Water Quality Control and Treatment Water Treatment

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Water Quality Control and Treatment Water Treatment

  1. 1. Water Quality Control and Treatment Water Treatment By Dr. Khamis AL-Mahallawi
  2. 2. Water treatment • Overview of the Water Treatment Process • Preliminary Treatment – Presedimentation – Aeration – Primary Sedimentation – Sedimentation and flotation zones • Aeration • Adsorption • Ion Exchange • Coagulation and Flocculation • Filtration • Membrane Processes / Electro-dialysis • Nanofiltration and reverse osmosis • Softening • Treatment – Groundwater types and treatment – Surface water treatment • Disinfection
  3. 3. Water Treatment Process abstraction - treatment – transport – storage - distribution
  4. 4. Water treatment processes Preliminary Treatment • Preliminary treatment is any physical, chemical or mechanical process used on water before it undergoes the main treatment process. • The purpose of preliminary treatment processes is to remove any materials which will interfere with further treatment. • Pretreatment may include screening, presedimentation, chemical addition, flow measurement, and aeration.
  5. 5. Preliminary Treatment / Screens • The screens are used to remove rocks, sticks, leaves, and other debris. • Very small screens can be used to screen out algae in the water. • All objects are removed by physical size separation • Screens on the outside of intakes are often cleaned by flushing water from the treatment plant backwards • There are two primary types of screens - bar screens and wire-mesh screens. • A bar screen is used to remove large debris. The spaces between the bars are two to four inches wide. • A wire-mesh screen is used to remove smaller debris. The gaps are about half an inch wide. • Water must be flowing slowly in order to pass through a wire-mesh screen - velocities should be no greater than 3.5 inches per second. A wire-mesh screen A bar screen
  6. 6. Preliminary Treatment / Presedimentation - Aeration Presedimentation is to settle out sand, grit, and gravel which will settle rapidly out of the water without the addition of chemicals at the beginning of the treatment process. Presedimentation depends on gravity and includes no coagulation and flocculation. Presedimentation will reduce the load on the coagulation/flocculation basin and on the sedimentation chamber, as well as reducing the volume of coagulant chemicals required to treat the water. Presedimentation basins are useful because raw water entering the plant from a reservoir is usually more uniform in quality than water entering the plant without such a holding basin Here in pretreatment, the purpose of sedimentation is to make the chemical treatment phase of the water treatment process more efficient by removing sediment from the raw water. In presedimentation basin, activated carbon may be added to the basin for taste, odor, and color problems, and some chemicals to control the growth of algae. Aeration removes carbon dioxide and hydrogen sulfide from the water. It also oxidizes the iron and manganese.
  7. 7. Preliminary Treatment / Monitoring • Flow Measurement : to adjust chemical feed rates, calculate detention times, and monitor the amount of water being treated. • It is also monitored for a variety of characteristics including pH, turbidity, total alkalinity, temperature, and coliform bacteria. • The pH and total alkalinity of the water will influence the amount of alkali to be added and can also influence the flocculation conditions • The level of turbidity will influence the amount of polymer (coagulant) added to the water. • Temperature is also measured since cold water does not floc as well as warm water and requires the addition of more polymer
  8. 8. Primary Sedimentation Notes: • sedimentation may not be necessary in low turbidity water of less than 10 NTU • In this case, coagulation and flocculation are used to produce pinpoint (very small) floc which is removed from the water in the filters • Sedimentation is a treatment process in which the velocity of the water is lowered below the suspension velocity and the suspended particles settle out of the water due to gravity. • The process is also known as settling or clarification • Settled solids are removed as sludge, and floating solids are removed as scum • The efficiency or performance of the process is controlled by: detention time, temperature, tank design, and condition of the equipment.
  9. 9. Primary Sedimentation / Location in the Treatment Process • The most common form of sedimentation follows coagulation and flocculation and precedes filtration. • This type of sedimentation requires chemical addition (in the coagulation/flocculation step) and removes the resulting floc from the water. • sedimentation following coagulation/flocculation is meant to remove most of the suspended particles in the water before the water reaches the filters, • Sedimentation at this stage in the treatment process should remove 90% of the suspended particles from the water, including bacteria. • The purpose of sedimentation here is to decrease the concentration of suspended particles in the water, reducing the load on the filters. • Sedimentation can also occur as part of the pretreatment process, where it is known as presedimentation.
  10. 10. Types of sedimentation basins Rectangular basins: have a variety of advantages - predictability, cost-effectiveness, and low maintenance. In addition, rectangular basins are the least likely to short-circuit, especially if the length is at least twice the width. A disadvantage of rectangular basins is the large amount of land area required. Double-deck rectangular basins: This type of basin conserves land area - has higher operation and maintenance costs. Square or circular sedimentation basins with horizontal flow are known as clarifiers. This type of basin is likely to have short-circuiting problems. Solids-contact clarifiers , also known as upflow solids-contact clarifiers or upflow sludge- blanket clarifiers combine coagulation, flocculation, and sedimentation within a single basin. found in packaged plants and in cold climates where sedimentation must occur indoors
  11. 11. Sedimentation and flotation zones • All sedimentation basins have four zones - the inlet zone, the settling zone, the sludge zone, and the outlet zone. • In a clarifier, water typically enters the basin from the center rather than from one end and flows out to outlets located around the edges of the basin. But the four zones can still be found within the clarifier A rectangular sedimentation basin
  12. 12. Sedimentation and flotation zones/Inlet Zone Purposes of the inlet zone of a sedimentation basin are • to distribute the water and to control the water's velocity as it enters the basin. • inlet devices act to prevent turbulence of the water. • The incoming flow must be evenly distributed across the width of the basin to prevent short-circuiting. • Short-circuiting is a problematic circumstance in which water bypasses the normal flow path through the basin and reaches the outlet in less than the normal detention time. • If the water velocity is greater than 0.5 ft/sec, then floc in the water will break up due to agitation ) of the water.
  13. 13. Sedimentation and flotation zones/Inlet Zone Two types of inlets. 1. The stilling ( ) wall, also known as a perforated baffle wall , spans the entire basin from top to bottom and from side to side. Water leaves the inlet and enters the settling zone of the sedimentation basin by flowing through the holes evenly spaced across the stilling wall. 2. The second type of inlet allows water to enter the basin by first flowing through the holes evenly spaced across the bottom of the channel and then by flowing under the baffle in front of the channel. The combination of channel and baffle serves to evenly distribute the incoming water
  14. 14. Sedimentation and flotation / Settling Zone • water enters the settling zone where water velocity is greatly reduced. • the bulk of floc settling occurs and this zone will make up the largest volume of the sedimentation basin. • For optimal performance, the settling zone requires a slow, even flow of water. • The settling zone may be simply a large expanse of open water. But in some cases, tube settlers and lamella plates, such as those shown below, are included in the settling zone. • Tube settlers and lamella plates • Water flows up through slanted tubes or along slanted plates. Floc settles out in the tubes or plates and drifts back down into the lower portions of the sedimentation basin. • Clarified water passes through the tubes or between the plates and then flows out of the basin. Actual Area vs. Effective Area
  15. 15. Sedimentation and flotation / Settling Zone Why Tube settlers and lamella plates: To increase the settling efficiency and speed in sedimentation basins. Each tube or plate functions as a miniature sedimentation basin, greatly increasing the settling area. Tube settlers and lamella plates are very useful in plants where site area is limited, or to increase the capacity of shallow basins. Adding inclined settling surface technology to an existing clarifier can increase water treatment flow by as much as 75%.
  16. 16. Sedimentation and flotation / Settling Zone
  17. 17. Sedimentation and flotation / Settling Zone Horizontal Parallel Plate Clarifiers
  18. 18. Sedimentation and flotation / Settling Zone Vertical Parallel Plate Clarifiers
  19. 19. Traditional Circular Clarifiers / Settling Zone
  20. 20. Sedimentation and flotation / Outlet Zone Outlet Zone is designed to: • prevent short-circuiting of water in the basin. • ensure that only well-settled water leaves the basin and enters the filter. • control the water level in the basin. • ensure that the water flowing out of the sedimentation basin has the minimum amount of floc suspended in it. • A typical outlet zone begins with a baffle in front of the effluent. • This baffle prevents floating material from escaping the sedimentation basin and clogging the filters. • The weirs serve to skim the water evenly off the tank
  21. 21. Sedimentation and flotation / Sludge Zone • The sludge zone is found across the bottom of the sedimentation basin. • Velocity should be very slow to prevent resuspension of sludge. • The tank bottom should slope toward the drains • Sludge removal by ( automated equipment or manually at least twice per year). • The best time of cleaning when water demand is low, (April and October). • Many plants have at least two sedimentation basins so that water can continue to be treated while one basin is being cleaned, maintained, and inspected. • If sludge is not removed from enough, the effective volume of the tank will decrease, reducing the efficiency of sedimentation. • Sludge built up on the bottom of the tank may become septic (anaerobically). • Septic sludge may result in taste and odor problems or may float to the top of the water and become scum or resuspended to be carried over to the filters.
  22. 22. Aeration Types of Aerators • air into the water • water into the air
  23. 23. Aeration Efficiency Surface contact between air and water • Smaller bubble size, greater surface contact with water. • Smaller drop size, greater surface contact with the air.
