Nanofiltration Systems

Nanofiltration (NF) systems remove dissolved contaminants from water using semi-permeable membranes with pore sizes between 0.5-2 nanometers, targeting hardness, organics, and multivalent ions while allowing monovalent salts to partially pass through. These pressure-driven membrane systems operate at 75-200 psi and typically achieve 85-95% rejection of calcium and magnesium hardness in municipal applications. NF provides selective separation ideal for water softening and color removal, but requires pretreatment to prevent fouling and generates 15-25% concentrate waste requiring disposal.

• Groundwater Hardness/TDS Reduction: NF systems treat high-hardness groundwater (300-800 mg/L as CaCO3) while retaining beneficial minerals. Positioned after well pumps and ahead of clearwell storage, these systems reduce hardness by 80-90% without complete demineralization. Selected over RO for lower energy consumption (150-250 psi vs 400+ psi) and reduced brine production.
• Surface Water Color/Organics Removal: Municipal plants use NF for removing natural organic matter and color compounds that cause taste/odor issues. Installed after conventional treatment but before final disinfection, NF reduces TOC by 85-95% and eliminates DBP precursors. Preferred over GAC for consistent performance and lower long-term costs.
• Arsenic/Nitrate Compliance: Small communities (0.5-5 MGD) deploy NF for arsenic removal below 10 ppb and nitrate reduction below 10 mg/L. Systems operate at 15-25 gpm/ft² flux with 85-95% contaminant rejection. Selected for reliable compliance without chemical addition requirements.

Daily Operations: Operators monitor feed/permeate pressure differential (typically 8-15 psi), recovery rate (75-85%), and permeate quality parameters. Adjustments include feed pressure optimization via VFD control and antiscalant dosing (2-5 mg/L) based on water chemistry. Flow totalizers track daily production and recovery efficiency trends.

​

Maintenance: Weekly membrane performance normalization calculations identify cleaning needs. Monthly CIP procedures require 4-6 hours using alternating caustic (pH 11-12) and acid (pH 2-3) solutions. Operators need Level II certification for chemical handling. Annual membrane integrity testing and element replacement requires confined space training for vessel entry.

​

Troubleshooting: Rapid flux decline indicates scaling or fouling requiring immediate CIP. Permeate quality degradation signals membrane damage or O-ring failure. Salt passage increase above 15% baseline indicates replacement needs. Warning signs include unusual pressure patterns, high differential pressure

• Membrane Elements: Thin-film composite polyamide membranes in 8" diameter spiral-wound configuration. Standard 40" length elements provide 400 ft² surface area. Municipal installations typically use 200-400 MWCO membranes operating at 150-250 psi. Manufacturers include Filmtec, Hydranautics, and Toray with 3-5 year service life.
• High-Pressure Pumps: Multi-stage centrifugal pumps deliver 150-300 psi operating pressure. Sized for 75-85% recovery rates with 15-40% recirculation. Stainless steel construction with VFD control for flux optimization.
• Pressure Vessels: FRP or stainless steel housings accommodate 1-7 membrane elements per vessel. Standard 8" diameter vessels rated for 300 psi with end ports for element loading.
• Cleaning System: CIP skids with heated chemical circulation for membrane cleaning every 30-90 days using caustic/acid solutions at controlled pH and temperature.

• Hydraulic Loading: 8-15 gfd (gallons per square foot per day) for typical municipal applications, with 10-12 gfd being optimal for consistent performance and reasonable membrane life
• Operating Pressure: 80-150 psi for most NF membranes, with 100-120 psi typical for municipal hardness removal and organics reduction applications
• Recovery Rate: 75-85% for municipal systems, balancing concentrate disposal costs with membrane fouling potential. Higher recoveries increase scaling risk
• Flux Rate: 12-20 gallons per square foot per day (gfd) sustainable flux, with 15 gfd common for new installations. Lower flux extends membrane life but increases capital costs
• Pretreatment Requirements: SDI15 <3.0, turbidity <0.5 NTU, and chlorine residual <0.1 mg/L to prevent membrane damage
• Membrane Replacement: Design for 3-5 year membrane life under typical municipal conditions, with annual replacement costs of $0.50-0.75 per 1,000 gallons treated
• System Availability: Target 95-98% uptime with proper redundancy and maintenance protocols

• What recovery rate can the source water support without excessive scaling? Requires detailed water chemistry analysis including LSI, calcium, sulfate, and silica concentrations. Wrong decision leads to frequent membrane cleaning (monthly vs. quarterly) and premature replacement. Need comprehensive water quality data over seasonal variations.
• Should the system include intermediate pumping or single-stage design? Depends on total system pressure drop and energy optimization. Systems >20 MGD typically benefit from intermediate pumping to reduce energy costs by 15-25%. Requires detailed hydraulic modeling and life-cycle cost analysis comparing capital vs. operating expenses.
• What level of pretreatment is justified for the specific source water? Balance between pretreatment capital costs and membrane replacement frequency. Poor pretreatment decisions can reduce membrane life from 4 years to 18 months. Requires pilot testing or similar source water performance data.
• How much redundancy is needed for continuous operation? Municipal systems typically require N+1 redundancy for critical applications. Insufficient redundancy risks regulatory violations during maintenance. Consider peak demand periods and acceptable downtime for the specific utility.

• MasterFormat 40 31 26 - Membrane Filtration Water Treatment Equipment: Primary section covering nanofiltration systems, membranes, and associated equipment
• May also reference 40 20 00 for water treatment equipment general requirements and 40 31 00 for filtration systems overview

• Material/Equipment Verification: Verify NSF/ANSI 61 certification for all wetted components, Confirm membrane compatibility with disinfection residuals, Validate pressure vessel ASME code compliance
• Installation Requirements: Level concrete pads within 1/8" over 20 feet, Adequate crane access for membrane vessel installation, Minimum 6-foot service clearances around pressure vessels
• Field Challenges: Membrane preservation solution disposal coordination, Precise piping alignment for multi-vessel arrays, Clean-in-place system integration complexity
• Coordination Issues: 16-20 week lead times for custom pressure vessels, Electrical coordination for VFD-controlled high-pressure pumps

• DOW FilmTec NF270/NF90 elements lead municipal applications with proven hardness/organics removal
• Toray UTC-70 series offers reliable performance in 4-8" configurations
• Hydranautics ESNA1-LF2 provides low-fouling characteristics for challenging source waters
• Koch Fluid Systems SelRO elements target specific municipal contaminant profiles
• All maintain strong municipal references across 1-50 MGD installations

• Conventional lime softening costs 40-60% less for hardness removal but requires larger footprint and sludge handling
• Ion exchange provides selective removal at similar capital cost but higher O&M due to regeneration chemicals
• Ultrafiltration followed by GAC handles organics removal at 30% lower cost but cannot address hardness or TDS reduction needs

Manufacturer relationships matter - establish direct technical support contacts beyond sales reps for troubleshooting membrane fouling issues. Standardize on single membrane supplier across multiple projects to leverage volume pricing (typically 15-20% savings on replacement elements). Consider phased capacity installation - install full infrastructure but delay some membrane vessels, reducing initial capital by 20-30% while maintaining expansion capability.