💧Membrane Technology for Water Treatment Unit 12 – Municipal Water Treatment Applications

Introduction to Municipal Water Treatment

• Municipal water treatment involves a series of processes designed to remove contaminants and ensure safe, potable water for public consumption
• Contaminants in raw water can include suspended solids, dissolved organic matter, pathogens (bacteria, viruses), and various chemical pollutants (pesticides, heavy metals)
• Conventional water treatment typically consists of coagulation, flocculation, sedimentation, filtration, and disinfection steps
• Membrane technology has emerged as an advanced alternative or complement to conventional treatment, offering high removal efficiency and compact footprint
• Membrane processes, such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), can effectively remove a wide range of contaminants based on their pore size and rejection characteristics
• Membrane treatment enables compliance with increasingly stringent water quality regulations and can address emerging contaminants of concern (pharmaceuticals, personal care products)
• Integration of membrane technology into existing water treatment plants requires careful consideration of pre-treatment, system design, and operational aspects to ensure optimal performance and longevity

Membrane Filtration Basics

• Membrane filtration relies on semi-permeable membranes that allow water to pass through while rejecting contaminants based on size, charge, and other properties
• The driving force for membrane filtration can be pressure, concentration gradient, or electrical potential difference
• Pressure-driven membrane processes include MF, UF, NF, and RO, with increasing rejection capabilities and operating pressures
  • MF membranes have pore sizes ranging from 0.1 to 10 microns and can remove suspended solids, bacteria, and protozoa
  • UF membranes have pore sizes between 0.01 and 0.1 microns and can additionally remove viruses and some dissolved organic matter
  • NF membranes have pore sizes around 0.001 microns and can reject multivalent ions and larger dissolved molecules
  • RO membranes are considered non-porous and can remove monovalent ions and small dissolved compounds
• Membrane performance is characterized by permeate flux (volume of water produced per unit membrane area and time) and rejection (percentage of contaminant removed)
• Concentration polarization and membrane fouling are major challenges in membrane filtration, requiring appropriate pre-treatment and cleaning strategies
• Membrane modules can be configured in various geometries, such as hollow fiber, spiral wound, and plate-and-frame, to optimize surface area and hydrodynamic conditions

Types of Membranes Used in Water Treatment

• Membranes used in water treatment can be classified based on their material, pore size, and configuration
• Polymeric membranes are the most common type, made from materials such as polyethersulfone (PES), polyvinylidene fluoride (PVDF), and polyamide (PA)
  • PES and PVDF membranes are widely used for MF and UF applications due to their mechanical strength, chemical resistance, and hydrophilicity
  • PA membranes are the standard material for NF and RO, offering high salt rejection and permeability
• Ceramic membranes, made from materials like alumina and titanium dioxide, offer high chemical and thermal stability but are more expensive than polymeric membranes
• Mixed matrix membranes incorporate inorganic particles (zeolites, metal-organic frameworks) into a polymeric matrix to enhance performance and selectivity
• Surface modification techniques, such as grafting and coating, can improve membrane hydrophilicity, fouling resistance, and targeted removal of specific contaminants
• Membrane pore size and distribution play a crucial role in determining the rejection spectrum and permeate quality
  • Tight UF membranes with molecular weight cut-offs (MWCO) below 100 kDa can achieve high virus removal
  • NF membranes with MWCO between 200 and 1000 Da are effective for removing hardness, dissolved organic carbon, and synthetic organic compounds
• Membrane configuration affects the module design, packing density, and cleaning efficiency
  • Hollow fiber membranes offer high packing density but are prone to fouling and difficult to clean
  • Spiral wound modules are compact and cost-effective but have limited tolerance for suspended solids
  • Plate-and-frame configurations allow easy cleaning and replacement but have lower packing density

Pre-treatment Processes

• Pre-treatment is essential to maintain membrane performance, minimize fouling, and extend membrane lifespan
• Screening is the first step to remove large debris, leaves, and other coarse materials that can damage or clog membranes
  • Coarse screens (1-10 mm openings) are used for initial removal, followed by fine screens (0.1-1 mm) for further protection
• Coagulation and flocculation help destabilize colloidal particles and form larger flocs that can be easily removed by sedimentation or filtration
  • Common coagulants include aluminum sulfate (alum), ferric chloride, and polyaluminum chloride (PACl)
  • Organic polymers (polyacrylamide) can be used as flocculant aids to improve floc strength and settleability
• Sedimentation or clarification removes suspended solids and flocs formed during coagulation/flocculation
  • Conventional sedimentation basins have a retention time of 2-4 hours to allow gravitational settling
  • High-rate clarifiers, such as plate settlers and tube settlers, can achieve faster settling rates and smaller footprints
• Granular media filtration, using sand, anthracite, or garnet, provides a final polishing step before membrane filtration
  • Dual-media filters (sand and anthracite) are common, offering deep bed filtration and longer filter runs
  • Pressure filters are often used for pre-treatment to pressurized membrane systems
• Chemical conditioning, such as pH adjustment and scale inhibitor addition, may be necessary to control scaling and protect membranes from chemical degradation
• Cartridge filters (1-10 microns) are used as a final safeguard to remove any remaining suspended solids and protect membranes from physical damage

