Ultrafiltration
Ultrafiltration (UF) is one of the most technical means established in treating water because of its permeability ability and its aspect of being perfect in rejecting microbes and other particles that makes water less useful. One of the most important aspects of assessing the quality of a separation process through any membrane is its ability to select a compound from another compound (Sandu et al., 2021). Lower membrane areas are critical as they enhance higher permeability values that allow the separation of mixtures and improve their purity. However, the inability of membranes to absorb the associated contaminants tends to decrease any membrane’s permeability and selectivity ability.
It is worth noting that the ability of a membrane to filter contaminants depends on selectivity and permeability of the membrane. Selectivity is the ability to separate contaminants, solutes, and particles with different chemical or physical properties from a mixture of unwanted compounds (Sandu et al., 2021). These membranes can select other solutes and contaminants depending on their pore sizes. It is worth noting that these filtering membranes’ permeability and selective ability depend on their porosity and the affinity between the substances in the feed solution. The permeability of a membranous substance is categorized based on the size of pores and membrane features. This aspect means that these differences are critical in achieving permeability and adjustable response concerning external stimuli.
One of the major restrictions that have affected the wide use of membrane systems is the decline of flux usually caused by NOM. Due to these increasing restrictions on regulation for drinking water, quality membrane systems have shown to be the technology of choice to provide safe drinking water. Over the years, low-pressure membranes such as ultrafiltration (UF) and MF (microfiltration) substitute conventional treatment systems that allow high-quality water at competitive prices. Groundwater tends to contain large quantities of dissolved compounds due to the interactions of hydrological cycles with the geosphere and biosphere. It is worth noting that these inorganic compounds intend to be harmful to human consumption.
Natural Organic Matter (NOM)
Natural Organic matter is one of the generic terms used for a mixture of organic matter and other slightly water-soluble components. As pointed out by (Levchuk, Màrquez, & Sillanpää, 2018) these naturally occurring matter is commonly found in surface water as well as in underground water. According to Bhatnagar & Sillanpää (2017), the most available routes for NOM include sediments, soil, and natural water. Natural water contains some humic substances that tend to contribute to the brown or yellow color of NOM (Bhatnagar & Sillanpää, 2017). These humic substances are sourced from dead plant tissues, dead organisms, and the excrement of various living things. Research studies have identified that these NOM mostly consist of humic (Humic Acid), Fulvic Acid (FA), and other non-humic fractions that comprise carbohydrate proteins and other amino acids (Matilainen et al 2011). It is worth noting that the fractions of NOM are classified as FA, Humic Acid (HA), and humans based on their solubility level (Bhatnagar & Sillanpää, 2017). As pointed out by Sandu et al, (2021), all the FA fractions are soluble at all PH values, while HA tends to be insoluble at PH less than 2. However, according to Levchuk, Màrquez, & Sillanpää, (2018), the HA fractions tend to be soluble at higher PH values. On the other hand, humans are among the fractions of NOM that are insoluble in water in its acidic or basic state.
NOM is an extremely complex mixture made up of organic compounds that vary greatly in chemical and physical properties. As pointed out by (Matilainen et al 2011), these organic substances have been identified to occur in natural forms or sourced from various activities conducted by a human. In most cases, the organic matter is found in colloidal particulate and dissolved forms in different ground and surface waters, let alone the water from rain (Bhatnagar & Sillanpää, 2017). Although exposure to these organic substances in the environment is never associated with direct health effects on humans, the presence and characteristics of NOM tend to have a significant impact on water used for drinking hence the necessity of treating it before consumption.
The naturally occurring matter plays a critical role in treating water in several ways. According to (Matilainen et al 2011), this organic matter can increase complaints among water consumers because of undesirable tastes, odours, and watercolour. It is worth noting the organic matter tends to deteriorate the pathogen log activity hence increasing the coagulant demand that is often associated with the art making water not fit for human consumption (Bhatnagar & Sillanpaa, 2017). NOM reduces the effectiveness of adsorption and ion exchange process hence affecting the quality of water thereof (Bhatnagar & Sillanpaa, 2017).
Carbon is the major constituent of NOM and tends to dissolve in different proportions that define its physical and chemical characteristics (Sandu et al., 2021). As pointed out by Levchuk, Màrquez, & Sillanpää, (2018), the color of these organic matter can be dissolved substances such as water depending on the measure of Fulvic and humic acids. Research studies have identified two the organic matter can be sourced from two sources that include the allochthonous source, which is derived from the terrestrial ecosystem, and the autochthonous source, which is derived from the microorganism and plants that grow within a water body (Levchuk, Màrquez, & Sillanpää, 2018). It is worth noting that the properties and the concentration of these naturally occurring substances tend to be highly variable as they are effective in various hydrological and biogeochemical processes that affect their sources. There are special cases where anthropogenic human activities contribute to an increase of NOM in any water body.
