Electrodialysis Equipment

Electrodialysis (ED) equipment removes dissolved salts from water using ion-selective membranes and direct current electrical fields to separate ionic contaminants. The process passes water through alternating cation and anion exchange membranes while applying voltage, creating concentrated brine and diluted product streams. Typical municipal ED systems achieve 85-95% salt removal efficiency at 75-85% water recovery rates. The primary limitation is high electrical energy consumption (2-8 kWh per thousand gallons) and the need for extensive pretreatment to prevent membrane fouling, making it cost-prohibitive for most municipal applications compared to reverse osmosis.

• Brackish Water Desalination: ED systems treat brackish groundwater (1,000-5,000 mg/L TDS) for potable supply in coastal and inland municipalities. Systems typically process 0.5-10 MGD, positioned after pre-filtration and ahead of disinfection. Selected over RO when TDS is moderate and energy costs favor ED's linear relationship between salinity and power consumption.
• Concentrate Management: ED reduces RO concentrate volumes by 50-80% before discharge or deep well injection. Installed downstream of RO systems at 10-50 MGD plants where discharge limits are restrictive. More cost-effective than evaporators for moderate TDS concentrate streams.
• Selective Ion Removal: Monovalent-selective ED removes sodium and chloride while retaining beneficial minerals like calcium and magnesium. Used at 1-15 MGD plants treating hard brackish water where customers prefer mineralized finished water over RO permeate requiring remineralization.

Daily Operations: Operators monitor stack voltage, current density, and differential pressure across membrane stacks every 2-4 hours. Key parameters include product water conductivity (target <500 µS/cm), concentrate flow rates, and power consumption (typically 1-3 kWh/1,000 gallons). Automatic polarity reversal cycles run every 15-60 minutes to prevent scaling and fouling.

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Maintenance: Weekly membrane cleaning with dilute acid (pH 2-3) and caustic (pH 11-12) solutions removes scale and biofilm. Monthly electrode inspection and quarterly membrane stack inspection required. Operators need Level II certification for electrical safety around DC power systems. Full membrane replacement typically occurs every 3-5 years at $50,000-200,000 per stack.

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Troubleshooting: Increasing voltage at constant current indicates membrane fouling or scaling. Rising pressure drop signals channel blockage requiring immediate cleaning. Declining current efficiency below 85% suggests membrane degradation. Warning signs include unusual odors (chlorine production from electrode degradation) and visible membrane discoloration during inspections.

• Ion Exchange Membranes: Alternating cation and anion exchange membranes (typically 600-1,200 pairs) create desalination and concentration chambers. Membranes sized 0.5-2.0 m² each, constructed from cross-linked polymer films. Selection based on current density requirements (150-400 A/m²) and fouling resistance.
• Electrode Assembly: Titanium electrodes with mixed metal oxide coating generate DC current for ion transport. Sized for 2-8 V/cell pair operation at design current density. Electrode life typically 5-10 years depending on water quality and current loading.
• Stack Assembly: Houses membrane pairs in parallel flow configuration. Municipal stacks contain 200-800 cell pairs, processing 50-500 gpm per stack. Constructed with corrosion-resistant materials (PVC, CPVC) and designed for 100-150 psi operation.
• Power Supply: DC rectifiers convert AC power to controlled DC current. Sized for 200-2,000 kW depending on plant capacity and salinity reduction requirements.

• Flow Rate Capacity: 0.1-25 MGD per train, with modular stacking for larger plants. Standard municipal modules: 0.5, 1.0, 2.5, 5.0 MGD configurations.
• Feed Water TDS Range: 500-5,000 mg/L optimal; 200-10,000 mg/L operational limit. Performance degrades below 200 mg/L due to insufficient conductivity.
• Operating Pressure: 15-60 psi across stack. Pressure drop typically 10-25 psi per stage at design flow rates.
• Current Density: 150-400 A/m² membrane surface area. Higher densities increase energy consumption exponentially above 300 A/m².
• Recovery Rate: 85-95% typical for municipal brackish water. Lower recoveries (75-85%) required for high-fouling feedwater.
• Power Consumption: 1.5-4.0 kWh/1,000 gallons treated, depending on feed TDS and target product quality.
• Membrane Life: 5-7 years typical replacement cycle under proper pretreatment conditions.
• Temperature Range: 35-113°F operational; 68-86°F optimal efficiency range.
• pH Operating Range: 2-9 for standard membranes; 4-8 for maximum membrane life.
• Pretreatment Requirements: <0.1 NTU turbidity, <0.5 mg/L free chlorine, SDI <3.0.

• 1. Single-stage vs. Multi-stage Configuration? Single-stage adequate for <2,000 mg/L feed TDS targeting 500 mg/L product. Multi-stage required for higher removals or feed concentrations >3,000 mg/L. Wrong choice results in 20-40% higher energy costs or failure to meet product water targets. Need: detailed feed water analysis, product quality requirements.
• 2. Constant Current vs. Constant Voltage Operation? Constant current provides stable performance but requires sophisticated controls. Constant voltage simpler but efficiency drops 15-25% as membranes age. Decision impacts 5-year operating costs by $100,000-500,000 for 5 MGD plant. Need: operational complexity tolerance, maintenance capabilities.
• 3. Concentrate Disposal Method? Discharge to sewer requires <3,000 mg/L TDS limit in most municipalities. Deep well injection needs geological assessment. Evaporation ponds require 2-5 acres per MGD. Wrong choice can add $200,000-1M in infrastructure costs. Need: local discharge regulations, geological data.
• 4. Pretreatment Extent? Minimal pretreatment (cartridge filtration) acceptable for high-quality groundwater. Extensive pretreatment (coagulation, sedimentation, filtration) required for surface water sources. Insufficient pretreatment reduces membrane life by 50-70%. Need: comprehensive water quality analysis including organics, scaling potential.

• Division 46 - Water and Wastewater Equipment
• Section 46 71 13

• Material/Equipment Verification: Verify membrane specifications and warranty terms; Confirm power supply compatibility and efficiency ratings; Check pretreatment system integration requirements
• Installation Requirements: Level concrete pads with vibration isolation; Dedicated electrical room for DC power supplies; Chemical feed systems for cleaning solutions
• Field Challenges: Membrane stack alignment during installation; DC power supply commissioning and testing; Integration with existing SCADA systems
• Coordination Issues: 16-24 week lead times for custom stack configurations; Early coordination with electrical contractor for DC systems

• Evoqua Water Technologies - ADVION ED systems for 0.1-10 MGD applications
• Saltworks Technologies - SaltMaker ED units, popular for smaller municipal plants (0.5-5 MGD)
• Suez/GE Water - HERO ED systems with integrated controls for larger installations
• PCCell GmbH - European manufacturer with growing North American presence in specialized applications

• Reverse Osmosis - Lower capital cost, higher energy use. Preferred for <2 MGD plants or high TDS applications.
• Ion Exchange - Better for selective removal, higher chemical costs. Choose for specific contaminants like nitrates.
• Nanofiltration - Middle ground for moderate TDS reduction. Roughly 30% lower capital than ED but limited to <1,500 mg/L TDS applications.

Work closely with membrane manufacturers on cleaning protocols - improper CIP procedures void warranties quickly. Specify redundant power supplies for critical applications; single-point failures are expensive. Consider modular designs allowing staged construction. Negotiate spare membrane pricing upfront - replacement costs can surprise operators. Plan for higher electrical demand than initially calculated; real-world current densities often exceed design values by 15-20%.