6.3 Applications in desalination and water purification
4 min read•august 7, 2024
is a game-changer for clean water. It's used to desalinate seawater, treat brackish water, and reclaim wastewater. These applications are crucial for addressing water scarcity and meeting growing demand for fresh water.
RO also excels at removing tricky contaminants like boron, pharmaceuticals, and heavy metals. But it's not without challenges. Dealing with the salty waste (brine) produced is a major hurdle that requires careful management to minimize environmental impacts.
Desalination and Water Treatment
Seawater Desalination
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- involves removing salt and other dissolved solids from seawater to produce potable water
- Typically uses RO membranes with high salt rejection (>99%) to overcome high osmotic pressure of seawater
- Requires extensive pre-treatment to prevent (media filtration, , )
- Post-treatment needed to remineralize water and adjust pH (limestone contactors, chemical dosing)
- Key challenges include high energy consumption, membrane , and brine disposal
- devices (pressure exchangers) can reduce energy usage by ~50%
- Membrane fouling controlled through pre-treatment and chemical cleaning
- Brine usually discharged back to sea, but can impact marine ecosystems
Brackish Water Treatment
- Brackish water has lower salinity than seawater (1,000-10,000 ppm TDS) and is often found in groundwater or estuaries
- Requires less energy and pre-treatment compared to seawater
- Can use RO membranes with lower salt rejection and operating pressures
- Brackish water RO plants common in inland areas with limited freshwater resources (Middle East, Southwest US)
- Permeate often blended with raw water to improve taste and mineral content
- Concentrate disposal can be challenging in inland locations far from the ocean
- Deep well injection and evaporation ponds are common disposal methods
Wastewater Reclamation
- Wastewater reclamation involves treating municipal or industrial wastewater to produce water for non-potable (irrigation, industrial) or indirect potable reuse
- Typically uses (MBR) followed by RO/NF and UV/
- Strict regulations on contaminant removal to protect public health
- Gaining acceptance as a drought-proof water supply in water-scarce regions (Singapore, Southern California, Australia)
- RO provides robust removal of dissolved contaminants, but is susceptible to fouling by organic matter and bacteria
- Fouling controlled through MBR pre-treatment and RO membrane cleaning
- Concentrate disposal options include discharge to wastewater treatment plant or ocean outfall
Contaminant Removal
Boron Removal
- Boron is a naturally occurring trace element that can be toxic to crops at high concentrations
- Particularly problematic in seawater desalination due to high boron concentration in seawater (~4.5 mg/L)
- WHO guideline for boron in drinking water is 2.4 mg/L
- Boron exists as uncharged boric acid () at neutral pH, which is poorly rejected by RO membranes
- Increasing feed pH above of boric acid ( = 9.14) converts it to charged borate ion (), which is well rejected by RO
- Two-pass RO with pH adjustment commonly used for in seawater desalination
- Alternative boron removal methods include ion exchange, adsorption (boron-selective resins), and electrocoagulation
Pharmaceutical Removal
- Pharmaceuticals and personal care products (PPCPs) are emerging contaminants of concern in wastewater and drinking water
- Include antibiotics, hormones, painkillers, antidepressants, etc.
- Potential adverse effects on aquatic life and human health
- RO and NF membranes can effectively remove most PPCPs through a combination of size exclusion, charge repulsion, and adsorption
- Rejection highly dependent on compound properties (molecular weight, charge, hydrophobicity) and membrane characteristics (pore size, surface charge, hydrophobicity)
- Polar, uncharged compounds (carbamazepine, caffeine) more challenging to remove than charged compounds (ibuprofen, diclofenac)
- Advanced oxidation processes (UV/H2O2, ozonation) can degrade PPCPs that are poorly removed by RO/NF
- Can be used as pre-treatment or post-treatment in combination with membranes
Heavy Metal Removal
- Heavy metals (arsenic, lead, cadmium, mercury) are toxic even at low concentrations and can accumulate in the body
- Can enter water sources through natural processes (arsenic in groundwater) or anthropogenic activities (industrial discharges, mining)
- Regulated in drinking water to protect public health (e.g. US EPA maximum contaminant level for arsenic is 10 )
- RO and NF membranes can remove dissolved heavy metals through size exclusion and charge repulsion (Donnan exclusion)
- Metals exist as hydrated ions or complexes in water, with size larger than membrane pores
- Charged metals (arsenate, chromate) rejected by electrostatic repulsion with negatively charged membrane surface
- Other methods include adsorption (activated alumina, iron oxide), ion exchange, and chemical precipitation
- Can be used as pre-treatment before RO/NF to reduce fouling and extend membrane life
Waste Management
Brine Management
- Brine (concentrate) is the high-salinity waste stream produced by RO/NF, containing concentrated salts and other rejected contaminants
- Typically 20-50% of feed flow for seawater RO, 10-20% for brackish water RO
- Can have salinity 1.5-2x higher than feed water, posing disposal challenges
- Most common brine disposal methods are surface water discharge and deep well injection
- Surface discharge involves releasing brine to the ocean or other water bodies, relying on dilution to minimize environmental impacts