Membrane System Design for Municipal Applications

• Designing a membrane system for municipal water treatment involves considering factors such as water quality, treatment objectives, plant capacity, and site constraints
• Pilot testing is often conducted to evaluate membrane performance, optimize operating conditions, and gather data for full-scale design
  • Pilot plants typically run for several months to capture seasonal variations in water quality and assess long-term membrane fouling and cleaning effectiveness
• Membrane system capacity is determined based on the design flow rate, permeate recovery, and redundancy requirements
  • Permeate recovery, the ratio of permeate flow to feed flow, typically ranges from 80-95% for low-pressure membranes (MF/UF) and 50-85% for high-pressure membranes (NF/RO)
  • Redundant membrane units or trains are included to allow for maintenance, cleaning, and unexpected downtime without interrupting the water production
• Membrane skids are designed to accommodate the required number of membrane modules, piping, valves, and instrumentation
  • Skid layout should optimize flow distribution, minimize pressure drop, and facilitate access for operation and maintenance
• Feed water pumps are selected to provide the necessary pressure and flow rate for the membrane system
  • Variable frequency drives (VFDs) are often used to adjust pump speed and maintain constant permeate flow or pressure
• Backwash and cleaning systems are integrated into the design to periodically remove accumulated foulants and restore membrane permeability
  • Backwash intervals and duration are optimized based on the feed water quality and fouling propensity
  • Clean-in-place (CIP) systems typically include chemical storage tanks, metering pumps, and heating elements to prepare and circulate cleaning solutions
• Instrumentation and control systems are essential for monitoring and automating membrane operations
  • Key parameters to monitor include feed, permeate, and concentrate flow rates, pressures, and water quality indicators (turbidity, conductivity)
  • Supervisory control and data acquisition (SCADA) systems enable remote monitoring, data logging, and control of the membrane system
• Integration with other treatment processes, such as pre-treatment and post-treatment, should be considered to ensure compatibility and optimize overall plant performance

Operational Considerations and Challenges

• Operating a membrane system for municipal water treatment requires careful monitoring, maintenance, and optimization to ensure consistent performance and regulatory compliance
• Membrane fouling is a major operational challenge that can reduce permeate flux, increase energy consumption, and shorten membrane lifespan
  • Fouling can be caused by suspended solids, organic matter, biological growth, and scaling (mineral precipitation)
  • Monitoring transmembrane pressure (TMP) and permeate flow can help detect fouling and trigger cleaning cycles
• Backwashing is a routine operation to remove reversible fouling and maintain membrane permeability
  • Backwash frequency and duration should be optimized based on the feed water quality and fouling rate
  • Air scouring can be used in conjunction with backwashing to enhance foulant removal and reduce chemical cleaning frequency
• Chemical cleaning is necessary to remove irreversible fouling and restore membrane performance
  • Cleaning agents are selected based on the type of foulant and membrane material compatibility
  • Common cleaning chemicals include acidic solutions (citric acid) for scaling, alkaline solutions (sodium hydroxide) for organic fouling, and oxidants (sodium hypochlorite) for biological fouling
  • Cleaning effectiveness should be monitored by tracking permeability recovery and comparing with benchmark values
• Membrane integrity monitoring is critical to ensure the removal of pathogens and maintain water quality
  • Direct integrity testing methods, such as pressure decay tests and marker-based tests, can detect membrane breaches and assess the log removal value (LRV) of the system
  • Indirect integrity monitoring, using turbidity or particle counters, provides continuous assessment of filtrate quality and can trigger alarms or shutdown in case of membrane failure
• Energy optimization is important to minimize operating costs and carbon footprint of membrane systems
  • Energy recovery devices, such as turbochargers and pressure exchangers, can recover energy from the concentrate stream and reduce net energy consumption
  • Optimizing feed pressure, recovery rate, and cleaning intervals can help balance energy efficiency and membrane performance
• Concentrate management is a challenge in membrane treatment, particularly for high-recovery systems
  • Concentrate disposal options include surface water discharge, deep well injection, evaporation ponds, and zero liquid discharge (ZLD) systems
  • Regulations on concentrate quality and disposal vary by location and can impact the feasibility and cost of membrane treatment

