Sodium Hypochlorite Generating Systems

Sodium Hypochlorite Generating Systems produce disinfectant on-site by electrolyzing salt brine solution, eliminating the need for bulk chemical deliveries and storage. These systems pass direct current through a dilute sodium chloride solution (typically 0.3-3% concentration) in electrolytic cells, generating sodium hypochlorite solution at 0.8% available chlorine concentration. Municipal installations typically produce 5-500 pounds per day of available chlorine, with power consumption averaging 3-4 kWh per pound of chlorine produced. The primary trade-off is higher capital cost and electrical demand compared to bulk hypochlorite purchase, though operational savings and safety benefits often justify the investment for plants using over 200 pounds per day.

• Primary Disinfection (Post-Secondary Treatment): OSHG systems feed 0.5-1.5% sodium hypochlorite solution directly into chlorine contact basins, providing 2-8 mg/L dosing for 2-15 MGD plants. Selected over gas chlorine for safety and over liquid hypochlorite for cost control and storage concerns

• Distribution System Residual Maintenance: Systems dose 0.1-0.4 mg/L at clearwell discharge or booster stations to maintain 0.2 mg/L minimum residual. Preferred for remote locations where liquid delivery is expensive or unreliable

• Process Water Disinfection: Units treat filter backwash water, thickener overflow, or centrate returns at 5-15 mg/L before plant recycle. Eliminates cross-contamination risks from these high-pathogen streams

• Biofilm Control: Low-dose continuous feed (0.2-0.5 mg/L) into wet wells, clarifier channels, or filter underdrain systems prevents biofilm buildup in 5-25 MGD facilities with extended detention times

Daily Operations: Operators monitor production rate displays, salt inventory levels, and hydrogen dilution airflow. Typical adjustments include production rate changes (±20% normal range) based on plant flow and residual targets. Cell voltage monitoring indicates electrode condition - gradual increases signal cleaning needs.

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Maintenance: Weekly cell inspection and monthly acid cleaning (5-10% HCl solution) maintains efficiency. Quarterly electrode voltage testing and annual rectifier calibration required. Safety equipment includes acid-resistant PPE, hydrogen gas detector, and emergency eyewash. Requires basic electrical troubleshooting skills and chemical handling certification.

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Troubleshooting: High cell voltage indicates scaling (clean with acid) or electrode degradation (replace after 5-7 years). Low production with normal voltage suggests brine concentration issues. Hydrogen alarm activation requires immediate shutdown - check dilution blower operation and ventilation adequacy. Warning signs include unusual electrode discoloration or excessive heating.

• Electrolytic Cell Stack: Titanium electrodes with ruthenium-iridium coating generate hypochlorite from brine solution. Cell capacity ranges 5-500 lbs Cl₂/day. Electrode spacing and current density determine efficiency and electrode life (5-7 years typical)

• Brine Preparation System: Dissolves salt to 3-4% solution using eductor or mechanical mixer. Storage tank sizing: 7-day salt supply minimum. Includes level controls and concentration monitoring for consistent feed quality

• Power Supply/Rectifier: Converts AC to DC current (6-12 VDC typical) with current control for production rate adjustment. Sizing: 3-5 kWh per pound of chlorine produced. Includes remote monitoring capabilities

• Hydrogen Dilution Blower: Dilutes hydrogen gas byproduct below 2% LEL using 20-40 CFM airflow per cell. Critical safety component with flow monitoring and automatic shutdown interlocks

• Storage/Feed System: Polyethylene tanks (500-5,000 gallons) with chemical feed pumps. Includes day tank for immediate use and bulk storage for operational flexibility

• Production Capacity: 5-2,000 lbs/day available chlorine (2.3-910 kg/day), sized for 1.5-2.0x average day demand to handle peak flows and provide redundancy

• Salt Consumption: 3.0-3.5 lbs salt per lb available chlorine produced, requiring 15-7,000 lbs/day salt feed depending on system size

