Water Treatment Plant Operation Volume 2 Chapter 16

When operating an electrodialysis unit, any process problems discovered must be acted upon immediately. This is because electrodialysis is a continuous process that relies on the proper functioning of various components, such as the membranes and electrodes. If any problems are left unaddressed, they can negatively impact the efficiency and effectiveness of the unit, leading to poor separation performance and potential damage to the equipment. Therefore, immediate action is necessary to ensure the smooth operation of the electrodialysis unit.

Water is transmitted through the membrane at a much more rapid rate than minerals. This means that water molecules can easily pass through the membrane, while minerals may have difficulty doing so. This is because the membrane is selectively permeable, allowing for the passage of water molecules but restricting the movement of larger molecules such as minerals. As a result, water can be transported across the membrane more quickly compared to minerals, making the statement true.

Osmosis is a process where a fluid, such as water, moves from an area of lower concentration to an area of higher concentration through a semipermeable membrane. This movement occurs in order to equalize the concentration on both sides of the membrane. Therefore, the statement that osmosis is the passage of a fluid from a weak solution into a more concentrated solution across a semipermeable membrane is true.

The statement is true because the water permeability and mineral permeability are indeed characteristics of the specific membrane used in its processing. These characteristics determine the membrane's ability to allow water and minerals to pass through it.

Reverse osmosis is a process that uses a semi-permeable membrane to remove impurities from water. In this process, water is forced through the membrane, leaving behind contaminants. Divalent ions, such as calcium and sulfate, have a higher charge and larger size compared to monovalent ions like sodium and chloride. Due to their larger size and higher charge, divalent ions are less likely to pass through the membrane, making them better rejected by the reverse osmosis unit. Therefore, the statement that divalent ions are better rejected than monovalent ions in a reverse osmosis unit is true.

Water supplies can indeed be classified according to mineral quality. This classification is based on the presence and concentration of minerals in the water. Different water sources can have varying levels of minerals such as calcium, magnesium, and iron. These minerals can affect the taste, odor, and overall quality of the water. Therefore, it is accurate to say that all available water supplies can be classified based on their mineral content.

Reverse osmosis modules have a large surface area due to their membrane structure. This surface area allows for the attachment and growth of bacterial slams and molds. Bacterial slams and molds can accumulate on the membrane, leading to fouling and decreased efficiency of the reverse osmosis system. Therefore, the statement is true.

The purpose of demineralization is to separate minerals from water. This process involves removing minerals such as calcium, magnesium, and iron, which can cause scaling and other issues in various industrial applications. Demineralization is commonly used in industries such as power plants, chemical processing, and water treatment to ensure the purity of water and prevent mineral buildup in equipment and pipes.

Under ideal conditions for a reverse osmosis unit, where the feed water is pure and there is no fouling of the membrane surface, there should be no change in water flocks with time. This means that the reverse osmosis process is effectively removing impurities and maintaining a consistent quality of water.

In reverse osmosis, as the temperature of the water increases, the flux actually increases. This is because higher temperatures increase the kinetic energy of the water molecules, causing them to move more rapidly. As a result, there is a higher rate of diffusion through the membrane, leading to an increase in flux. Therefore, the statement that the flux decreases with increasing temperature is incorrect.

Membrane compaction could be the cause of a decrease in flux in a reverse osmosis unit. Compaction refers to the squeezing of the membrane which can lead to reduced pore size and increased resistance to water flow. This can result in a decrease in the amount of water that can pass through the membrane, leading to a decrease in flux.

Scaling or fouling of the membrane by both organic and inorganic materials is the most common problem in Electrodialysis (ED) operation. This occurs when the membrane becomes coated or clogged with deposits, which can be made up of various substances such as minerals, salts, or organic compounds. Scaling or fouling reduces the efficiency of the ED process and can lead to decreased performance and increased maintenance requirements. It is important to regularly clean and maintain the membrane to prevent scaling or fouling and ensure optimal operation of the ED system.

Concentrated polarization refers to a buildup of retained particles on the surface of a membrane. This occurs when the feed closest to the membrane contains a high concentration of particles, causing them to accumulate and create a layer on the membrane surface. This buildup can negatively affect the performance and efficiency of the membrane, leading to reduced filtration and potentially clogging the membrane.

