Microfiltration Systems

Microfiltration systems remove suspended solids, bacteria, and pathogens from water using membrane barriers with pore sizes ranging from 0.1 to 0.4 microns. These pressure-driven systems force water through hollow fiber or flat sheet membranes, achieving turbidity removal to consistently below 0.1 NTU while providing 4-log virus removal when combined with coagulation. Municipal applications typically operate at 10-25 psi transmembrane pressure with flux rates of 20-60 gallons per square foot per day. The primary trade-off is membrane fouling requiring regular backwashing and chemical cleaning, increasing operational complexity and energy consumption compared to conventional filtration.

• Primary Treatment Clarification Enhancement: MF systems are increasingly used downstream of primary clarifiers in 2-15 MGD plants to remove residual suspended solids before secondary treatment. The 0.1-0.4 micron membranes capture particles that escaped clarification, reducing BOD loading on downstream biological processes by 15-25%. Systems typically process 80-95% of plant flow with bypass capability

• Tertiary Filtration for Reuse: Most common application in 5-50 MGD facilities, replacing conventional sand filters downstream of secondary clarifiers. MF provides consistent <1 NTU effluent for irrigation reuse or indirect potable reuse pretreatment. Systems operate at 35-65 GFD flux rates with backwash frequencies of 20-45 minutes

• Solids Thickening Reject Water Treatment: Smaller plants (0.5-5 MGD) use MF to treat centrate and filtrate from dewatering operations before returning to plant headworks. This prevents solids recycling and reduces peak hydraulic loading by 10-20%

• Emergency Backup Filtration: Packaged MF units provide temporary tertiary treatment during conventional filter maintenance or upset conditions, maintaining permit compliance

Daily Operations: Operators monitor transmembrane pressure trends (typically 2-8 psi normal operation), recording readings every 2-4 hours. Turbidity monitoring requires daily calibration checks and grab sample verification. Flow distribution between trains requires manual balancing during peak demand periods. Chemical feed tank levels and pump stroke counts need daily verification to maintain proper dosing rates.

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Maintenance: Membrane modules require integrity testing every 6 months using bubble point or pressure decay methods. Chemical cleaning intervals range from 2-8 weeks depending on feed water quality, requiring confined space entry procedures and acid/caustic handling protocols. Backwash pump maintenance occurs quarterly with bearing lubrication and impeller inspection. PPE requirements include chemical-resistant gloves, eye protection, and respiratory protection during cleaning operations.

• Membrane Modules: Hollow fiber or flat sheet configurations in PVDF or PES materials. Hollow fiber modules range from 40-80 ft² surface area per module, with 500-2000 modules per train depending on capacity. Selection factors include cleaning tolerance, flux requirements, and replacement cost ($150-400 per module)

• Backwash System: High-rate pumps (typically 2-3x forward flow rate) with air scour blowers providing 3-8 SCFM per ft² membrane area. Includes backwash tanks (15-30 minutes storage) and automated valve sequencing. Critical for maintaining 20-40 GFD sustainable flux rates

• Chemical Feed Systems: Sodium hypochlorite (0.5-2.0 mg/L) for biofouling control and citric acid (200-500 mg/L) for iron/calcium scaling. Includes day tanks, metering pumps, and automated dosing based on transmembrane pressure or time intervals

• Instrumentation Package: Transmembrane pressure monitoring (0-30 psi range), turbidimeters (<0.1-10 NTU), and flow meters with totalizing capability. PLC-based control systems manage backwash cycles, chemical dosing, and alarm functions

• Membrane Flux Rate: 15-40 GFD (gallons per square foot per day) for typical municipal applications. Conservative design uses 20-25 GFD for surface water, 25-35 GFD for groundwater. Higher flux rates increase fouling potential and cleaning frequency

• Transmembrane Pressure (TMP): Operating range 5-15 psi, with alarm typically set at 20 psi. Clean water TMP should be 2-4 psi. Rapid TMP rise indicates membrane fouling requiring backwash or chemical cleaning

• Recovery Rate: 90-95% for municipal systems. Lower recovery increases waste stream volume but reduces fouling. Systems typically operate at 92-94% recovery for optimal balance

• Backwash Frequency: Every 15-60 minutes depending on feed water quality. Surface water requires more frequent backwash (15-30 min) than groundwater (30-60 min)

• Chemical Enhanced Backwash (CEB): Every 6-24 hours using 100-500 ppm sodium hypochlorite or 2000-5000 ppm citric acid. Frequency depends on organic loading and iron/manganese content

• Membrane Pore Size: 0.1-0.4 microns nominal, with 0.2 microns most common for municipal applications providing 4-log virus removal credit

• Air Scour Rate: 3-8 SCFM per square foot of membrane area during backwash cycles

• What membrane configuration suits the site constraints and maintenance capabilities? Hollow fiber systems (Pall, Evoqua) require 15-20% less footprint than flat sheet (Kubota, Toray) but need more skilled maintenance. Submerged systems (GE ZeeWeed) use less energy but require larger tanks. Wrong choice impacts 20-year lifecycle costs by $500K-2M for 10 MGD plants

• Should the system include pre-treatment for organics removal? Raw water TOC >4 mg/L typically requires coagulation upstream to prevent rapid membrane fouling. Without proper pre-treatment, CEB frequency doubles and membrane life drops from 7-10 years to 3-5 years. Need jar testing and pilot data for waters with seasonal algae blooms

• What level of automation and remote monitoring is justified? Basic SCADA costs $50K-100K but prevents membrane damage from operator errors. Advanced systems with predictive analytics cost $200K-400K but reduce chemical usage 15-25% and extend membrane life. Decision depends on staffing levels and operator experience

• How much redundancy is required for continuous operation? N+1 configuration adds 20-30% capital cost but ensures compliance during maintenance. Smaller plants (<5 MGD) often use 50% redundancy while larger systems use 25% spare capacity based on regulatory requirements and consequence of failure

• Division 46 - Water and Wastewater Equipment

• Section 46 71 13 - Membrane Bioreactors (primary for submerged configurations)

• Material/Equipment Verification: Membrane material compatibility with chlorine residuals, Skid materials meeting NSF-61 standards, CIP chemical compatibility verification

• Installation Requirements: Crane access for membrane module replacement, Dedicated electrical room with HVAC for controls, Chemical feed system integration points

• Field Challenges: Membrane handling requires clean environment, Precise leveling critical for uniform distribution, 12-16 week lead times typical

• Coordination Issues: Early coordination with controls contractor for SCADA integration, Chemical feed system timing with startup

• Evoqua - ZeeWeed 1000/500 series (municipal plants 1-50 MGD)

• Pentair X-Flow - Aquaflex A35/A55 modules (0.5-20 MGD applications)

• Suez - Ultraflo membrane systems (2-100 MGD capacity)

• Koch Membrane - PURON submerged systems (municipal retrofits)

• Conventional Sand Filtration - 30-40% lower capital cost, preferred for high turbidity raw water or existing infrastructure upgrades

• Ultrafiltration - Similar performance but higher flux rates, 10-15% higher cost, better for space-constrained sites

• Ceramic Membranes - 2-3x capital cost but longer life and chemical tolerance, justified for challenging water quality or industrial discharge impacts

Establish direct technical support relationships with membrane manufacturers early - their field service engineers provide invaluable startup assistance and troubleshooting expertise. Negotiate spare membrane module pricing upfront, as replacement costs can be 15-20% of original equipment cost. Consider phased installations for larger plants to optimize operations before full buildout. Many operators report significant learning curves, so plan extended commissioning periods.