  24. 24. Aeration• Aeration is the process of bringing water and air into close contact. • Aeration is the process to remove dissolved gases, such as carbon dioxide, hydrogen sulfide, and to oxidize dissolved metals such as iron. It can also be used to remove volatile organic chemicals (VOC). It happened by: • Exposing drops or thin sheets of water to the air or • introducing small bubbles of air and letting them rise through the water. the aeration is accomplished the desired results by: • Sweeping ) or scrubbing action caused by the turbulence of water and air mixing together • Oxidizing certain metals and gases
  25. 25. CHEMICAL SUBSTANCES AFFECTED BY AERATION The constituents that are commonly affected by aeration are: • Volatile organic chemicals, such as benzene, found in gasoline, or trichloroethylene, dichloroethylene, and perchloroethylene, examples of solvents are used in dry cleaning or industrial processes. • Carbon dioxide • Hydrogen sulfide (rotten-egg odor) • Methane (flammable) • Iron (will stain clothes and fixtures) • Manganese (black stains) • Various chemicals causing taste and odor
  26. 26. CHEMICAL SUBSTANCES AFFECTED BY AERATION / CARBON DIOXIDE • Surface waters have a low CO2 content ( 0 to 2 mg/l). • Deep lake or reservoir can have high CO2 content due to the respiration of microscopic animals and lack of abundant plant growth at the lake bottom. • aerators remove CO2 by the physical scrubbing or sweeping action caused by turbulence. • aeration can reduce the CO2 content to 4.5 mg CO2 /l • Concentration of CO2 in groundwater are usually higher than in surface water. • Water from a deep well normally contains less than 50 mg/l, but a shallow well can have a much higher level, up to 50 to 300 mg/l.
  27. 27. CARBON DIOXIDE REMOVAL • The most appropriate treatment for carbon dioxide may be aeration, addition of an alkali, or a combination of the two • CO2 gas dissolves easily in water, resulting in carbonic acid: • H2O + CO2 <===> H2CO3 • CO2 is neutralized through the addition of an alkali (basic, ionic salt), such as lime (Ca(OH)2) or soda ash (Na2CO3). • Lime reacts with carbon dioxide, removing the carbon dioxide from the water as shown below: • CO2 + Ca(OH)2 <===> CaCO3 + H2O CO2 above 5 to 15 mg/l in raw water can cause three operating problems: • It increases the acidity of the water, making it corrosive by forming a “weak” acid, H2CO3. • It tends to keep iron in solution, thus making iron removal more difficult. • It reacts with lime added to soften water, causing an increase in the amount of lime needed for the softening reaction.
  28. 28. HYDROGEN SULFIDE • A poisonous gas (Brief exposures--less than 30 minutes in concentrations as low as 0.03 percent by volume in the air) - rotten-egg odor • H2S occurs mainly in groundwater supplies. • It may be caused by the action of iron or sulphur reducing bacteria in the well. • Occasional disinfection of the well can reduce the bacteria producing the H2S • H2S in a water supply will disagreeably alter the taste of coffee, tea, and ice. • H2S is corrosive to piping, tanks, water heaters, and copper alloys that it contacts. Operational problems: • Disinfection of the water can become less effective because of the chlorine demand exerted by the hydrogen sulfide. – H2S + Cl2 + O2- → S + H2O + 2Cl- – H2S + 4Cl2 + 4 H2O → H2SO4 + 8 HCl • There could be corrosion of the piping systems and the water tanks.
  29. 29. H2S Removal by Aeration method • Hydrogen sulfide is physically removed by agitating ) the water via bubbling or cascading ) and then separating or "stripping )" the hydrogen sulfide in a container. H2S + O2 = water (H2O) + elemental sulfur • Aeration is most effective when hydrogen H2S are lower than 2.0 mg/l. • At higher concentrations, this method may not remove all of the offensive odor unless the air is used to oxidize hydrogen sulfide chemically into solid sulfur, which is then filtered. • In a typical aeration system, ambient air is introduced into the water using an air compressor or blower. • Well-designed aeration tanks maintain a pocket of air in the upper third or upper half of the tank. • If the tank does not maintain an air pocket, sulfur odor may return. • When sulfur levels exceed 10 mg/l, larger aeration tanks, repressurization systems, chlorination systems, or a combination may be needed.
  30. 30. METHANE • Methane gas can be found in groundwater. • It may be formed by the decomposition of organic matter. • It can be found in water from aquifers that are near natural-gas deposits. • Methane is a colorless gas that is highly flammable and explosive. • When mixed with water, methane will make the water taste like garlic. • The gas is only slightly soluble in water and therefore is easily removed by the aeration of the water.
  31. 31. IRON AND MANGANESE • Iron and manganese minerals are found in soil and rock. • Iron and manganese can dissolve into groundwater as it percolates through the soil and rock. • Iron in the ferrous form and manganese in the manganous form are objectionable. • more than 0.3 mg/l of iron will cause yellow to reddish-brown stains of plumbing fixtures or almost anything that it contacts. • If the concentration exceeds 1 mg/l, the taste of the water will be metallic and the water may be turbid. • Manganese even at levels as low as 0.1 mg/l, will cause blackish staining of fixtures and anything else it contacts. • If the water contains both iron and manganese, staining could vary from dark brown to black. • Consumer complaints are laundry is stained and that the water is red or dirty. • iron and manganese should not be aerated unless filtration is provided.
  32. 32. Iron and Manganese Removal • Iron and manganese in well waters occur as soluble ferrous and manganous bicarbonates. • In the aeration process, the water is saturated with oxygen to promote the following reactions • The oxidation products, ferric hydroxide and manganese dioxide, are insoluble. • After aeration, they are removed by clarification or filtration • Occasionally, strong chemical oxidants such as chlorine (Cl2 or potassium permanganate (KMnO4 may be used following aeration to ensure complete oxidation 4Fe(HCO3)2 + O2 + 2H2O = 4Fe(OH)3 - + 8CO2 ferrous bicarbonate oxygen water ferric hydroxide carbon dioxide 2Mn(HCO3)2 + O2 = 2MnO2 + 4CO2 - + 2H2O manganese bicarbonate oxygen manganese dioxide carbon dioxide water
  33. 33. TASTE AND ODOR & DISSOLVED OXYGEN TASTE AND ODOR • Aeration is effective in removing tastes and odors that are caused by volatile materials • Volatile materials (e.g Methane and hydrogen sulfide) have low boiling point and will vaporize very easily. • Many taste and odor problems in surface water could be caused by oils and by- products that algae produce. • Since oils are much less volatile than gases, aeration is only partially effective. DISSOLVED OXYGEN • Oxygen is injected into water through aeration to remove the flat taste. • The amount of oxygen that the water can hold is dependent on the temp. • The colder the water, the more oxygen the water can hold. • Water that contains excessive amounts of oxygen can become very corrosive. • Excessive oxygen can cause air binding of filters.
  34. 34. TYPES OF AERATORS Aerators fall into two general categories. • introduce air into the water or water into the air. • The water-to-air method is designed to produce small drops of water that fall through the air • The air-to-water method creates small bubbles of air that are injected into the water stream. • All aerators are designed to create a greater amount of contact between the air and water to enhance the transfer of the gases.
  35. 35. WATER INTO AIRCascade Aerators • consists of a series of steps that the water flows over. • aeration is accomplished in the splash zones. • The aeration action is similar to a flowing stream. • Splash areas are created by placing blocks across the incline. • Cascade aerators used to oxidize iron and to partially reduce dissolved gases. • the oldest and most common type of aerators. Cone Aerators • are used primarily to oxidize iron and manganese prior to filtration. • the water pumped to the top of the cones and then allowed to cascade down through the aerator.
  36. 36. WATER INTO AIR Slat ( ) and Coke Aerators • similar to the cascade and cone types. • They usually consist of three-to-five stacked trays, which have spaced wooden slats in them. • The trays are filled with fist-sized ( pieces of coke, rock, ceramic balls, limestone, or other materials. • The primary purpose of the materials is provide additional surface contact area between the air and water. Spray Aerators spray aeration is successful in oxidizing iron and manganese and is successful in increasing the dissolved oxygen of the water.
  37. 37. WATER INTO AIR Draft Aerators: the air is induced by a blower. Types: • external blowers mounted at the bottom of the tower to induce air from the bottom of the tower. • Water is pumped to the top and allowed to cascade down through the rising air. • The other, an induced-draft aerator, has a top-mounted blower forcing air from bottom vents up through the unit to the top. • Both types are effective in oxidizing iron and manganese before filtration.
  38. 38. AIR INTO WATER • These are not common types used in water treatment. • The air is injected into the water through a series of nozzles submerged in the water. • It is more commonly used in wastewater treatment for the aeration of activated sludge. Air-into-water • Diffuser • Draft tube
  39. 39. AIR INTO WATER Pressure Aerators • Uses a pressure vessel. • The water to be treated is sprayed into the high-pressure air, allowing the water to quickly pick up dissolved oxygen. • A pressure aerator commonly used in pressure filtration is a porous stone installed in a pipeline before filtration. • The air is injected into the stone and allowed to stream into the water as a fine bubble, causing the iron to be readily oxidized. • The higher the pressure, the more readily the transfer of the oxygen to the water. • more O2 is available, more readily the oxidation of the Fe or Mn.