The most common serious problem resulting from concentration polarization is the increasing tendency of precipitation of sparingly soluble salts and deposition of particle matter on the membrane surface. This occurs when the concentration of solutes near the membrane surface becomes too high, leading to the formation of solid precipitates and the deposition of particles on the membrane. This can reduce the efficiency of the membrane and hinder its performance. Compaction of the membrane, leakage of salts and particulate matter, and reduction and maintenance requirements are not directly related to concentration polarization.

When treating seawater brackish water with high organic content, provisions should be made for flushing reverse osmosis units. This is because seawater and brackish water often contain high levels of organic matter, which can lead to fouling and clogging of the reverse osmosis membranes. Flushing the units helps to remove any accumulated organic material and maintain the efficiency of the system.

The term flux is used to describe the rate of water flow through a permeable membrane. It refers to the amount of water that passes through the membrane per unit of time.

Recovery in the demineralization process is defined as the percentage of feed flow that is recovered as product water. This means that it measures the amount of water that is successfully treated and converted into usable product water, compared to the total amount of feed water that enters the process. A higher recovery rate indicates a more efficient demineralization process, as it means a larger percentage of the feed water is being converted into usable product water.

Sodium hexametaphosphate is the most frequently used scale inhibitor to prevent the clogging of the reverse osmosis membrane. It is a chemical compound that helps to prevent the formation of scale or deposits on the membrane surface, which can reduce its efficiency and lifespan. Sodium hexametaphosphate works by binding to the calcium and magnesium ions in the water, preventing them from forming scale. This helps to maintain the performance and longevity of the reverse osmosis membrane.

The water flux does not increase as the mineral count content of the feed increases. In fact, the opposite is true. The osmotic pressure contribution decreases with increasing mineral content, leading to a decrease in water flux. Therefore, the statement is false.

The common causes of reverse osmosis membrane damage include membrane compacting, membrane fouling, membrane hydrolysis due to bacterial degradation, membrane hydrolysis due to pH outside operating limits, and membrane hydrolysis due to temperature outside operating limits. These factors can lead to a decrease in the efficiency and effectiveness of the membrane, resulting in damage and reduced performance.

The increased salinity of many rivers and lakes can be attributed to agricultural runoff and waste discharge. Agricultural runoff occurs when excess fertilizers and pesticides are washed into nearby water bodies, increasing the salinity levels. Waste discharge from industries and households also contributes to the salinity as it introduces pollutants and chemicals into the water. These factors disrupt the natural balance of the water bodies and lead to an increase in salinity levels.

Acid is added as a pretreatment step before demineralization and using cellulose acetate membranes to slow down the process of hydrolysis. Hydrolysis is a chemical reaction that occurs when water breaks down the chemical bonds in a substance. By slowing down hydrolysis, the acid helps to preserve the integrity of the cellulose acetate membranes and prevent them from breaking down too quickly. This ensures that the membranes can effectively separate the minerals from the water during the demineralization process.

The correct answer is Hollow fine fiber, Spiral wound, Tubular. These are the types of commercial available membrane configurations that have been used in reverse osmosis operating plants. Hollow fine fiber membranes consist of small, hollow fibers that allow water to pass through while retaining impurities. Spiral wound membranes are made up of flat sheets wound around a central tube, creating a spiral structure. Tubular membranes are cylindrical in shape and allow water to flow through the center while retaining contaminants on the outside. These configurations are commonly used in reverse osmosis plants for water purification.

The impurities that can be deposited or grow on the membrane surface of a reverse osmosis unit include bacteria, dissolved in organics, dissolved organics, and suspended solids. These impurities can accumulate on the membrane over time, leading to fouling and reduced efficiency of the reverse osmosis process. Regular cleaning and maintenance of the membrane are necessary to prevent the buildup of these impurities and ensure the optimal performance of the reverse osmosis unit.

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Seawater and brackish water contain appreciable quantities of chloride, magnesium, sodium, and sulfate ions. These ions are commonly found in seawater due to various natural processes. Chloride ions come from the dissolution of salts in the water, while magnesium and sulfate ions originate from the weathering of rocks and minerals. Sodium ions are present due to the weathering of rocks and the input from rivers and other water sources. These ions are essential for the survival of marine organisms and play a crucial role in maintaining the balance of the marine ecosystem.

