Ion-exchange Vessels

Ion-exchange vessels remove dissolved ions from water by passing it through specialized resin beds that exchange unwanted ions for more desirable ones. These cylindrical pressure vessels, typically 6-12 feet in diameter for municipal applications, contain cation or anion exchange resins that selectively capture contaminants like hardness minerals, nitrates, or heavy metals. Modern municipal IX systems achieve 90-99% removal efficiency for target ions, with typical service flows of 2-8 gpm/ft² through the resin bed. The primary limitation is the need for chemical regeneration using salt brine or caustic solutions, creating concentrated waste streams that require disposal.

• Softening for Membrane Pretreatment (2-25 MGD): Ion-exchange vessels using strong acid cation resin remove calcium and magnesium upstream of RO/NF systems. Typically 2-4 vessels in lead-lag configuration, regenerated with 10% salt brine. Selected to prevent membrane scaling and extend membrane life from 3-5 years to 7-10 years.
• Nitrate Removal (0.5-15 MGD): Strong base anion resin vessels treat groundwater exceeding 10 mg/L nitrate-nitrogen. Usually 3-4 vessels with one in regeneration using 4-6% salt/2% caustic solution. Selected over biological treatment for consistent performance and lower O&M complexity.
• PFAS Treatment (1-50 MGD): Single-use anion exchange vessels using specialized resins like Purolite A-694 or ResinTech SIR-700 for per- and polyfluoroalkyl substances removal. Vessels operate until breakthrough, then entire resin bed requires disposal. Selected for 90%+ PFAS removal efficiency when GAC proves insufficient.
• Arsenic Removal (0.5-10 MGD): Iron-based media in pressure vessels (often Bayoxide E33 or GFH) treat groundwater exceeding 10 ppb arsenic. Pre-chlorination oxidizes As(III) to As(V) for better removal. Selected over membrane systems for lower energy costs and simpler operation in smaller plants.

Daily Operations: Operators monitor differential pressure (typically 5-15 psi across bed), effluent quality via online analyzers or grab samples, and flow rates. Hardness breakthrough triggers regeneration cycle - usually 1-5 grains/gallon for softening applications. Service run lengths vary from

• Pressure Vessel: ASME-coded steel or fiberglass tanks, 4-12 feet diameter, 6-20 feet tall for municipal applications. Internal rubber lining or epoxy coating prevents corrosion. Sizing based on 2-8 gpm/ft² loading rate and 4-8 hour empty bed contact time. Selection considers regeneration frequency and resin bed depth requirements.
• Resin Bed: Typically 3-6 feet deep, ranging from 50-500 cubic feet per vessel. Strong acid cation (Purolite C-100E), strong base anion (Rohm & Haas IRA-402), or specialty resins. Graded support media prevents resin loss. Selection based on target contaminant, capacity requirements, and regeneration chemical compatibility.
• Distribution System: Underdrain collectors with 0.2-0.5 mm slots prevent resin carryover while ensuring uniform flow distribution. Hub-and-lateral or Johnson screen designs common. Upper distributor for backwash and regeneration. Materials typically 316SS or PVC based on chemical exposure.
• Regeneration System: Chemical feed pumps, solution tanks, and piping for brine, acid, or caustic delivery. Eductor systems for brine preparation. Flow rates typically 0.5-1.0 gpm/ft² for regeneration. Safety features include containment, ventilation, and emergency shower stations for caustic handling.

• Flow Rate Parameters: Service flow rates: 2-15 gpm/ft² (typical 4-8 gpm/ft²), Backwash flow rates: 4-12 gpm/ft² (varies by resin type), Fast rinse rates: 2-4 gpm/ft²
• Vessel Sizing: Diameter range: 24"-120" for municipal applications, Height-to-diameter ratio: 2:1 to 4:1 (typically 3:1), Minimum bed depth: 30" (36" preferred for softening), Freeboard: 50-100% of bed depth for backwash expansion
• Operating Pressures: Design pressure: 125-150 psi (fiberglass vessels), Operating pressure: 25-80 psi, Minimum pressure drop across bed: 2-5 psi at service flow
• Resin Loading: Softening: 1.0-1.5 ft³ resin per gpm continuous flow, Nitrate removal: 0.8-1.2 ft³ resin per gpm, PFAS removal: 2-4 ft³ resin per gpm (depending on influent levels)
• Regeneration Requirements: Salt dosage: 6-15 lbs NaCl per ft³ resin (softening), Contact time: 30-60 minutes, Regeneration flow rate: 0.5-2 gpm/ft²

• Single-bed vs. multi-bed configuration for capacity >5 MGD: Multi-bed systems provide redundancy and allow continuous operation during regeneration. Single beds require storage or parallel units. Threshold: >5 MGD typically justifies multiple vessels. Wrong decision leads to inadequate capacity during regeneration cycles or excessive capital costs.
• Cocurrent vs. countercurrent regeneration for softening applications: Countercurrent systems achieve 20-30% better salt efficiency (6-8 lbs/ft³ vs. 10-15 lbs/ft³) but require specialized internal distributors. Threshold: >2 MGD capacity or high hardness (>300 mg/L) favors countercurrent. Poor choice impacts long-term operating costs significantly.
• Fiberglass vs. steel vessel construction for operating pressures >60 psi: Steel vessels required for high-pressure applications or aggressive water chemistry. Fiberglass suitable for most municipal applications <100 psi. Threshold: >80 psi operating pressure or pH <6.5 requires steel. Wrong material selection leads to premature failure or unnecessary costs.
• Internal vs. external regeneration system for space-constrained sites: External brine systems allow better control but require additional space and piping. Internal systems are compact but limit regeneration flexibility. Site footprint <1000 ft² typically drives internal system selection.

• Primary: Division 40 - Process Integration
• Section 40 21 13 - Liquid-Liquid Separation

• Material/Equipment Verification: Verify resin specifications match water quality requirements, Confirm vessel ASME code compliance and pressure ratings, Check regenerant chemical compatibility with internals
• Installation Requirements: Crane access for vessel placement (typical 8-12 week lead times), Adequate clearance for resin changeout (minimum 8-10 ft overhead), Proper foundation design for loaded vessel weight
• Field Challenges: Resin shipping/loading logistics coordination, Underdrain system installation verification
• Coordination Issues: Chemical feed system integration timing, Control system programming dependencies

• Pentair Water Solutions: X-Flow IX series vessels, strong municipal track record with 2-50 MGD softening plants
• Evoqua Water Technologies: MEMCOR IX systems, popular for 5-100 MGD facilities with automated regeneration
• ResinTech: Standard FRP vessels (RT-series), cost-effective for smaller 0.5-10 MGD applications
• US Water Services: Custom steel vessels, preferred for large 20+ MGD installations requiring robust construction

• Membrane softening (nanofiltration): Preferred for high-TDS waters >500 mg/L, 20-30% higher capital cost but lower chemical usage
• Lime softening: Cost-effective for large plants >20 MGD with high hardness, 40-50% lower operating costs but higher sludge handling requirements
• Reverse osmosis: Best for combined softening/TDS reduction, typically 2-3x capital cost but produces high-quality permeate for blending applications

Establish relationships with resin suppliers early - they often provide free technical support and can optimize regeneration cycles for your specific water chemistry. Consider bulk resin purchasing agreements for multiple vessels to reduce costs by 15-20%. Always specify spare resin (typically 10% extra) during initial procurement rather than emergency purchasing later. Request manufacturer startup assistance - the investment pays off in optimized performance and reduced troubleshooting time.