  40. 40. AIR STRIPPING • can be quite effective in removing volatile organic chemicals (VOCs) from water. • A major concern is that VOCs may be carcinogens. • Air stripping capable of removing up to 90 percent of the most highly volatile VOCs. • water flow over cascade aerators or in specially designed air-stripping towers. • water is allowed to flow down over a support medium or packing contained in the tower, while air is being pumped into the bottom of the tower.
  41. 41. VOC removable by air stripping
  42. 42. AIR STRIPPING Diffused aeration air stripper Low profile sieve tray air stripper
  43. 43. COMMON OPERATING PROBLEMS • Aeration raises the dissolved oxygen content of the water. • If too much oxygen is injected into the water, the water becomes supersaturated • Supersaturation may cause corrosion whenever water and oxygen come into contact with metallic surfaces • False Clogging Of Filters-Air Binding (the spaces between the filter media particles begin to fill with bubbles). • Air binding causes the filter to plugged and in need of backwashing. • Slow removal of the hydrogen sulfide from the towers, algae production, clogged filters, and overuse of energy. • Hydrogen sulfide is most efficiently removed, not by oxidation, but by the physical scrubbing action of aeration. • This removal is dependent on the pH of the water. At a pH of 6 or less, the hydrogen sulfide is easily removed. • high pH, the hydrogen sulfide will ionize, preventing removal by aeration.
  44. 44. OPERATIONAL TESTING Three basic control tests are involved in the operation of the aeration process: • Dissolved oxygen (to estimate whether the process is over or under aerated) • pH (give an indication of the amount of carbon dioxide removal. pH increases as the carbon dioxide is removed. pH can also be used to monitor the effective range for hydrogen sulfide, iron, and manganese removal) • Temperature (is important as the saturation point of oxygen increases as the temperature decreases. • As water temperature drops, the operator must adjust the aeration process to maintain the correct DO level.
  45. 45. Dissolved oxygen
  46. 46. Adsorption
  47. 47. Adsorption • Adsorption: is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to surface • This process creates a film of the adsorbate (the molecules or atoms being accumulated) on the surface of the adsorbent. • It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid. • The term sorption encompasses both processes, while desorption is the reverse of adsorption. It is a surface phenomenon.
  48. 48. Types of Adsorption • Types of Adsorption: Depending on the nature of the forces between adsorbate & adsorbent, Adsorption is of two types. • 1. Physical Adsorption or physisorption: Forces of attraction between adsorbate and adsorbent are Van der Waal‟s forces, they are weak. • 2. Chemical Adsorption or chemisorption: Forces of attraction between adsorbate and adsorbent are as strong as chemical bonds, they are strong. This type of adsorption cannot be easily reversed.
  49. 49. Adsorption • Adsorption, is sorption processes in which certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column. • Adsorption systems treat water by adding a substance, such as activated carbon or alumina, to the water supply. • Adsorbents attract contaminants by chemical and physical processes that cause them to „stick‟ to their surfaces for later disposal.
  50. 50. Adsorption • the most commonly-used adsorbent is activated carbon • Powdered activated carbon is often used when temporary quality problems arise; it can simply be added to the water and discarded with waste sludge. • Granular activated carbon is often arranged in a bed through which source water is slowly passed or percolated. • Activated alumina treatment is used to attract and remove contaminants, like arsenic and fluoride, which have negatively charged ions.
  51. 51. Physical adsorption (Physisorption) • In physical adsorption, the forces of attraction between the molecules of the adsorbate and the adsorbent are of the weak van der Waals' type. • van der Waals forces is the sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules • A van der Waals bonding already illustrates the difference in bonding between two molecules and one molecule with a solid. • A van der Waals bonding between two molecules can be described as the interaction between two point dipoles. • Since the forces of attraction are weak, the process of physisorption can be easily reversed by heating or decreasing the pressure of the adsorbate (as in the case of gases). The attraction is not fixed to a specific site and the adsorbate is relatively free to move on the surface
  52. 52. Chemical adsorption (Chemisorption) • In chemisorption, the forces of attraction between the adsorbate and the adsorbent are very strong; • the molecules of adsorbate form chemical bonds with the molecules of the adsorbent present in the surface. • Adsorption is generally accompanied by release of energy, that is, most adsorption processes are exothermic in nature. • Enthalpy is a measure of the total energy of a thermodynamic system. • It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure. • Enthalpy of Physisorption, which is the enthalpy change for the adsorption of one mole of an adsorbate on an adsorbent surface, is in the range of 20 kJ/mole to 40kJ/mole • Enthalpy values for chemisorption, are an order of magnitude high, that is, 200 kJ/mole to 400 kJ/mole
  53. 53. Comparison between Physisorption and Chemisorption Physisorption Chemisorption Forces of attraction are vander Waals‟ forces Forces of attraction are chemical bond forces Low enthalpy of adsorption (20 - 40 k.J/mole) High enthalpy of adsorption (200 - 400 k.J/mole) This process is observed under conditions of low temperature This process takes place at high temperatures It is not specific It is highly specific Multi-molecular layers may be formed Generally, monomolecular layer is formed This process is reversible This process is irreversible
  54. 54. Adsorption Equillibria • Adsorption equilibrium is a dynamic concept achieved when the rate at which molecules adsorb onto a surface is equal to the rate at which they desorb. • Most of the adsorption theories have been developed for gas solid systems because the gaseous state is better understood than the liquid. • Till now the statistical theories developed for gas – solid systems were applied for liquid solid systems with little confidence for designing of the equipment. • The most commonly used equilibrium models to understand the adsorption systems was Freundlich and Langmuir isotherm equation
  55. 55. ADSORPTION EQUILIBRIA / Isotherm • Adsorption is usually described through isotherms • If the adsorbent and adsorbate are contacted long enough an equilibrium will be established between the amount of adsorbate adsorbed and the amount of adsorbate in solution. The equilibrium relationship is described by isotherms. • isotherms is the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant temperature. • The quantity adsorbed is nearly always normalized by the mass of the adsorbent to allow comparison of different materials.
  56. 56. Some general isotherms are shown in the figure below
  57. 57. Isotherm models • The figures below show that there are four common models for isotherms.
  58. 58. Freundlich: K Freundlich isotherm coefficient and n • The amount of adsorbate adsorbed is a function of the liquid-phase concentration and called adsorption equilibrium isotherm. • Different functions can be used to describe the adsorption equilibrium. • Freundlich isotherm data can be used. • This adsorption isotherm empirical equation is. • in which: • qmax = loading capacity (g/kg) • cs = equilibrium concentration (g/m3) • x = adsorbed amount of compound (g) • m = mass of activated carbon (kg) • K = Freundlich constant ((g/kg).(m3/g)n) • n = Freundlich constant (-) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Concentration MassSorption n = 0.5 n = 0.2 n = 2 n = 1 n = 5 n = 0.1
  59. 59. Freundlich: K Freundlich isotherm coefficient and n • The Freundlich constants K and n are influenced by water temperature, pH, type of carbon and the concentration of other organic compounds. • Using laboratory experiments the Freundlich constants can be determined for a single substance with a specific type of activated carbon. • Taking logs and rearranging: log q = log K + n log Cs. • When these graphs are plotted on a logarithmic scale the Freundlich constant K can be determined from the intersection of the graph with the y- axis. • The slope of the line is equal to the Freundlich constant n. • The higher the K-value, the better the adsorption.
  60. 60. Freundlich: K Freundlich isotherm coefficient and n
  61. 61. Langmuir • In 1916, Irving Langmuir published a new model isotherm for gases adsorbed to solids. It is a semi-empirical isotherm derived from a proposed kinetic mechanism. It is based on four assumptions: – The surface of the adsorbent is uniform, that is, all the adsorption sites are equivalent. – Adsorbed molecules do not interact. – All adsorption occurs through the same mechanism. – At the maximum adsorption, only a monolayer is formed: molecules of adsorbate do not deposit on other, already adsorbed, molecules of adsorbate, only on the free surface of the adsorbent. • These four assumptions are seldom all true
  62. 62. Langmuir • Langmuir suggested that adsorption takes place through this mechanism: where Ag is a gas molecule and S is an adsorption site. • Letting xo represent the total concentration of available sites on a given amount of fresh solid substrate, we can define a fractional coverage (θ) as (1) where x is the concentration of occupied sites. • The rate of adsorption va will be proportional to the concentration of gas or liquid (c) above the surface and the fraction of the surface that is not covered (1- θ), yielding a rate equation (2) where ka is the rate constant for adsorption.