Thin-film composite membranes have several advantages over cellulose acetate membranes. Firstly, they have a higher rejection rate, meaning they can effectively remove more impurities and contaminants from the water. Additionally, thin-film composite membranes have a higher flux rate, which means they can process water at a faster rate. Moreover, these membranes have a lower salt passage rate increase after three years of use, indicating that they maintain their performance and efficiency over a longer period of time. However, it is important to note that thin-film composite membranes are also subject to biological attack and hydrolysis or compaction, which can affect their durability and lifespan.

Plastic pipes offer several benefits when used with electro dialysis units. Firstly, they are easy to construct, making installation and maintenance simpler and more efficient. Additionally, plastic pipes are resistant to corrosion and can withstand saline environments, ensuring their durability and longevity. Lastly, plastic pipes are generally more cost-effective compared to stainless steel pipes, making them a cost-efficient choice for electro dialysis units.

In a reverse osmosis unit, operators should be able to collect samples during the cleaning, flushing, rinsing, and servicing modes of operation. These modes involve different processes that are necessary for maintaining the efficiency and functionality of the unit. During cleaning, any accumulated contaminants or particles are removed from the system. Flushing helps to remove any remaining cleaning agents or residues. Rinsing ensures that the system is thoroughly cleaned and prepared for operation. Lastly, servicing involves maintenance activities such as replacing filters or conducting repairs. Collecting samples during these modes allows operators to monitor the effectiveness of the processes and ensure the quality of the water produced.

The daily operation and maintenance items that should be performed on electro-dialysis units include checking the pressure drop across the cartridge filter to ensure it is functioning properly, filling out the lock sheet to document any maintenance activities, inspecting stacks for excess external leakage to prevent any potential issues, and verifying that electrodes are bumping and flowing properly to ensure efficient operation. Voltage probing the membrane stacks is not mentioned as a daily maintenance task in this context.

Demineralization plants involve the use of various chemicals, electrical equipment, and hydraulic systems. These aspects require proper handling to ensure the safety and protection of personnel. Improper handling of chemicals can lead to accidents or exposure to harmful substances. Electrical equipment should be handled with caution to prevent electrical shocks or fires. Hydraulic systems should be operated properly to prevent leaks or malfunctions that can cause accidents. Overall, proper handling of these aspects is crucial to maintaining a safe working environment in demineralization plants.

Electro dialysis is a process that uses an electric field to separate ions and molecules from a solution. In this case, the free treatment is required to remove the materials of chlorine residual, iron, and manganese. Chlorine residual refers to the remaining chlorine in water after disinfection, which needs to be removed to make the water safe for consumption. Iron and manganese are common contaminants found in water sources and can cause discoloration and unpleasant taste. Therefore, electro dialysis is necessary to remove these materials and ensure the water is clean and safe.

The important considerations regarding electrical dialysis feed water quality include bacteriological quality, biological quality, iron, and silica. Bacteriological quality refers to the presence of bacteria or microorganisms in the water, which can contaminate the dialysis process. Biological quality is also important as it includes the presence of viruses and other pathogens that can affect the safety of the dialysis treatment. Iron can cause scaling and damage to the dialysis equipment, while silica can also cause scaling and affect the performance of the dialysis process. Therefore, monitoring and maintaining these factors are crucial for ensuring the quality of the dialysis feed water.

The advantages of demineralization by electro dialysis (E D) process include the efficient removal of most organic constituents, removal of all total dissolved solids (TDS), waste brine containing only salts that have been removed, a well-developed technology with available equipment and membranes, and zero energy requirements. This process is effective in removing both organic and inorganic impurities from water, resulting in high-quality demineralized water. The waste brine produced during the process only contains the salts that have been removed, making it easier to handle and dispose of. The technology and equipment used in electro dialysis have been well-established and refined, ensuring reliable and efficient operation. Additionally, the process requires no external energy input, making it cost-effective and environmentally friendly.

The correct answer is concentration gradient across the membrane and mineral permeability constant. The concentration gradient refers to the difference in concentration of the mineral on either side of the membrane. This gradient drives the movement of the mineral through the membrane. The mineral permeability constant refers to the ability of the mineral to pass through the membrane. If the mineral has a high permeability constant, it will pass through the membrane more easily. Therefore, the flux of the mineral through the membrane depends on both the concentration gradient and the mineral permeability constant.