- Requires outfall design to maximize mixing and dispersion (diffusers, multi-port risers)
- Potential impacts on marine ecosystems due to elevated salinity, temperature, and contaminants
- Deep well injection pumps brine into confined saline aquifers, hundreds to thousands of meters underground
- Geologically suitable sites with porous and permeable rock formations needed to store large brine volumes
- Risk of groundwater contamination due to improper well construction or seismic activity
- Surface discharge involves releasing brine to the ocean or other water bodies, relying on dilution to minimize environmental impacts
- Emerging technologies aim to minimize brine volumes and recover valuable resources
- (ZLD) uses thermal evaporation and crystallization to produce solid salts and distilled water
- High energy cost and complex operation limit widespread adoption
- Selective salt recovery produces marketable products (NaCl, Mg(OH)2, CaCO3) through precipitation, evaporation, or membrane processes
- Brine mining extracts valuable minerals (lithium, uranium) from geothermal brines or oil/gas produced water using adsorption or ion exchange
- (ZLD) uses thermal evaporation and crystallization to produce solid salts and distilled water
Key Terms to Review (25)
Advanced Oxidation: Advanced oxidation refers to a group of chemical processes that generate highly reactive species, primarily hydroxyl radicals, which are used to degrade organic contaminants in water. This method is effective for breaking down a wide range of pollutants, making it a valuable tool in water purification and desalination processes.
Boron removal: Boron removal refers to the processes employed to reduce boron concentrations in water, which is essential for protecting aquatic life and ensuring compliance with water quality standards. The presence of boron in water can cause toxicity to plants and aquatic organisms, making its removal crucial in water treatment processes, particularly in desalination and purification applications.
Brackish water treatment: Brackish water treatment involves the processes used to purify water that has a higher salinity than freshwater but lower salinity than seawater. This type of water is often found in estuaries and coastal areas and is essential for various applications, including agricultural irrigation, drinking water supply, and aquaculture. Effective brackish water treatment plays a crucial role in the broader fields of desalination and water purification, allowing for the recovery of usable water resources from saline sources.
Brine Management: Brine management refers to the processes and strategies used to handle, treat, and dispose of brine, which is the highly concentrated saltwater produced during desalination and water purification processes. Effective brine management is crucial in minimizing environmental impact and ensuring compliance with regulations, as improper disposal can lead to harm to marine ecosystems and freshwater resources.
Cellulose acetate: Cellulose acetate is a thermoplastic material derived from cellulose, which is obtained from plant fibers. It is commonly used in membrane technology due to its favorable properties such as high permeability and selectivity, making it suitable for various applications in water treatment and desalination processes. Its biodegradability also contributes to its appeal as an environmentally friendly material for membrane fabrication.
Cleaning protocols: Cleaning protocols refer to the systematic procedures used to restore the performance of membranes in water treatment processes by removing fouling agents and contaminants. These protocols are crucial for maintaining membrane integrity, optimizing separation efficiency, and ensuring the longevity of membrane systems across various applications.
Energy recovery: Energy recovery refers to the process of capturing and utilizing the energy that would otherwise be wasted in water treatment systems, particularly in desalination and purification processes. This concept is essential for improving overall system efficiency and reducing operational costs, which can be crucial for sustainability in water management.
EPA Regulations: EPA regulations refer to the set of rules and standards established by the Environmental Protection Agency (EPA) to protect human health and the environment. These regulations guide water treatment processes, including those involving membrane technology, ensuring safe drinking water, pollution control, and proper management of hazardous substances.
Flux: Flux refers to the rate at which a substance passes through a membrane per unit area, typically expressed in units like liters per square meter per hour (L/m²/h). It is a fundamental concept in membrane technology, influencing the efficiency and performance of various separation processes.
Fouling: Fouling refers to the accumulation of unwanted materials on the surface of a membrane, which leads to a decline in performance and efficiency. This phenomenon is critical to understanding how membranes function in various applications, as fouling can significantly impact both the effectiveness of the separation process and the operational longevity of the membrane system.
Heavy metal removal: Heavy metal removal refers to the processes used to eliminate toxic metals such as lead, mercury, cadmium, and arsenic from water sources. These metals can pose significant health risks to humans and the environment, making their removal crucial in desalination and water purification efforts. Various techniques like membrane filtration, adsorption, and chemical precipitation are employed to effectively treat contaminated water.