  63. 63. Langmuir • The rate of desorption is simply proportional to the fraction of the surface that is already occupied, so the rate equation is • (3) • and kd is the rate constant for desorption. Setting equations (2) and (3) equal yields an equilibrium statement that can be written as (4) • The ratio of rate constants in equation (4) is equal to an equilibrium constant (K = ka/kd). • Upon substitution and further rearrangement, the fractional coverage is given by (5)
  64. 64. Langmuir • Equation (5) is plotted for an arbitrary value of K, (is often called a Langmuir absorption isotherm) • The surface sites becomes saturated as the concentration rises. • The magnitude of K quantifies the relative affinity that a given solute has for surface adsorption. • Like all equilibrium constants, K is temperature dependent. fractional coverage
  65. 65. BET Isotherm • Often molecules do form multilayers, that is, some are adsorbed on already adsorbed molecules and the Langmuir isotherm is not valid. • In 1938 Stephen Brunauer, Paul Emmett, and Edward Teller developed a model isotherm that takes that possibility into account. Their theory is called BET theory, after the initials in their last names. • They modified Langmuir's mechanism as follows: • A(g) + S ⇌ AS • A(g) + AS ⇌ A2S • A(g) + A2S ⇌ A3S and so on
  66. 66. Characteristics and general requirements • Adsorbents are used in the form of spherical pellets, rods, moldings ), or monoliths ( ) with hydrodynamic diameters between 0.5 and 10 mm. • They must have high abrasion resistance ), high thermal stability and small pore diameters, which results in higher exposed surface area and hence high surface capacity for adsorption. • The adsorbents must also have a distinct pore structure which enables fast transport of the gaseous vapors. Most industrial adsorbents fall into one of three classes: • Oxygen-containing compounds – Are typically hydrophilic and polar, including materials such as silica gel and zeolites. • Carbon-based compounds – Are typically hydrophobic and non-polar, including materials such as activated carbon and graphite. • Polymer-based compounds - Are polar or non-polar functional groups in a porous polymer matrix.
  67. 67. Silica gel • a chemically inert, nontoxic, polar and dimensionally stable (< 400 C) amorphous form of SiO2. • Prepared by the reaction between sodium silicate and acetic acid, which is followed by a series of after-treatment processes such as aging, pickling. • After treatment methods results in various pore size distributions. • Silica is used for drying of process air (e.g. oxygen, natural gas) and adsorption of heavy (polar) hydrocarbons from natural gas • silica gel is a solid • it has an average pore size of 2.4 nanometers and has a strong affinity for water molecules .
  68. 68. Zeolites • Natural or synthetic crystalline aluminosilicates. • Havin a repeating pore network and release water at high temperature. • Zeolites are polar in nature. They are manufactured by hydrothermal synthesis of sodium aluminosilicate or another silica source in an autoclave followed by ion exchange with certain cations (Na+, Li+, Ca2+, K+, NH4+). • The channel diameter of zeolite cages usually ranges from 2 to 9 Å. • The ion exchange process is followed by drying of the crystals, which can be pelletized with a binder to form macroporous pellets. • Zeolites are applied in drying of process air, CO2 removal from natural gas, CO removal from reforming gas, air separation… • Non-polar (siliceous) zeolites are synthesized from aluminum-free silica sources or by dealumination of aluminum-containing zeolites. • The dealumination process is done by treating the zeolite with steam at elevated temperatures, typically greater than 500 C. • This high temperature heat treatment breaks the aluminum-oxygen bonds and the aluminum atom is expelled from the zeolite framework.
  69. 69. Activated carbon • non-polar and cheap, highly porous, amorphous solid consisting of microcrystallites with a graphite lattice • Usually prepared in small pellets or a powder. • Its main drawbacks (it is reacts with oxygen at moderate temp. (over 300 C). • manufactured from carbonaceous material, including coal, peat, wood, or nutshells (e.g., coconut). • The manufacturing process consists of two phases, carbonization and activation. • The carbonization process includes drying and then heating to separate by- products, including tars and other hydrocarbons from the raw material, as well as to drive off any gases generated. • The process is completed by heating the material over 400 C in an oxygen-free atmosphere that cannot support combustion.
  70. 70. Activated carbon • The carbonized particles are then "activated" by exposing them to an oxidizing agent, usually steam or carbon dioxide at high temperature. • This agent burns off the pore blocking structures created during the carbonization phase and so, they develop a porous, three-dimensional graphite lattice structure. • The size of the pores developed during activation is a function of the time that they spend in this stage. • Longer exposure times result in larger pore sizes. • The most popular aqueous phase carbons are bituminous based because of their hardness, abrasion resistance, pore size distribution, low cost • Activated carbon is used for adsorption of organic substances and non-polar adsorbates and it is also usually used for waste gas (and waste water) treatment. • It is the most widely used adsorbent??? most of its chemical (eg. surface groups) and physical properties (eg. pore size distribution and surface area) can be tuned according to what is needed.
  71. 71. Activated carbon Activated Carbon Granular Activated Carbon Powder
  72. 72. Activated carbon
  73. 73. Characteristics of Different Adsorbents Table. Characteristics of Different Adsorbents DisadvantagesUseCharacteristicsType Difficult to regenerate Removal of organic pollutants Hydrophobic, favors organics over water Activated Carbon Low total capacity Air separation, dehydration Hydrophillic, polar, regular channels Zeolites Trace removal not effective Drying gas streamsHigh capacity, hydrophilic Silica gel Trace removal not effective Drying gas streamsHigh capacity, hydrophilic Activated alumina
  74. 74. Factors Affecting the Rate of Adsorption 1. Surface area of the adsorbent: • The rate of adsorption increases with increase in surface area of the adsorbent • Rate of adsorption ά 1/ Diameter of adsorbent for powdered activated carbon 2. Nature of solute (adsorbate): • Solubility of solute: Adsorption ά (1/Solubility of solute in solvent) • Chain length of molecules: Adsorption ά chain length of molecule • Molecular size of solute: Increase in molecular size of solute favors adsorption • Geometry of the molecule: Branched- lesser tendency to get removed, Coiled- more easily removed because easily entrapped in the interstices. • Degree of ionization: Adsorption ά (1/ Dissociation constant) this is the reason why salts are not much removed by activated carbon 3. Surface tension of the solvent: • Those substances which lower the surface tension of solvent in which they are dissolved become concentrated in surface layer (eg: organic substances), while those substances which raise the surface tension are less concentrated in the surface than in the bulk of solution. (eg.inorganic ions)
  75. 75. Ion Exchange • The ion exchange process percolates water through bead-like spherical resin materials (ion-exchange resins). • Ions in the water are exchanged for other ions fixed to the beads. • The two most common ion-exchange methods are softening and deionization. • Ion exchange materials are insoluble substances containing loosely held ions which are able to be exchanged with other ions in solutions which come in contact with them. • Exchanges take place without any physical alteration to the ion exchange material. • Ion exchangers are insoluble acids or bases which have salts which are also insoluble, and this enables them to exchange either positively charged ions (cation exchangers) or negatively charged ones (anion exchangers). • Many natural substances such as proteins, cellulose, living cells and soil particles exhibit ion exchange properties which play an important role in the way the function in nature.
  76. 76. Ion Exchange • Ion exchange uses a resin that removes charged inorganic contaminants like arsenic, chromium, nitrate, radium, uranium, and fluoride. • It works best with particle-free water and can be scaled to fit any size treatment facility. • Ion exchange is most often used to remove hardness (cation resin) or nitrate (anion resin). • In both instances, it can be regenerated with salt water. • The use of ion exchange to remove radionuclides (an atom with an unstable nucleus) is complicated by the fact that these materials accumulate in the resin and occur at high levels in the regenerant, greatly complicating operations. • Activated carbon is generally preferred for removing organic contaminants, whereas ion exchanges often best for removing inorganic soluble molecules
  77. 77. Types of Ion Exchange Resin / Acid Cation Resins Strong Acid Cation Resins • Behaving like strong acid • highly ionized in both the acid (R-SO3H) and salt (R-SO3Na) form. • Metal salt can be converted to the corresponding acid. • They can be used for entire range of pH and are utilized for water softening (calcium and magnesium removal). • The regeneration takes place by contact with a strong acid solution (hydrochloric or sulphuric) Weak Acid Cation Resins • Carboxylic acid (COOH) acts as the ionizable group in weak acid cation resins. • They show more affinity for hydrogen ions. This results in the regeneration of the hydrogen form with less acid than is required for strong acid resins.
  78. 78. Base AnionTypes of Ion Exchange Resin / Resins Strong Base Anion Resins • They are suitable for entire pH range. • They deionize water in hydroxide (OH) form. • Acidic nature of the water can be removed and pure water can be obtained. • The reaction can be put forward as: • for which sodium hydroxide is used as the regenerant Weak Base Anion Resins • In weak base resins, intensity of ionization is affected by pH. • They are incapable to split salts but can absorb acids
  79. 79. Types of Ion Exchange Resin
  80. 80. How ion exchange resins work • The resins are prepared as spherical beads 0.5 to 1.0 mm in diameter. • These appear solid even under the microscope, but on a molecular scale the structure is quite open. • This means that a solution passed down a resin bed can flow through the cross- linked polymer, bringing it into intimate contact with the exchange sites. • The affinity of sulphonic acid resins for cations varies with the ionic size and charge of the cation. • Generally the affinity is greatest for large ions with high valency. • For dilute Solutions the order of affinity for some common cations is approximately:
  81. 81. The general chemical reaction of ion exchange Cation exchange: nRA + Bn+ = RnBn+ + nA+ Anion exchange: nRA + Bn- = RnBn- + nA- Where: R : ionized resin molecule with an attached functional group; A+ / A- : exchangeable ion; Bn+ / Bn- : cation/anion dissolved in the liquid; n+ /n- : electrical charge of the ion B.
  82. 82. The general chemical reaction of ion exchange • Suppose a resin has greater affinity for ion B than for ion A. If the resin contains ion A and ion B is dissolved in the water passing through it, then the following exchange takes place, the reaction proceeding to the right: • When the resin exchange capacity nears exhaustion, it will be in the BR form. • A mass action relationship applies : • Q is the equilibrium quotient, and is a constant specific for the pair of ions and type of resin. • This expression indicates that if a concentrated solution containing ion A is now passed through the exhausted bed, the resin will regenerate into the AR form ready for re-use, whilst ion B will be eluted into the water. • All large scale applications for ion exchange resins involved such exhaustion and regeneration cycles.
