12.4 Removal of emerging contaminants and micropollutants
4 min read•august 7, 2024
Emerging contaminants like , , and PFAS are sneaking into our water. These tiny troublemakers are tough to get rid of with regular treatment methods. They're causing a stir because we're not sure how they affect our health long-term.
Water treatment pros are stepping up their game to tackle these new threats. They're using fancy techniques like advanced oxidation and special membranes to filter out the bad stuff. It's a constant battle to keep our water clean and safe from these sneaky pollutants.
Emerging Contaminants
Pharmaceuticals and Personal Care Products
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- Pharmaceuticals include prescription and over-the-counter drugs (antibiotics, , painkillers) that enter water systems through human excretion and improper disposal
- Personal care products consist of a wide range of chemicals found in cosmetics, fragrances, and hygiene products (triclosan, parabens, phthalates) that can persist in the environment
- These contaminants are not effectively removed by conventional wastewater treatment processes leading to their presence in surface and groundwater sources
- Potential health effects of long-term exposure to low levels of these contaminants are not yet fully understood but are a growing concern
Endocrine Disruptors and Microplastics
- Endocrine disruptors are chemicals that interfere with the normal functioning of the endocrine system by mimicking or blocking hormones (bisphenol A, , dioxins)
- These substances can have adverse effects on reproductive, developmental, and neurological systems even at very low concentrations
- Microplastics are small plastic particles (<5 mm) that originate from the breakdown of larger plastic waste or are intentionally added to products (microbeads in cosmetics)
- Microplastics can adsorb and concentrate other pollutants acting as vectors for their transport and bioaccumulation in aquatic organisms
PFAS and Nanomaterials
- PFAS are a group of synthetic chemicals used in various industrial and consumer applications (non-stick coatings, firefighting foams, stain-resistant fabrics) known for their persistence and bioaccumulation potential
- These substances have been linked to adverse health effects including cancer, immune system dysfunction, and developmental issues
- are engineered materials with at least one dimension in the nanoscale range (<100 nm) that exhibit unique properties compared to their bulk counterparts
- The increasing use of nanomaterials in consumer products (silver nanoparticles in clothing, titanium dioxide in sunscreens) raises concerns about their potential environmental and health impacts as they can easily pass through conventional treatment processes
Removal Techniques
Advanced Oxidation Processes
- (AOPs) involve the generation of highly reactive species, primarily hydroxyl radicals (OH•), to degrade recalcitrant organic contaminants
- Common AOP techniques include UV/H2O2, ozone/H2O2, and Fenton processes (Fe2+/H2O2) that can effectively mineralize a wide range of pollutants
- AOPs can be used as a pre-treatment step to improve the biodegradability of wastewater or as a polishing step to remove residual contaminants after conventional treatment
- Factors influencing AOP efficiency include pH, oxidant dose, and the presence of scavengers (carbonate, bicarbonate) that can compete for hydroxyl radicals
Membrane Selectivity and Adsorption
- Membrane processes, such as nanofiltration (NF) and (RO), can effectively remove emerging contaminants based on their molecular size, charge, and hydrophobicity
- The selectivity of NF and RO membranes is determined by the membrane pore size, surface charge, and material properties (polyamide, cellulose acetate)
- Adsorption processes, such as activated carbon filtration, rely on the affinity of contaminants to the adsorbent surface driven by hydrophobic and electrostatic interactions
- Factors affecting adsorption efficiency include adsorbent properties (surface area, pore size distribution), contaminant characteristics (molecular size, polarity), and solution chemistry (pH, ionic strength, competing solutes)
Rejection Mechanisms
- is a primary rejection mechanism in membrane processes where contaminants larger than the membrane pore size are physically retained while smaller molecules pass through
- occurs when the membrane surface and contaminant have the same charge (both negative or both positive) leading to electrostatic exclusion of the contaminant
- involves the adsorption of contaminants onto the membrane surface followed by their diffusion through the membrane matrix
- Solute-solute interactions and concentration polarization can also influence the rejection of emerging contaminants in membrane processes
Monitoring and Regulation
Trace Contaminant Analysis
- Monitoring emerging contaminants requires advanced analytical techniques due to their low concentrations (ng/L to µg/L range) and complex matrix interactions
- Liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) is widely used for the detection and quantification of polar organic contaminants (pharmaceuticals, pesticides)
- Gas chromatography-mass spectrometry (GC-MS) is employed for the analysis of volatile and semi-volatile compounds (PCBs, PAHs) after appropriate sample preparation steps
- Sample pre-concentration techniques, such as solid-phase extraction (SPE) and liquid-liquid extraction (LLE), are often necessary to achieve the required detection limits
Regulatory Considerations
- Regulatory frameworks for emerging contaminants are still evolving as more information becomes available on their occurrence, fate, and toxicity
- The US EPA has established health advisories for some PFAS compounds (PFOA, PFOS) and is considering their inclusion in the Safe Drinking Water Act
- The European Union has included several emerging contaminants (diclofenac, 17-beta-estradiol) in the Watch List of the Water Framework Directive to gather monitoring data and support future prioritization
- Risk assessment approaches, such as the development of water quality criteria and maximum contaminant levels (MCLs), are being adapted to address the challenges posed by the diverse nature and limited toxicological data available for many emerging contaminants
Key Terms to Review (23)
Adsorption-diffusion mechanism: The adsorption-diffusion mechanism refers to the process by which contaminants adhere to a solid surface (adsorption) and then migrate through a porous medium (diffusion) towards an equilibrium state. This mechanism is crucial for understanding how emerging contaminants and micropollutants are effectively removed from water, as it dictates the interaction between the contaminant molecules and the adsorbent materials used in treatment systems.
