Sulfur Dioxide Gas Feed Systems

Sulfur dioxide gas feed systems inject SO2 gas into water streams for dechlorination, primarily removing residual chlorine from treated effluent before discharge to protect aquatic life. The system meters pressurized SO2 gas through rotameters or mass flow controllers, then dissolves it in water via ejectors or diffusers to create sulfurous acid, which rapidly neutralizes free and combined chlorine. Typical dechlorination efficiency exceeds 99% with SO2:Cl2 stoichiometric ratios of 1:1 to 1.5:1. The primary limitation is handling hazardous SO2 gas, requiring extensive safety systems, leak detection, and emergency scrubbing equipment that significantly increases capital and operating costs compared to alternative dechlorination methods.

• Dechlorination at Water Treatment Plants: SO2 systems neutralize residual chlorine before discharge to receiving waters, typically sized for 1.5-3.0 ppm SO2 per ppm Cl2. Located downstream of clearwells, upstream of discharge structures. Selected over sodium bisulfite for cost effectiveness at larger plants (>5 MGD) and lower chemical storage requirements.
• Wastewater Effluent Dechlorination: Primary application in municipal plants with chlorine disinfection, removing residual chlorine to meet NPDES permit limits (<0.1 mg/L). Systems connect directly to effluent channels before outfall structures. Preferred for plants treating >2 MGD due to lower operating costs compared to liquid alternatives.
• Process Water Dechlorination: Used at water reclamation facilities treating chlorinated potable water for industrial reuse or groundwater recharge. Systems typically sized for 10-50 lb/day SO2 capacity, connecting upstream of RO systems or recharge basins where residual chlorine would damage membranes or violate injection standards.

Daily Operations: Operators monitor cylinder weights, solution tank levels, and residual chlorine measurements downstream of injection points. Typical adjustments include SO2 feed rate changes based on influent chlorine levels and flow variations. Systems require 15-30 minutes daily attention, with automated controls handling most routine operations.

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Maintenance: Weekly cylinder changeouts and monthly calibration of feed equipment and detection systems. Requires specialized training for SO2 handling, full-face respirators, and emergency response procedures. Annual maintenance includes regulator rebuilds and scrubber system testing. Most utilities require two certified operators for safety during cylinder changes.

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Troubleshooting: Common failures include plugged injectors from crystallization, regulator diaphragm failures, and solution pump cavitation. Warning signs include erratic feed rates, high residual chlorine readings, and SO2 odors. Equipment typically lasts 10-15 years with proper maintenance, though regulators may require rebuild every 5-7 years in high-usage applications.

• Gas Cylinders and Manifolds: 150-lb or 2000-lb cylinders containing liquefied SO2, with automatic switchover manifolds for continuous operation. Cylinder selection based on daily usage - 150-lb cylinders for <20 lb/day systems, 2000-lb for higher demands. Manifolds include pressure regulators, check valves, and cylinder scales.
• Vacuum Regulators: Control SO2 gas flow using water-powered ejectors creating vacuum conditions for safety. Typically sized for 5-200 lb/day capacity with bronze or stainless steel construction. Critical for preventing SO2 leaks under positive pressure conditions.
• Solution Tanks and Injection Systems: Fiberglass or polyethylene tanks (50-500 gallons) where SO2 dissolves in water creating sulfurous acid solution. Includes circulation pumps, solution strength monitors, and injection pumps sized for specific flow rates and residual targets.
• Safety Equipment: Includes SO2 leak detection systems, emergency scrubber systems, and ventilation controls. Detection systems typically set at 2-5 ppm alarm levels with automatic system shutdown capabilities.