  83. 83. Uses • To remove unwanted ions from a solution passed through it (heavy metals from metal wastes - salts from fruit juices) • To accumulate a valuable mineral from the water which can later be recovered from the resin. • Strong cation resins in the hydrogen form are used for the hydrolysis of starch and sucrose. • Used in the laboratory to remove interfering ions during analysis or to accumulate trace quantities of ions from dilute solutions. • A cation resin in the hydrogen form can be used to determine the total concentration of ions in a mixture of salts. The sample passing through a column is converted to the equivalent quantity of acid and the amount readily found by titration. • Earliest applications of ion exchange was the separation of rare earth elements (Promethium (element 61) and five new elements in the actinide series).
  84. 84. WATER TREATMENT The two major types of treatment applied to water are: • Water softening - the replacement of ‟hard‟ ions such as Ca2+ and Mg2+ by Na+ • Demineralisation - the complete removal of dissolved minerals.
  85. 85. Water softening • Softening is used primarily as a pretreatment method to reduce water hardness prior to reverse osmosis (RO) processing. • The softeners contain beads that exchange two sodium ions for every calcium or magnesium ion removed from the "softened" water. • In water softening a cation resin in the sodium form is used to remove hard metal ions (calcium and magnesium) from the water along with troublesome traces of iron and manganese • These ions are replaced by an equivalent quantity of sodium, so that the total dissolved solids content of the water remains unchanged as does the pH and anionic content. • At regular time intervals the resin is cleaned. This involves passing influent water back up through the resin. In water softening the regenerant is a strong solution of sodium chloride.
  86. 86. Example of Water softening A) Sodium cation exchange: • Ca+2 + 2Na.R = Ca.R + 2Na+ • Mg+2 + 2Na.R = Mg.R + 2Na+ Regeneration: using strong brine (NaCl) • Mg. R + 2NaCl = 2Na.R + MgCl2 • Ca. R + 2NaCl = 2Na.R + CaCl2
  87. 87. Example of Water softening
  88. 88. Demineralisation = deionization • Complete deionization can be achieved by using two resins. • The water is first passed through a bed of cation exchange resin in hydrogen ion form • During service, cations in the water are taken up by the resin while hydrogen ions are released. • Thus the effluent consists of a very weak mixture of acids. • Then water passes through second anion exchange resin in the hydroxide form. • Here the anions are exchanged for hydroxide ions, which react with the hydrogen ions to form water. • twin bed units will reduce the total solids content to approximately 1-2 mg L-1. • it is usual to pass water leaving the cation unit through a degassing tower. • degassing tower removes the carbonic acid produced from carbon dioxide and bicarbonate in the feed water and reduces the load on the anion unit. • Without degassing the carbonic acid would be taken up by the anion bed after conversion to carbonate.
  89. 89. Mixed resin • Mixed resin produces water with much lower levels of dissolved material than can be achieved by distillation. • In laboratories, mixed resin is often used in disposable cartridges. These are only used once, but larger mixed resin units can be regenerated. • After exhaustion the bed is subjected to an up flow of water. • Anionic resin beads are less dense than the cationic ones and they rise to the top so that the bed is separated into two layers of resin. • Each is regenerated in situ with the appropriate regenerant then rinsed with clean water.
  90. 90. Cation and anion exchange and regeneration i) Hydrogen cation exchange: M+a + aH.R M.Ra + aH+ Examples: Ca+2 + 2H.R Ca.R2 + 2H+ Na+ + H.R Na.R + H+ Regeneration: using strong acid Ca. R + H2SO4 2H.R + CaSO4 2Na. R + H2SO4 2H.R + Na2SO4 ii) Hydroxyl anion exchange: A-b + bR.OH Rb.A + bOH Examples: NO3 -+ R.OH R.NO3 - + OH CO3 -2 + 2 R.OH R2.CO3 -2 + 2OH Regeneration: using strong base (caustic soda) R.NO3 - + NaOH R.OH + NaNO3 R2.CO3 -2 + 2NaOH 2R.OH + Na2CO3
  91. 91. Typical ion exchange installation
  92. 92. The ion exchange system in water treatment a) Configuration of the ion exchanger • Cylindrical steel tank (Diameter 1-2 m - Height 3-4 m ) • ion exchange bed occupies 1-3 meters of the tank height • The water inters from the top (downflow) at the rate of 0.5 to 7 L/s.m2 . • When breakthrough is reached the tank is taken off-line and backwashed by applying water form the bottom upwards to remove any suspended solids. • After backwashing the regeneration solution is pumped from the bottom up wards at the rate of 0.7 to 1.5 L/s.m2. • The same influent distributor is used to drain the upflow backwash water and the regeneration solution ( brine, acid or base). • At the end of the regeneration the bed is washed with clean water to remove the residual of the regeneration solution • An under drain piping system is installed at the bottom to collect the treated water, and used to pump the upflow backwash water and the regeneration solution.
  93. 93. The ion exchange system in water treatment b) Pretreatment: • The influent to the ion exchanger should be filtered to remove turbidity. • Dissolved Organic matter should be removed by GAC before the IE because the organic may coat the resin and reduce its exchange capacity. • The IE is efficient for TDS less than 1000 mg/L. c) Sizing ion exchanger : depends on the following factors: • i) Contact time • ii) Hydraulic loading rate • iii) resin depth • iv) number of columns. d) Multiple tanks Operation: • Ion exchange tanks can be operated in parallel or in series. • Multiple units permit one or more units to remain in operation while one unit is taken out of service for backwashing and generation or maintenance.
  94. 94. Exchange capacity of ion exchange resins • The total solid phase concentration “q0” is termed ion exchange capacity. • For cation exchange resins, “q0” is in the range of 200 to 500 meq/100 mg of resin. • During the exchange, the resin should be electrically neutral thus the all the exchange sites should be occupied either by the original ion (such as Na+) or by the replacing ions ( such as Ca+2 and Mg+2) and the ion exchange occupancy should be equal to “q0” at any time.
  95. 95. Exchange capacity of ion exchange resins • the Thomas kinetic equation is used for ion exchange columns. • C = effluent concentrat ion of the ions, mg/l or meq/l • Co = influent concentrat ion of the ions, mg/l or meq/l • k1= rate constant, L/d . eq • q0 maximum solid phase concentrat ion of exchange solute, eq/kg of resin • M = mass of resin, kg • V throughput volume, L ( ) • Q flow rate, L/d • To apply this equation it is necessary to perform a laboratory column test or pilot scale column to obtain the breakthrough curve.
  96. 96. Ion Exchange process analysis in the Fixed bed a) Mass transfer inside the Ion exchange bed: • When the polluted water is pumped on the ion exchange bed, the pollutant ions replace the exchangeable ions in the resin. • The area of the ion exchange bed in which the exchange occurs is called the mass transfer zone (MTZ) • No further adsorption occurs below the MTZ and the water leaving the MTZ zone contains the minimum concentration value of the pollutant that the bed can produce. • With time a zone of saturation is created above the MTZ in which the resin has reached its maximum exchange capacity and no further replacement occurs. • The equilibrium concentration Ce of the pollutant in water in this zone is the same as C0. • The Zone below the MTZ essentially clean zone and no adsorbed material on it. • With time the saturation zone depth increases and the MTZ is pushed down until we reach to a point where the clean zone disappears and breakthrough occurs.
  97. 97. Breakthrough curve • Breakthrough is said to have occurred when the effluent concentration reaches to 5% of the influent concentration (i.e, Cb = 0.05C0). • After additional time the MTZ start to decrease until it disappears and the bed is called exhausted. • Exhaustion of the bed is assumed to have occurred when the effluent Concentration is equal to 95% of the influent concentration (i.e, C = 0.95C0) • The area above the breakthrough curve is equal to the mass of the pollutant adsorbed in the column
  98. 98. Calculation of the length of the MTZ ***
  99. 99. Detection of resin exhaustion • A resin is considered to be exhausted when the ions in the resin have mostly been replaced by the ions that are being removed from the solution. • Exhaustion of demineraliser is usually detected by an electrical conductivity cell installed at the outlet. • When the conductivity rises to indicate ionic breakthrough, a regeneration cycle can be initiated automatically. • With small units it is possible to incorporate a pH indicator on the anion resin of a mixed bed cartridge. • Exhaustion can be followed down the side of a transparent cartridge as the alkaline anion resin is converted to the neutralised salt form.
  100. 100. Advantages and disadvantages in the use of Ion-Exchange Resins The advantages of ion exchange processes are: • very low running costs. • Very little energy is required, • The regenerant chemicals are cheap and if well maintained resin beds can last for many years before replacement is needed.
  101. 101. Disadvantages in the use of Ion-Exchange Resins Calcium sulphate fouling • Using Sulphuric acid as cation resin regenerant will react with calcium in water forming calcium sulphate precipitates. • This fouls the resin and blocks drain pipes with a build up of scale (hydrochloric acid must be substituted). Iron fouling • Aeration allows oxidation of Fe2+ to Fe3+ and consequent precipitation of ferric hydroxide which clogs resin beads and prevents ion exchange. Iron fouling is the commonest cause of softener failure. Adsorption of organic matter • the presence of dissolved organic material can become irreversibly adsorbed within the anion beads, reducing their exchange capacity. • Removal of organics prior to demineralisation is achieved by flocculation with alum or ferric salts followed by filtration.