• Power Requirements: 2.5-4.0 kWh per lb available chlorine, with systems ranging from 15 kW (small units) to 500 kW (large installations)

• Water Quality Requirements: Source water <500 mg/L TDS, <2.0 NTU turbidity, pH 6.5-8.5, <0.5 mg/L free chlorine residual

• System Pressure: Feed water pressure 40-80 psi, with booster pumps required if municipal pressure <60 psi

• Hypochlorite Strength: 0.6-0.8% available chlorine concentration typical, with 0.4-1.2% range depending on electrolytic cell design and current density

• Footprint: 100-1,500 sq ft depending on capacity, including salt storage (30-day minimum), electrical panels, and maintenance access

• Accuracy: ±2% production rate control with continuous monitoring and automatic adjustment capabilities

• What production capacity and redundancy level is required? Calculate 1.5x average day chlorine demand plus 25% safety factor. Systems <50 lbs/day can use single units; >100 lbs/day require N+1 redundancy. Undersizing results in inadequate disinfection during peak demands; oversizing increases capital and O&M costs by 15-30%

• Should the system be packaged or site-built? Packaged systems work for <500 lbs/day production with 50-100 sq ft footprints. Site-built required for >1,000 lbs/day with dedicated buildings. Decision depends on available space, local electrical codes, and maintenance capabilities

• What electrolytic cell technology and configuration? Monopolar cells offer 85-90% current efficiency for <200 lbs/day systems. Bipolar cells provide 90-95% efficiency for larger systems but require higher capital investment. Wrong choice affects 20-year lifecycle costs by $100,000-500,000

• How will salt storage and handling be managed? Bulk storage (30-90 days) reduces costs but requires material handling equipment and weather protection. Bag systems increase labor costs 3-5x but suit smaller facilities with limited space and maintenance staff

• Division 40 - Process Integration

• Section 40 05 00 - Common Work Results for Process Integration

• Section 40 30 00 - Chemical Treatment Equipment

• Primary specification under 40 30 00 for electrolytic generation equipment, controls, and chemical feed systems

• Secondary references in 40 05 00 for integration with existing chlorination systems

• Material/Equipment Verification: Verify electrode material (MMO coating specifications), Confirm rectifier capacity and efficiency ratings, Check brine tank material (HDPE/fiberglass only)

• Installation Requirements: 480V/3-phase power with isolation transformer, Ventilation for hydrogen gas (1 CFM per amp), Floor drains for brine spills

• Field Challenges: Salt delivery/storage coordination, Electrical rough-in often undersized for rectifier loads

• Coordination Issues: HVAC integration for ventilation requirements, Process piping tie-ins during plant operation

• Lead times: 16-20 weeks typical, 24+ weeks for custom configurations

• MIOX Corporation - MIX Series (0.5-500 ppd capacity), dominant in smaller municipal plants

• De Nora - ChlorTec systems (10-8,000 ppd), popular for medium-large facilities

• Evoqua/Chlorinsitu - OSEC systems (5-2,000 ppd), strong in retrofit applications

• Kemisan - KLORIGEN units (20-6,000 ppd), growing presence in North American market with competitive pricing

• Bulk sodium hypochlorite delivery - Lower capital cost ($50K vs $200K+), preferred for plants <2 MGD with good chemical delivery access

• UV disinfection with chloramination - Higher capital but eliminates chlorine storage concerns, growing preference for plants >10 MGD

• Calcium hypochlorite feed systems - Intermediate option at $75-100K, suitable where brine disposal is problematic but on-site generation desired

Establish direct manufacturer relationships early - field service response varies dramatically between vendors. MIOX and De Nora offer superior technical support but at premium pricing. Consider split-system designs (separate electrolyzer/rectifier) for easier maintenance access. Negotiate service contracts including electrode replacement - typical 3-year electrode life costs $15,000-25,000. Size brine storage for 30-day capacity minimum to avoid delivery coordination issues.