• Feed Rate Capacity: 10-500 lb/day SO2 for typical municipal dechlorination applications (0.5-50 MGD plants). Size based on maximum chlorine residual × flow rate × stoichiometric ratio (1.1-1.4 lb SO2/lb Cl2).
• System Pressure: Operating pressure 15-25 psig with pressure regulators maintaining ±2 psig stability. Cylinder pressure ranges from 300 psig (full) to 75 psig (near empty) at 70°F.
• Feed Accuracy: ±2% of set point for electronic mass flow controllers; ±5% for rotameter-based systems. Critical for maintaining proper dechlorination without overdosing.
• Turndown Ratio: Minimum 10:1 for variable demand applications. Electronic systems achieve 20:1 or better turndown.
• Ambient Temperature Range: -10°F to 120°F operation with automatic temperature compensation for mass flow measurement.
• Cylinder Configuration: Single cylinder (50-150 lb) for smaller plants; manifold systems with 2-6 cylinders (150 lb each) for larger facilities requiring continuous operation.
• Residual Detection: SO2 residual analyzers with 0.02-2.0 ppm measurement range and ±0.02 ppm accuracy for closed-loop control.

• What maximum daily SO2 demand determines system capacity? Calculate peak chlorine residual (typically 2-4 ppm) × maximum daily flow × 1.3 safety factor. Undersizing results in incomplete dechlorination and permit violations; oversizing increases capital costs and reduces turndown capability. Requires historical chlorine residual data and flow projections.
• Does variable flow require automatic proportional control or manual adjustment? Plants with >25% flow variation need automatic mass flow controllers with flow pacing. Manual systems cost 40% less but require constant operator attention and create compliance risks during flow changes. Decision depends on staffing levels and discharge permit flexibility.
• How many backup cylinders ensure continuous operation? Single-cylinder systems require weekly changeouts; dual-cylinder automatic switchover provides 2-week autonomy. Plants operating >16 hours/day or with limited weekend staffing need automatic switchover. Manual systems acceptable for batch treatment or facilities with 24/7 staffing.
• What residual monitoring level matches permit requirements? Continuous analyzers ($15K-25K) required for permits with instantaneous maximums; grab sampling adequate for daily average limits. Analyzer selection affects control system complexity and operating costs.

• Division 40 - Process Integration
• Section 40 23 19 - Chemical Feed Equipment for Water Treatment
• Primary specification covers SO2 gas feed systems, mass flow controllers, cylinder manifolds, and safety equipment
• May reference Division 23 (HVAC) for scrubber ventilation systems and Division 26 (Electrical) for control integration

• Material/Equipment Verification: Verify 316L stainless steel construction for all wetted parts, Confirm PTFE/Viton seals rated for SO2 service, Check analyzer compatibility with existing SCADA systems
• Installation Requirements: Dedicated ventilation system with 10+ air changes/hour, Emergency shower/eyewash within 10 feet of equipment, Separate electrical classification for hazardous area
• Field Challenges: SO2 cylinder handling requires trained personnel and lifting equipment, Scrubber water quality affects performance significantly
• Coordination Issues: 12-16 week lead times typical for engineered systems, Early coordination with fire marshal for storage requirements

• Evoqua Water Technologies - WALLACE SO2 systems with integrated scrubbers for 1-50 MGD plants
• Hach Company - CL17 and CL500 series analyzers with SO2 injection controls
• Capital Controls Group - Series 1000 SO2 feed systems with automatic switchover capabilities
• Tonka Equipment - Custom SO2 feed skids with PLC controls, popular in smaller municipal applications under 5 MGD

• Sodium bisulfite liquid feed - Safer handling, 2-3x higher chemical costs but lower capital investment. Preferred for plants <2 MGD.
• Activated carbon contactors - Higher capital cost (3-4x) but eliminates chemical handling entirely. Best for plants with space constraints.
• UV disinfection conversion - Eliminates dechlorination need completely. Capital cost 4-5x higher but trending toward cost-competitive lifecycle economics for new construction >5 MGD.

Establish relationships with local SO2 suppliers early - cylinder delivery logistics often determine system reliability more than equipment selection. Consider bulk storage (1-ton containers) for plants >10 MGD to reduce handling frequency and costs. Install redundant analyzers; single-point failures shut down chlorination systems. Budget 15-20% additional for scrubber pumps and pH adjustment - these auxiliary systems frequently require upgrades during commissioning.