Hollow Fiber: Hollow fiber refers to a type of membrane configuration commonly used in various filtration processes, where long, thin tubes made of porous materials allow fluid to flow through their lumens while filtering out contaminants. This structure maximizes surface area for filtration, making it an efficient choice for microfiltration and ultrafiltration applications in water treatment.
Membrane Bioreactor: A membrane bioreactor (MBR) is a technology that combines a biological treatment process with a membrane filtration system, allowing for the effective treatment of wastewater. This system uses membranes to separate treated water from the mixed liquor of microorganisms, providing higher quality effluent and enabling the recycling and reuse of water. MBRs are particularly valuable in applications where space is limited, as they offer a compact solution for water and wastewater treatment.
Membrane fouling: Membrane fouling refers to the accumulation of unwanted materials on the surface or within the pores of a membrane, leading to decreased performance and efficiency in filtration processes. This phenomenon affects various applications such as desalination, water purification, and even energy-efficient systems, as fouling can hinder flow rates and increase operational costs due to more frequent cleaning or replacement of membranes.
Microfiltration: Microfiltration is a membrane filtration process that separates particles in the size range of 0.1 to 10 micrometers from liquids, primarily used for removing suspended solids, bacteria, and some larger viruses. This technique plays a critical role in addressing water treatment challenges, offering an effective solution for the clarification of water and wastewater by utilizing the basic principles of membrane separation.
Nanofiltration: Nanofiltration is a pressure-driven membrane separation process that operates between ultrafiltration and reverse osmosis, effectively removing small solutes, divalent ions, and larger organic molecules while allowing monovalent ions and water to pass through. This selective permeability makes nanofiltration particularly useful in addressing various water treatment challenges by improving water quality and reducing contaminants.
NSF/ANSI Standards: NSF/ANSI Standards are guidelines and criteria developed by the National Sanitation Foundation (NSF) in collaboration with the American National Standards Institute (ANSI) to ensure the safety, quality, and performance of products and systems used in water treatment and purification. These standards play a crucial role in the desalination and water purification sectors by establishing requirements for materials, design, construction, and testing methods that protect public health and safety.
Pharmaceutical removal: Pharmaceutical removal refers to the process of eliminating pharmaceutical contaminants from water sources to ensure safe drinking water and protect aquatic ecosystems. This involves various techniques that can effectively degrade or remove synthetic compounds, such as hormones and antibiotics, which may pose health risks to humans and wildlife when present in significant concentrations in water supplies.
Polyamide: Polyamide refers to a type of synthetic polymer that contains amide bonds in its main chain. This material is widely used in the production of membranes due to its high strength, chemical resistance, and thermal stability, making it ideal for applications like nanofiltration and water purification processes.
Rejection Rate: Rejection rate refers to the efficiency of a membrane in separating solutes from a solvent during a filtration process. It indicates the percentage of a particular solute that is prevented from passing through the membrane, thereby influencing the overall performance of various membrane separation processes.
Reverse Osmosis: Reverse osmosis is a water purification process that uses a semipermeable membrane to remove ions, molecules, and larger particles from drinking water. It operates by applying pressure to overcome osmotic pressure, allowing water to flow from a concentrated solution to a diluted one, effectively filtering out contaminants and providing clean water.
Seawater desalination: Seawater desalination is the process of removing salt and other impurities from seawater to produce fresh water suitable for human consumption and irrigation. This technology plays a crucial role in addressing water scarcity, especially in arid regions where fresh water resources are limited, and connects to various applications in water purification and innovative methods like forward osmosis.
Spiral Wound: Spiral wound refers to a type of membrane module configuration commonly used in water treatment processes, particularly for reverse osmosis and nanofiltration. This design wraps the membrane sheets around a central permeate collection tube, maximizing surface area and improving efficiency while minimizing space requirements. Spiral wound modules are key components in various applications, enhancing the effectiveness of membrane filtration in producing clean water.
Ultrafiltration: Ultrafiltration is a membrane filtration process that separates particles based on size, typically retaining solutes with a molecular weight greater than 1,000 Daltons while allowing water and smaller solutes to pass through. This process effectively addresses various water treatment challenges, including the removal of suspended solids, colloids, and some organic compounds.
Zero Liquid Discharge: Zero Liquid Discharge (ZLD) is an advanced wastewater treatment process that aims to eliminate any discharge of liquid waste by recovering and reusing all wastewater generated. This process focuses on minimizing environmental impact by ensuring that all water is treated and either reused in the production process or evaporated, leaving behind only solid waste. ZLD systems are particularly valuable for industries and processes where water conservation is critical, promoting sustainability through the reuse of resources.