  102. 102. Disadvantages in the use of Ion-Exchange Resins Organic contamination from the resin • The resins themselves can be a source of non-ionized organic contamination. New commercial grade resin often contains organics remaining after manufacture. when removal is needed, the demineralised water can be passed through an ultra filtration membrane. Bacterial contamination • Resin beds do not act as filters for the removal of bacteria. • Resin beds can generate a culture media for continued growth. • Resins beds can be decontaminated with disinfectants such as formaldehyde • heat or oxidising disinfectants as chlorine must not be used as these damage resins. Chlorine contamination • chlorine damages resins. It is customary to treat such feeds by passing them through activated carbon which removes chlorine very efficiently. ENVIRONMENTAL IMPLICATIONS • The waste water for disposal after regeneration contains all the minerals removed from the water plus salt from the spent regenerants. • volume of it is equivalent to 1-5% of the treated water throughput.
  103. 103. Coagulation and Flocculation Coagulation refers to all the reactions and mechanisms that result in particle aggregation in the water being treated, including in situ coagulant formation (where applicable), particle destabilization, and physical interparticle contacts • Coagulant formation, particle destabilization, typically occur during and immediately after chemical dispersal in rapid mixing; • inter particle collisions that cause aggregate (floc) formation begin during rapid mixing but usually occur predominantly in the flocculation process. • The physical process of producing inter-particle contacts is termed flocculation. • Flocculation defined as the uses gentle stirring to bring suspended particles together so they will form larger more settleable clumps (groups) called floc.
  104. 104. A common classification of particles • Molecules sizes smaller than 1 nm • Colloids generally with dimensions between 1 nm - 1 μm • Suspended matter having sizes larger than 1 μm. • Colloids: humic acids, proteins, colloidal clay, silica and viruses. • Suspended matter: Bacteria, algae, silt, sand and organic debris. • Suspended matter-when it is larger than 5-10 μm can be removed quite easily by filtration or sedimentation and filtration. • The removal of colloids is possible
  105. 105. A common classification of particles
  106. 106. STABILITY OF PARTICLE SUSPENSIONS • Coagulation process is used to increase the rate or kinetics of particle aggregation and floc formation • The objective is to transform a stable suspension [i.e., one that is resistant to aggregation (or attachment to a filter grain)] into an unstable one. • there are forces that tend to pull the interacting surfaces together • The most important attractive force is called the London–van der Waals force. • It arises from spontaneous electrical and magnetic polarizations that create a fluctuating electromagnetic field within the particles and in the space between them • The most well-known repulsive force is caused by the interaction of the electrical double layers of the surfaces (“electrostatic” stabilization). • As particles approach one another on a collision course, the fluid between them must move out of the way. • The repulsive force caused by this displacement of fluid is called hydrodynamic retardation.
  107. 107. Electrostatic Stabilization • Origins of Surface Charge. Most particles in water, mineral and organic, have electrically charged surfaces, and the sign of the charge is usually negative. Three important processes for producing this charge are considered in the following discussion. • First, surface groups on the solid may react with water and accept or donate protons. • Second, surface groups can react in water with solutes other than protons. • Third, a surface charge may arise because of imperfections within the structure of the particle; this is called isomorphic replacement, or substitution
  108. 108. The Electrical Double Layer (EDL) • In a colloidal suspension, there can be no net imbalance in the overall electrical charge • the primary charge on the particle must be counterbalanced in the system. • If the particle is negatively charged, excess ions of opposite charge (positive) accumulate in this interfacial region • Ions of opposite charge accumulating in the interfacial region, together with the primary charge, form an electrical double layer. The diffuse layer results from: – electrostatic attraction of ions of opposite charge to the particle – electrostatic repulsion of ions of the same charge as the particle, and – thermal or molecular diffusion that acts against the concentration gradients produced by the electrostatic effects • Because of the primary charge, an electrostatic potential (voltage) exists between the surface of the particle and the bulk of the solution • This electric potential can be pictured as the electric pressure that must be applied to bring a unit charge having the same sign as the primary charge from the bulk of the solution to a given distance from the particle surface
  109. 109. The zeta potential • It is a scientific term for electrokinetic potential in colloidal systems (ζ-potential) • It represents the net charge between the primary charge and the counter charge in the EDL located between the surface and the shear plane • zeta potential is electric potential in the interfacial double layer at the location of the slipping plane versus a point in the bulk fluid away from the interface • zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle. • A value of 25 mV (positive or negative) can be taken as the arbitrary value that separates low-charged surfaces from highly-charged surfaces. • The zeta potential indicates the degree of repulsion between adjacent, similarly charged particles in a dispersion.
  110. 110. The zeta potential Zeta potential [mV] Stability behavior of the colloid from 0 to ±5, Rapid coagulation or flocculation from ±10 to ±30 Incipient instability from ±30 to ±40 Moderate stability from ±40 to ±60 Good stability more than ±61 Excellent stability • For molecules and particles that are small enough, a high zeta potential will confer stability, i.e., the solution or dispersion will resist aggregation. • When the potential is low, attraction exceeds repulsion and the dispersion will break and flocculate. • Colloids with high zeta potential (negative or positive) are electrically stabilized while colloids with low zeta potentials tend to coagulate or flocculate
  111. 111. a net electrostatic repulsion/attraction developed between colloids as a result of EDL The net resultant force is a result of: • Attractive potential energy (mostly van der Waals forces). • Repulsion potential energy (electrostatic forces)
  112. 112. Relative strength of forces Dipole-dipole interactions: electrostatic interactions of permanent dipoles in molecules. These interactions tend to align the molecules to increase the attraction (reducing potential energy). An example of dipole-dipole interactions can be seen in hydrogen chloride (HCl) Hydrogen bond : is the attractive interaction between a hydrogen atom and an electronegative atom, such as nitrogen, oxygen, or fluorine. The hydrogen bond is a strong electrostatic dipole-dipole interaction. It is stronger than a van der Waals interaction, produces interatomic distances shorter than sum of van der Waals radii Relative strength of forces Bond type Dissociation energy (kcal) Covalent 400 Hydrogen bonds 12-16 Dipole-dipole 0.5 - 2 London (van der Waals) Forces <1
  113. 113. Calculation of the zeta potential*** The magnitude of these forces is measured by the zeta potential, which is: where: • Z is the zeta potential, • q is the charge per unit area, • d is the thickness of the effective charge layer, and • D is the dielectric constant of the liquid. The greater the zeta potential, the greater are the repulsion forces between the colloids and, therefore, the more stable is the colloidal suspension
  114. 114. Brief There are two major forces acting on colloids: 1) electrostatic repulsion (simply, negative colloids repel other negatively charged colloids) 2) intermolecular, or van der Waals, attraction • Coagulants can be used to reduce the electrostatic repulsive forces. The electrostatic repulsion reduced by the addition of countercharged ions such as [Al3+]
  115. 115. Process Description Purpose to aid in the removal of nonsettleable solids from water. Coagulation is defined as: • the destabilization of colloidal solids; • the water treatment process which causes very small suspended solids to attract one another and form larger particles. Suspended particles in water resist settling for two primary reasons: 1. Particle size; and, 2. Natural forces between particles. • Suspended particles in water normally have a negative (-) charge. • Since these particles all have the same charge, they repel each other, keeping each other from settling. • This natural repelling force is called the zeta potential. • Coagulation neutralizes the forces (zeta potential), which cause suspended solids in water to repel each other and resist settling. • Once the repulsive forces have been neutralized these particles can stick together (agglomerate) when they collide. • The force which holds the floc together is called the van der Waals force.
  116. 116. Flocculation • After coagulation the destabilized particles can collide, aggregate so flocs can be formed. This step is called flocculation. • Flocculation: The process of agglomeration of the destabilized particles to such a size that separation by sedimentation and filtration is possible. • In flocculation one can make a distinction between peri-kinetic and ortho- kinetic flocculation. • Brownian motion is the driving force in the agglomeration of destabilized particles up to 1μm-level peri-kinetic flocculation). • Above ~ 1 μm the influence of Brownian motion on the collision rate of the particles can be neglected, then artificial mixing is necessary to get an efficient flocculation. That part of the flocculation process is called ortho-kinetic flocculation. • Flocculation uses gentle stirring to cause the particles to collide so that they can stick together, for a particle (floc) large enough and heavy enough to settle
  117. 117. Flocculation Practice in Water Treatment*** • Coagulation and Flocculation Process Design Where, • G = velocity gradient, s-1 • P = power input, W • V = volume of water in mixing tank, m3 • μ = dynamic viscosity, Pa.s • Viscosity is a measure of the resistance of a fluid which is being deformed. The SI physical unit of dynamic viscosity is the pascal-second (Pa·s), which is identical to N·m−2·s • Velocity gradient velocity gradient is the change in relative velocity between parallel planes with respect to the change in perpendicular distance throughout the depth of the material. Velocity gradient has the dimensions as rate of shear, which is reciprocal seconds.