Impact of Different External Electrical Potentials
Addressing the external challenges in global water scarcity requires treatment plants that use modular, multifunctional, scalable, resilient, energy-efficient, and chemical-free technologies. Mature and well-established technologies play a critical role in providing safe and clean water by reducing the augmenting and the environmental impacts on drinking water (Sandu et al., 2021). This membrane-based separation process includes both ultrafiltration and microfiltration, among others. However, removing emerging toxic contaminants in water tends to be challenging using various membranes due to the limits of conventional size, let alone the charge exclusion criteria used by these membranes to enhance effective separation (Mao & Zhang, 2019). The interaction of organic molecules, microorganisms, and inorganic salts deteriorate by shortening the membrane performance ability and lifetime. However, such challenges have led to the development of new membranes with additional functionalities and properties that goes beyond normal and conventional membranes.
Electrified membranes (EM) are said to have the ability to address the above challenges by introducing electro-activity as an additional membrane function. The EM is known for the recent extension of the role of membranes beyond the pure separation of water (Mao & Zhang, 2019). The membranes enhance this by exploiting electric-based phenomena that include adsorption and rejection, oxidation and reduction, electroporation, and electrophoresis. This is one aspect that EM has introduced beyond how typical membranes function. The other aspect worth noting is that some of these EM can effectively degrade or transform contaminates to enhance their rejection in the charged filtration process.
The functionality of these EMS uses porous flow-through electrodes that apply the electric potential difference to enhance the separation process. Compared to traditional membranes systems, the EM has effectively introduced reaction kinetics and electrode stability that allows contaminants to move to various poles, enhancing their separation from pure water (Mao & Zhang, 2019). Researchers have argued that the effective performance of Eclectic-membranes can be attributed to the co-occurring membranes that tend to enhance the movements of solutes within any solution. The improved ability and functionality of these membranes have effectively increased the contaminant removal in any flowing fluid, more-so water (Mao & Zhang, 2019). The other aspect that enhanced the functionality of these EM is their ability to target various contaminants and remove them from water. These targeted contaminants include heavy metals such as zinc, emerging organic pollutants, personal care products, uncharged molecules, pharmaceutical products, and pathogenic microbes.
Modern inventions of electric membranes have been focusing on improving their selectivity ability and increasing their sufficiency by limiting their energy consumption rate. Apart from contaminant removal, electric membrane can mitigate membrane scaling and foul through various electrochemical strategies that tend to be affected by external electric potential. For instance, biological and organic foulants can be effectively degraded using electrochemical strategies based on in situ generations of various oxidizing agents and species (Mao & Zhang, 2019). Furthermore, the chemical composition of these membranes and the solute of the contaminant in question have been affecting the functionality of these membranes. Therefore, the translation of the EM process to real-world applications calls for considerable improvements in contaminant selectivity, efficiency removal, process sustainability, and temporal stability.
The impact of the external electric potential of solutes and other contaminants in water has been enhancing the discoveries of modern EM that is more selective and effective in reducing even targeted pollutants from water. Traditional membranes were easily affected by these electric potentials, where most of them lowered their permeability or particular ability (Aizawa et al. 2017). Future developments are now focusing on coming up with EM that uses less energy but maintains its effectiveness in removing contaminants and other NOM that are known to affect the usability of water. Ionic strengths and pH on metal and polymer materials modified MF and UF membranes for removal of inorganic.
Removing heavy metal ions from wastewater is the prime importance of cleaning water fit for human consumption. The current issues associated with water purification have led to various techniques to remove these species from water (Aizawa et al., 2017). One of the methods embraced to a greater extent is polymer-enhanced ultrafiltration (PEUF), which has systematically been enhancing the water’s filtration process, enhancing the removal of inorganic matter in water. Researchers have identified that water pollution is one of the biggest concerns globally.
Modernity has been accounting for increased pollution in water. Large companies tend to produce many pollutants that are typical to pollute water. The separation of these inorganic requires effective modification of separation membranes to ensure drinking water is safe. Some inorganic compounds include arsenic, chromium, nitrate NOM, and other inorganic compounds (Aizawa et al., 2017). Most of these dyes and inorganic substances are considered the most dangerous contaminants because of their recalcitrant structures. They present a serious threat to global sustainability as they hinder light penetration, affecting the natural environment more than terrestrial. When combined with other industrial waste, the contaminants tend to cause an imbalance in soil and water content.
