weeks for custom configurations

• Acrison - Model 105 volumetric feeder, widely used in 1-50 MGD plants for polymer and lime feeding

• Hapman - Helix flexible screw feeders popular for smaller installations under 10 MGD

• Schenck Process - AccuRate series feeders common in larger facilities 25-100 MGD

• K-Tron - T35 pneumatic conveying feeders used with bulk storage systems

• Gravimetric feeders cost 40-60% more but provide superior accuracy for critical applications like fluoride dosing

• Liquid chemical systems eliminate dust issues but require heated storage for some chemicals and cost 2-3x more for equivalent capacity

• Pneumatic conveying systems work well with bulk storage (>20 tons) but add complexity and 50-75% higher capital costs compared to simple volumetric feeders

Establish relationships with local manufacturer representatives early - they provide valuable troubleshooting support and spare parts inventory. Consider purchasing spare auger/screw assemblies during initial procurement to avoid 4-6 week lead times during emergencies. Specify stainless steel construction even for non-corrosive chemicals; the 15-20% cost premium pays back through reduced maintenance. Install bypass piping around feeders to maintain chemical feed during maintenance periods.