  118. 118. Types of Flocculation Tanks Mechanical Flocculators Paddle wheel Type (vertical and Horizontal Types) Foil Type Mixing Blade
  119. 119. Types of Flocculation Tanks • Hydraulic Flocculators • The axial flow flocculators are typically used because they impart a nearly constant gradient in each compartment. • Flocculators are designed to have a minimum of three compartments to provide for tapered (to make smaller gradually) mixing. • The velocity gradient, G is tapered so that it is larger in the first compartment and less is the other compartments as the floc grows.
  120. 120. Chemicals are used to neutralize the zeta potential • These chemicals are coagulants, sometimes called primary coagulants, and coagulant aids. • Since most suspended particles in water carry a negative (-) charge, coagulants consist of chemicals that provide positively (+) charged ions. Common coagulants are: • 1. Metal Salts • a. Aluminum Salts (Alum (aluminum sulfate) - PACs (polyaluminum chlorohydrate, and other variations) • b. Iron Salts (Ferric Chloride - Ferric Sulfate - Ferrous Sulfate) • 2. Polymers (polyelectrolytes)
  121. 121. Common coagulants / Polymers • Polymers (polyelectrolytes) are extremely large molecules which produce thousands of charged ions when dissolved in water. 1. Cationic Polyelectrolytes - Have a positive (+) charge. Used as either a primary coagulant or as a coagulant aid. Cationic polymers: • allow reduced coagulant dose; • improve floc settling; • are less sensitive to pH; • improve flocculation of organisms such as bacteria and algae. 2. Anionic Polyelectrolytes- Have a negative (-) charge. Used primarily as a coagulant aid. Anionic polymers are used to: • increase floc size; • improve settling; • produce a stronger floc; • They are not materially affected by pH, alkalinity, hardness or turbidity. 3. Nonionic Polyelectrolytes- Balanced or neutral charge. • Used as a primary coagulant or coagulant aid.
  122. 122. Coagulant aids Coagulant aids are chemicals which are added to water during coagulation to improve coagulation by: • building a stronger, more settleable floc; • overcoming slow floc formation in cold water; • reducing the amount of coagulant required; • reducing the amount of sludge produced. • The key reason coagulant aids are used is to reduce the amount of alum used, which, in turn, decreases the amount of alum sludge produced. • Alum sludge is difficult to dewater and to dispose of.
  123. 123. Types of Coagulant Aids Activated Silica • increases the coagulation rate; • reduces the amount of coagulant needed; • widens the pH range for effective coagulation; • strengthens floc Weighting agents (Bentonite Clay, Powdered Limestone; Powdered Silica) provide additional particles that can enhance floc formation. They are used to treat water that is: • high in color; or, • low in turbidity; or, • low in mineral content.
  124. 124. Factors Which Affect How Well a Coagulant Work Factors Which Affect How Well a Coagulant Work (1) Mixing Conditions (2) pH (3) Alkalinity (4) Water Temperature (5) Turbidity • If the alkalinity concentration in the water is not high enough, and effective floc will not form when either alum or ferric sulfate is used. Metal salts (alum, ferric sulfate, ferric chloride) consume natural alkalinity. • Each mg/L of alum will consume 0.5 mg/l total alkalinity (as CaCO3). • Each mg/L ferric sulfate will consume 0.75 mg/L total alkalinity (as as CaCO3). • Each mg/L ferric chloride will consume 0.92 mg/L total alkalinity (as CaCO3). • It may be necessary to add alkalinity to the water (lime, soda ash, caustic soda) to the water in order for the metal salts to work properly. The doses should be confirmed with jar testing.
  125. 125. Coagulation/Flocculation Facilities Coagulation/Flocculation Facilities • Flash Mix - purpose is to distribute the coagulant rapidly and evenly throughout the water. • Water should be stirred violently for a brief time to encourage the greatest number of collisions between particles as possible. • Types of Mixers: Mechanical - Pumps and Conduits • Detention time should be 30 seconds or less (Design Criteria). • Flocculation - provides for gentle mixing to encourage floc formation. • Detention time of at least 30 minutes, with a detention time of 45 minutes preferred.
  126. 126. Process Control A. Chemical Selection B. Chemical Application / Solution Preparation C. Monitoring Process Effectiveness
  127. 127. Process Control / Chemical Selection • A. Chemical Selection - These raw water characteristics should be monitored in order to do a thorough job of chemical selection. 1. Temperature • Low water temperatures slow chemical reactions, causing decreased efficiency and slow floc formation. • Higher coagulant doses may be required to maintain acceptable results. 2. pH • Extremes can interfere with the coagulation/flocculation process. • The optimum pH depends on the specific coagulant. 3. Alkalinity • Low alkalinity causes poor coagulation. • May be necessary to add alkalinity (lime, caustic soda, soda ash). 4. Turbidity • Difficult to form floc with low turbidity water, may need to add weighting agents. 5. Color - Indicates presence of organic chemicals which can react with the coagulant, and with chlorine to form disinfection byproducts.
  128. 128. Process Control / Chemical Application B. Chemical Application: Solution Preparation • For Example when preparing potassium permanganate solutions, a three percent solution is best. Potassium permanganate has a limited solubility of about five percent at normal temperatures. In order to prepare the solution needed the following information is required: • Chemical required • Volume of water required • Specific gravity • Weight of solution • Concentration
  129. 129. Process Control / Monitoring Process Effectiveness C- Monitoring Process Effectiveness • (1) Jar Test • (2) pH • (3) Turbidity • (4) Temperature • (5) Alkalinity
  130. 130. Process Evaluation and Troubleshooting • Problems with coagulation and flocculation normally show up in subsequent processes (sedimentation and filtration). Actual plant performance should be monitored for: Poor or inadequate flash mixing • Gentle flocculation • Adequate flocculation time • Settled- and filtered water quality. Indication of poor or inadequate flash mixing: 1. Very small floc (pin floc) 2. High turbidity in settled water should be less than 1.0 NTU when the raw water turbidity is less than 1 NTU 95 percent of the time; less than 2.0 NTU when the raw water turbidity is more than 1 NTU 95 percent of the time. 3. Too frequent filter backwashing If any of these symptoms occur, check: (1) detention time and mixing energy in the flash mix (detention time should be less than 30 seconds); (2) stirring speed in the flocculator – should have a peripheral speed of between 0.5 and 2.0 feet per second; (3) flocculation detention time – should be at least 30 minutes, with 45 minutes being preferred.
  131. 131. Jar Test • Coagulation/flocculation is the process of binding small particles in the water together into larger, heavier clumps which settle out relatively quickly. • The larger particles are known as floc. • changing water characteristics require the operator to adjust coagulant dosages at intervals to achieve optimal coagulation. • Different dosages of coagulants are tested using a jar test, which mimics the conditions found in the treatment plant. • The first step of the jar test involves adding coagulant to the source water and mixing the water rapidly (as it would be mixed in the flash mix chamber) to completely dissolve the coagulant in the water. • Then the water is mixed more slowly for a longer time period (as flocculation basin conditions and allowing the forming floc particles to cluster together). • Finally, the mixer is stopped and the floc is allowed to settle out, as it would in the sedimentation basin.
  132. 132. Jar Test • A major goal of water treatment is turbidity removal. • The jar test is a simulation of the treatment processes that have been developed to accomplish turbidity removal • Alum, ferrous sulfate, and ferric chloride are three common coagulants • The best dose will also be a function of pH. The optimum pH for alum coagulation is usually between 5.5 and 6.5. • There is no way to “calculate” the best dose. It must be determined by trial and error; hence, the jar test. • The reaction chemistry varies according to the pH and alkalinity of the test sample. Alum coagulation proceeds according to the following equation • if there is enough alkalinity in the water to react with the amount of alum dosed: Al2(SO4)3 • 14H2O + 6HCO3 - ↔ 2Al(OH)3(s) + 6CO2 + 14H2O + 3SO4 -2 • If there is insufficient alkalinity, the reaction will proceed according to the equation: • Al2(SO4)3 • 14H2O ↔ 2Al(OH)3 + 3H2SO4 + 8H2O • An alkalinity test is usually performed before initiating a jar test to determine whether alkalinity supplements might be required.
  133. 133. JAR TEST PROCEDURE 1. measure the initial pH, alkalinity, and turbidity of the sample to be tested. Make any pH adjustment necessary. calculate for the maximum alum dose you plan to use, what alkalinity concentration is required to prevent significant pH reduction. Compare this required amount to the amount measured in step 1, and supplement the sample with alkalinity if necessary. Al2(SO4)3 • 14H2O + 6HCO3 - ↔ 2Al(OH)3(s) + 6CO2 + 14H2O + 3SO4 -2 Decide on six dosages of the chemical(s) include coagulants, coagulant aids, and lime 2. Prepare a stock solution of the chemical(s). 3. Collect a two gallon sample of the water to be tested. This should be the raw water. 4. Measure 1,000 mL of raw water and place in a beaker. Repeat for the remaining beakers. 5. Place beakers in the stirring machine. 6. With a measuring pipet, add the correct dosage of lime and then of coagulant solution to each beaker as rapidly as possible.