Post-treatment and Disinfection

• Post-treatment refers to the processes downstream of membrane filtration that are necessary to meet water quality goals and ensure public health protection
• Disinfection is a critical post-treatment step to inactivate any remaining pathogens and provide a residual in the distribution system
  • Chlorination is the most common disinfection method, using chlorine gas, sodium hypochlorite, or calcium hypochlorite
  • Chlorine dose is adjusted based on the water quality and desired residual, typically targeting 0.2-1.0 mg/L free chlorine
  • Chloramination, using a combination of chlorine and ammonia, can be used to maintain a longer-lasting residual and reduce disinfection byproduct formation
  • UV disinfection can be used as an alternative or supplement to chemical disinfection, providing effective inactivation of Cryptosporidium and Giardia without forming byproducts
• pH adjustment may be necessary to control corrosion, optimize disinfection effectiveness, and improve taste and odor
  • Acidic permeate from RO systems can be blended with bypass water or adjusted using alkaline chemicals (sodium hydroxide, soda ash)
  • Target pH range is typically 7.0-8.5, depending on the distribution system materials and water quality goals
• Remineralization is often required for RO permeate to restore mineral content and improve water stability
  • Calcium and magnesium can be added using calcite contactors, dolomite beds, or direct chemical injection (calcium chloride, magnesium sulfate)
  • Alkalinity can be increased using sodium bicarbonate or carbon dioxide injection to control pH and prevent corrosion
• Fluoridation may be mandated by local regulations to prevent dental caries
  • Fluoride can be added using sodium fluoride, sodium fluorosilicate, or fluorosilicic acid, targeting a concentration of 0.7-1.2 mg/L
• Corrosion control strategies are important to minimize lead and copper leaching from distribution system materials
  • Phosphate-based corrosion inhibitors (orthophosphate, hexametaphosphate) can form protective scales on pipe surfaces
  • pH and alkalinity adjustment can also help control corrosion by reducing the solubility of metal ions
• Blending of membrane permeate with other water sources (groundwater, surface water) can be used to optimize water quality and reduce post-treatment requirements
  • Blending ratios should be determined based on the source water qualities, treatment goals, and regulatory compliance
• Clearwell storage is often provided after post-treatment to ensure adequate contact time for disinfection and provide operational flexibility
  • Baffling and mixing in the clearwell should be designed to minimize short-circuiting and dead zones
  • Clearwell size is determined based on the disinfection contact time, plant capacity, and storage requirements for distribution system

Case Studies and Real-world Applications

• The Orange County Water District (OCWD) in California operates the world's largest potable reuse project using membrane technology
  • The Groundwater Replenishment System (GWRS) produces 100 million gallons per day (MGD) of high-quality water from treated wastewater using MF, RO, and UV/H2O2 advanced oxidation
  • The purified water is injected into the groundwater basin and serves as a seawater intrusion barrier and a reliable water supply for over 850,000 residents
• The Carlsbad Desalination Plant in San Diego County, California, is the largest seawater desalination facility in the United States
  • The plant uses UF pretreatment followed by RO to produce 50 MGD of potable water from Pacific Ocean seawater
  • The desalinated water is post-treated with calcite contactors for remineralization and blended with other water sources before distribution
• The Scottsdale Water Campus in Arizona showcases the integration of multiple membrane technologies for potable water production
  • The campus includes MF, RO, and electrodialysis reversal (EDR) systems to treat a blend of surface water, groundwater, and reclaimed wastewater
  • The advanced treatment processes allow for a diversified water portfolio and improved water quality, with a total production capacity of 70 MGD
• The Andijk III water treatment plant in the Netherlands demonstrates the use of ceramic membranes for surface water treatment
  • The plant uses CeraMac® ceramic MF membranes followed by UV disinfection to treat water from the IJssel Lake
  • Ceramic membranes offer high chemical and thermal resistance, enabling effective cleaning and long membrane lifetimes
• The Sulaibiya Wastewater Treatment and Reclamation Plant in Kuwait is one of the largest membrane-based wastewater reuse projects in the world
  • The plant uses UF and RO to treat secondary effluent from a wastewater treatment plant, producing 100 MGD of high-quality water for industrial and agricultural use
  • The project demonstrates the potential of membrane technology to alleviate water scarcity in arid regions and support sustainable water management practices
• The Twin Oaks Valley Water Treatment Plant in San Diego, California, uses membrane filtration as a pretreatment for conventional surface water treatment
  • The plant employs submerged UF membranes to remove suspended solids and pathogens from raw water before conventional coagulation, flocculation, and granular media filtration
  • The hybrid membrane-conventional treatment approach enhances the reliability and efficiency of the treatment process, with a total capacity of 100 MGD
• The Bundamba Advanced Water Treatment Plant in Queensland, Australia, is part of the Western Corridor Recycled Water Scheme, which supplies purified recycled water for industrial and agricultural use
  • The plant uses UF, RO, and UV disinfection to treat secondary effluent from the Bundamba Wastewater Treatment Plant
  • The high-quality recycled water is used for power station cooling, industrial processes, and irrigation, reducing the demand on potable water supplies

Emerging Technologies and Future Trends

• Advances in membrane materials and manufacturing processes are leading to the development of