Technology has been embraced to a greater extent to help and solve this problem. Some of the technical strategies that have been adopted include flocculation, advanced oxidation process, and pressurized liquid filtration techniques, among others. Traditional methods such as reverse osmosis and non-filtration have low permeability and require a lot of pressure to enhance their performance (Aizawa et al., 2017). Traditional methods require more energy and account for an increased cost of processing. Other methods such as UF and MF processes tend to have higher permeability and require less pressure to enhance their performance in removing some of these dangerous contaminants in water. However, despite their effectiveness in removing impurities in water, some of these processes are less effective when retaining small molecules with low molecular weight, such as dyes and other heavy elements such as chromium and arsenic compounds.
The ionic strength and the PH of these compounds affect how these inorganic compounds are filtered in water. For this reason, advanced filtration techniques such as PEUF have the binding abilities that enhance the removal of these polymers and inorganic compounds in water (Kumar & Ismail, 2015). The other merit of why PEUF is embraced to a greater extent is that it is environmentally friendly and prevents excessive polluting of soil and water after filtration is enhanced. PEUF is advanced because it is added into the feed of a contaminated solution hence capturing all the unwanted polymers effectively.
Effect of External Electrical Potentials on Membrane Properties
UF and MF are some membrane-based filtering technologies that utilize thin layers to separate contaminants, including solid, bacteria, NOM, and other products. However, despite their effectiveness in cleaning water, external electric potentials affect these membrane properties, which, in turn, affect their efficacy in removing contaminants in water and other solutions (Kumar & Ismail, 2015). Some of the membrane properties affected include:
Surface Charge
Separation membranes are critical as they help in selectively separating particles and solutes within a feed solution such as water. Depending on the separation model adopted, the goal of each process is to enhance the maximum separation of contaminants in the feed solution and enhance its purification later (Lebedev et al., 2018). However, the surface charges of these membranes can be affected, and in the long run, fewer or fewer contaminants are filtered out. The electrostatic interactions in these membranes trend to predict the parameters to be separated in the process. Membrane fouling is one aspect that affects the surface charge of these filtering membranes, hence reducing their effectiveness.
In most cases, fouling occurs when particles with different charges are deposited on the surface membrane, affecting the integrity of the pores to enhance the filtration process. However, the membrane integrity can be restored by cleaning it with solutions that restore the original surface charge, enhancing the effective and speedy separation of contaminants in the feed solution (Lebedev et al., 2018). The feed or the cleaning chemical should be used regularly to maintain the integrity of these membranes.
Potential Hydrophilicity
Hydrophilicity and the surface charge of a membrane are key factors that affect the fouling of a membrane. It is worth noting that an uncharged hydrophilic surface is considered resistant to fouling (Gu et al., 2019). The aspect means that any chemical compound that comes into contact with the membrane tends to alter its surface by detaching or attaching some functional moieties. In most cases, the chemical tends to bring a change in the surface charge of the membranes as well as its hydrophilic ability (Lebedev et al., 2018). Any effect on these properties affects how solutes and contaminants are filtered out in a solution. Therefore, much energy is required to separate solutes from the feed solution. However, the hydrophilicity of these membranes can be restored through thorough cleaning.
Membrane Surface Roughness
The roughness of a membrane plays a critical role in particle and solutes adhesion. The roughness of a membrane are determined by the contact angle and its chemical composition (Lebedev et al., 2018). The surface charge and the hydrophilicity of a membrane tend to affect the ability of membranes to absorb various chemicals and contaminants. Research studies have identified that rough surfaces tend to be more effective than smooth surfaces. The aspect is critical as it gives the membrane more time to absorb the solutes or contaminants. The aspect relates that when a feed solution is passed via a rough answer, it tends to take longer before it gives the membrane (Lebedev et al., 2018). Smooth surfaces increase the speed of the feed solution, and less time is taken for the fluid to pass across the membrane. In the very end, fewer solutes and contaminants are filtered out. Maintaining membranes’ integrity requires cleaning or replacements that ensure that all the target solutes and contaminants are not filtered out effectively.
Porosity
On the other hand, porosity refers to the ratio of the volume of pores compared to the importance of bulk that enhances separation. More bulk means high low porosity as small pores allow the feed solution to move effectively (Erofeev et al. 2019). On the other hand, less bulk means high porosity as there are large spaces where the fluid to move freely and at a higher speed. Highly porous membranes filter out fewer contaminants than less porous membranes (Erofeev et al., 2019). The increased porosity of a membrane usually occurs after excessive corrosion by the polymers and other humic acids that affect the integrity of these membranes. However, maintaining their integrity is key, as the more porous a membrane is, the less effective it becomes. The feed solution has naturally occurring organic matter and other inorganic compounds that require considerable time to be filtered out effectively (Erofeev et al., 2019). Excessive use of membranes is often associated with the art of increasing their porosity. This accounts for the reason as to why after using a membrane for an extended period, its selective ability and permeability tend to lower. However, membrane washing has been identified as one of the most effective approach that helps in restoring the integrity of deteriorates filtering units.
References
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