Advanced oxidation processes: Advanced oxidation processes (AOPs) are a set of chemical treatment procedures designed to remove organic contaminants from water by generating highly reactive species, typically hydroxyl radicals, which can effectively degrade pollutants. These processes are crucial for breaking down emerging contaminants and micropollutants that traditional water treatment methods often fail to eliminate, making them vital in ensuring water quality and safety.
Biofouling: Biofouling is the accumulation of microorganisms, algae, and other biological materials on surfaces submerged in aquatic environments, often leading to negative impacts on membrane performance and efficiency in water treatment systems. It can significantly affect separation mechanisms and process parameters, influencing the design and operational aspects of membrane technologies.
Charge Repulsion: Charge repulsion is a fundamental electrostatic phenomenon where like charges (either positive or negative) push away from each other due to their similar electric charge. This principle plays a crucial role in the behavior of membranes during the removal of emerging contaminants and micropollutants, as it influences how particles interact with membrane surfaces, potentially affecting filtration efficiency and fouling rates.
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.
Cross-flow filtration: Cross-flow filtration is a membrane filtration process where the feed solution flows parallel to the membrane surface, allowing for continuous separation of particles and solutes while minimizing membrane fouling. This technique optimizes the filtration efficiency by maintaining a shear force along the membrane, promoting a more stable operation and enhancing the longevity of the membrane.
Dead-End Filtration: Dead-end filtration is a membrane separation process where the feed stream flows perpendicular to the membrane surface, and the filtered liquid passes through the membrane while the remaining feed is retained. This method leads to the accumulation of retained particles on the membrane surface, resulting in fouling and requiring periodic cleaning or replacement of the membrane.
Drinking Water Standards: Drinking water standards are regulatory benchmarks that ensure the safety and quality of drinking water supplied to consumers. These standards set permissible levels for various contaminants and pollutants, including emerging contaminants and micropollutants, thereby safeguarding public health and the environment. Establishing these standards is crucial for guiding water treatment processes and technologies aimed at effective contaminant removal.
EPA Guidelines: EPA guidelines refer to the standards and recommendations set by the Environmental Protection Agency (EPA) to protect human health and the environment. These guidelines establish criteria for water quality, waste management, and pollutant control, directly influencing how technologies like membrane processes are implemented in water treatment, design, and operation.
Flux Rate: Flux rate is the measure of the amount of fluid that passes through a membrane surface per unit time, typically expressed in liters per square meter per hour (L/m²/h). This term is crucial in evaluating membrane performance and efficiency in various applications, particularly in water treatment processes, where understanding how quickly water can be processed is key to system design and operation.
Hormones: Hormones are chemical messengers produced by glands in the endocrine system that regulate various physiological processes in the body. They play a crucial role in maintaining homeostasis and influencing functions such as growth, metabolism, and reproduction. In the context of water treatment, hormones can act as emerging contaminants, raising concerns about their presence and effects on aquatic ecosystems and human health.
Microplastics: Microplastics are tiny plastic particles, typically less than 5 millimeters in size, that result from the breakdown of larger plastic debris or are manufactured as small particles for various applications. They pose significant challenges in water treatment due to their prevalence in aquatic environments and their potential to harm both ecosystems and human health.
Nanofiltration membranes: Nanofiltration membranes are semi-permeable membranes that operate between ultrafiltration and reverse osmosis, allowing selective separation of molecules based on size and charge. They effectively remove divalent ions, organic compounds, and some larger monovalent ions while allowing smaller molecules, like water and monovalent ions, to pass through. This selective permeability makes them valuable in various applications, particularly in water treatment processes.
Nanomaterials: Nanomaterials are materials that have at least one dimension at the nanoscale, typically between 1 to 100 nanometers. These materials exhibit unique physical and chemical properties due to their small size, high surface area, and quantum effects, making them valuable in various applications, particularly in water treatment technologies and the removal of pollutants. Their enhanced performance compared to conventional materials makes them essential for innovative solutions in environmental engineering.
Pesticides: Pesticides are chemical substances used to kill or control pests, which can include insects, weeds, fungi, and other organisms that can harm crops and human health. These chemicals play a crucial role in agriculture and public health, but their presence in the environment can lead to contamination of water sources, raising concerns about their impact on ecosystems and human safety.
Pharmaceuticals: Pharmaceuticals are chemical compounds used in the diagnosis, treatment, or prevention of diseases and medical conditions. They can enter water systems through various pathways, such as human excretion, improper disposal, and agricultural runoff, leading to environmental contamination and potential health risks.
Polymeric Membranes: Polymeric membranes are selective barriers made from organic polymers that allow certain substances to pass while blocking others, primarily used in separation processes. These membranes are crucial in various applications, including water treatment, where they facilitate the removal of contaminants and impurities.
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.
Size Exclusion: Size exclusion is a separation process that allows particles to be separated based on their size, with larger particles being excluded from entering certain spaces or membranes. This principle is crucial in the removal of emerging contaminants and micropollutants, as it enables specific filtration processes to effectively trap larger harmful substances while allowing smaller, less harmful ones to pass through. The efficiency of size exclusion can significantly impact water treatment effectiveness by enhancing the overall quality of the treated water.
Temperature: Temperature is a measure of the average kinetic energy of particles in a substance, reflecting how hot or cold that substance is. In membrane technology, temperature plays a vital role in influencing the performance, efficiency, and characteristics of membranes, impacting processes such as filtration and transport phenomena.
Transmembrane Pressure: Transmembrane pressure (TMP) is the pressure difference between the two sides of a membrane, driving the flow of fluid through it. This pressure difference is crucial for the operation of membrane processes, affecting fluid dynamics, mass transfer, and overall separation efficiency in various applications.
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.