  134. 134. JAR TEST PROCEDURE 7. With the stirring paddles lowered into the beakers, start the stirring machine and operate it for one minute at a speed of 80 RPM. 8. Reduce the stirring speed to 20 RPM and continue stirring for 30 minutes 9. Stop the stirring apparatus and allow the samples in the beakers to settle for 30 minutes. 10. Determine which coagulant dosage has the best flocculation time and the most floc settled out. 11. Test the turbidity of the water in each beaker using a turbidometer 12. If lime or a coagulant aid is fed at your plant in addition to the primary coagulant, you should repeat the jar test to determine the optimum dosage of lime or coagulant aid. Use the concentration of coagulant chosen in steps 10 and 11 and alter the dosage of lime or coagulant aid. 13. Using the procedure outlined in step 11, measure the turbidity of water at three locations in the treatment plant - influent, top of filter, and filter effluent. 14. Prepare a graph of alum dose vs. remaining turbidity in order to identify the dosage that produced optimum turbidity removal
  135. 135. JAR TEST
  136. 136. Jar Test Example Low alkalinity High alkalinity
  137. 137. Explanation of Jar Test Example • Water A had low alkalinity and required less coagulant to achieve good coagulation and flocculation than the higher alkalinity of Water B • Plots of turbidity versus coagulant dose showed a continual decrease in turbidity with an increase in coagulant dose. • Water A, with FeCl3, showed a decrease followed by an increase (at 40 mg/L) in turbidity. • This dictates that adsorption and charge neutralization is taking place due to the colloids restabilizing and not coagulating. • Addition of coagulant to the low alkalinity waters lead to a drop in the pH of Water A, which enhanced adsorption and charge neutralization. • higher coagulant doses are needed with high alkalinity waters where pH remains fairly constant. • Although slightly less alum than FeCl3 was needed to reach an optimum level, the residual turbidity when using the alum coagulant did not fall below 1 NTU. • This means that even though alum may require a slightly smaller dose, it still may not be able to meet the desired effluent regulations without the additional help of a filter or polymer
  138. 138. Filtration • After separating most floc, the water is filtered as the final step to remove remaining suspended particles and unsettled floc 1. Rapid sand filters • use relatively coarse sand and other granular media to remove particles and impurities that have been trapped in a floc through the use of flocculation chemicals-typically salts of aluminium or iron. • Water and flocs flows through the filter medium under gravity or under pumped pressure • Water moves vertically through sand which often has a layer of activated carbon or anthracite coal (a hard, compact variety of mineral coal). • The top layer removes organic compounds • Most particles pass through surface layers but are trapped in pore spaces or adhere to sand particles • To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called backflushing or backwashing) • compressed air may be blown up through the bottom of the filter to break up the compacted filter media to aid the backwashing process
  139. 139. Rapid sand filters
  140. 140. Rapid sand filters / Advantages and disadvantages Advantages • Much higher flow rate than a slow sand filter; • Requires relatively small land area • Less sensitive to changes in raw water quality, e.g. turbidity • requires less quantity of sand • Disadvantages • Requires greater maintenance than a slow sand filter. For this reason, it is not usually classed as an "appropriate technology,". • Generally ineffective against taste and odour problems. • Produces large volumes of sludge for disposal. • Requires on-going investment in costly flocculation reagents. • treatment of raw water with chemicals is essential • skilled supervision is essential • cost of maintenance is more • it cannot remove bacteria
  141. 141. Slow sand filters • Slow "artificial" filtration (a variation of bank filtration) to the ground, Water purification plant • The filters are carefully constructed using graded layers of sand with the coarsest sand, along with some gravel, at the bottom and finest sand at the top. • Drains at the base convey treated water away for disinfection • effective slow sand filter may remain in service for many weeks or even months • produces water with a very low available nutrient level and low disinfectant levels • Slow sand filters are not backwashed; they are maintained by having the top layer of sand scraped off • A 'large-scale' form of slow sand filter is the process of bank filtration in a riverbank.
  142. 142. Slow sand filters
  143. 143. Slow sand filters / Advantages and disadvantages • Advantages • require little or no mechanical power, chemicals or replaceable parts, • require minimal operator training and only periodic maintenance, • often an appropriate technology for poor and isolated areas. • simple design • disadvantages • Due to the low filtration rate, slow sand filters require extensive land area for a large municipal system. • Many municipal systems in grown cities installed rapid sand filters, due to increased demand for drinking water.
  144. 144. Membrane filtration • is a treatment process based on the physical separation of compounds from the water phase with the use of a semi-permeable membrane. • Most of the membranes used are synthetic membranes made of organic polymers. Can be divided into two catego-ries based on the pore sizes of the membrane: • micro- and ultrafiltration (MF and UF) remove colloidal substances and microorganisms • nanofiltration and reverse osmosis (NF and RO) remove colloidal substances and microor-ganisms but also dissolved substances like micropollutants and ions • Micro- and ultrafiltration remove substances from the water phase by a sieve mechanism • Microfiltration removes bacteria and the larger viruses (to a size of 0.05 μm). • Ultrafiltration also removes bacteria, but because of the smaller pore size all the larger viruses are removed • The removal of suspended solids of MF and UF is at least 99%.
  145. 145. Membrane filtration • The removal of microorganisms is referred to in log units. • A removal of one log unit corresponds to a 90% removal. The removal of 4 log units cor-responds to a 99.99% removal. • molecular weight cut-off (MWCO) can also be used as an indication of the ability of membranes to reject compounds • MWCO is the molecular weight of spherical molecules which are 90% rejected by the membrane‟s pores. • The unit of MWCO is Dalton (1 Dalton is the mass of one hydrogen atom = 1.66x10-27kg) • The MWCO for MF/UF is in the range of 10,000 to 500,000 Dalton (10 to 500 kD).
  146. 146. Different filtration processes and size of compounds removed
  147. 147. Different filtration processes and size of compounds removed
  148. 148. Different filtration processes and size of compounds removed
  149. 149. MF/UF for drinking water • In drinking water treatment, UF can be used in different stages of the process: – as a pre-treatment of surface water to remove suspended solids, heavy metals, bacteria and viruses in order to prevent pollution of the dunes, or to prevent clogging of the NF/RO membranes – as treatment of backwash water from rapid sand filtration – treatment of surface water as the first step in drinking water production. In some cases, the MF/UF installation is preceded by a conventional coagulation/flocculation/floc removal treatment process in order to reduce the risk of membrane fouling
  150. 150. Log-removal capacity of MF and UF for different microorganisms
  151. 151. Principle • The membrane is the barrier responsible for the separation of compounds out of the water phase. The membrane is semi-permeable. • The pore size determines the removal of different compounds. • The removed compounds remain at the raw waterside of the membrane and accumulate on the membrane. • Three water streams can be distinguished: – the dirty water or raw water is called feed water – the water passing the membrane is called the permeate or product water. – the water with the rejected particles is called concentrate or retentate.
  152. 152. Possibilities for the use of MF and UF for drinking water production
  153. 153. Membrane and different flows
  154. 154. Dead-end filtration mode • In dead-end filtration, all the feed water is through the membrane. • The suspended solids remain on the feed side of the membrane. • As a consequence, the resistance of permeation will increase in time. • The water flux decreases if the pressure is constant, or the pressure increases if the flux is constant. • Periodically the membrane has to be cleaned in order to reduce the resistance of permeation. • The period of permeation is called filtration time. A filtration run is the filtration time together with the cleaning time (also called filtration cycle).
  155. 155. Inside-out filtration • In a configuration with inside-out filtration, feed water enters the inside of the capillaries or tubular membranes. • The water is pushed from the inside to the outside of the membrane. • Permeate is collected outside the membrane and transported to the permeate tube.
  156. 156. Membrane fouling • During filtration the resistance increases as a result of fouling of the membrane surface. • The resistance increases because the pores in the membrane are blocked and because caked suspended matter is built up on the membrane surface. • This resistance increase is referred to as fouling. • The definition of fouling is: the deposition of suspended or dissolved substances on the membrane surface or in front of the pores or in the membrane pores. • fouling can be subdivided into different mechanisms: - membrane resistance - pore blocking - adsorption in the pores - cake resistance - high concentration of dissolved substances near the surface.
  157. 157. Membrane fouling • Due to the accumulation of solids on and in the membrane during dead-end filtration, the total resistance increases with time. • The mass of a cake layer is difficult to measure. The thickness of the cake layer depends on The trans membrane pressure TMP. • The thickness of the cake layer is in the range of several micrometers, depending on the rejected compounds.
  158. 158. Cleaning • As a result of the dead-end mode, the membrane has to be cleaned often in order to remove the rejected compounds. • The cleaning intervals can be constant in time or can be determined by a maximum pressure. • If possible, cleaning of membranes should be avoided because during the cleaning no permeate is produced. • Also, permeate and energy are used for the cleaning. With specific cleanings chemicals are also used. • Different methods or a combination of methods can be used to clean a membrane module: - forward flush (FF) - back flush (BF) - air flush (AF) - chemical enhanced flush (CEF) or enhanced back flush (EBF) - cleaning in place (CIP) or chemical soaking
  159. 159. Forward flush (FF) and back flush (BF) The forward flush is a turbulent cross-flow along the feed side of the membrane surface This is the opposite of the filtration mode where the flow is through the membrane (flow direction perpendicular to the membrane surface) In the back flush, filtration direction is reversed After removing the particles from the pores and the membrane surface, the particles and the cake have to be transported out of the module. A combined back flush and forward flush can be used. First, a back flush is used to clean the pores and to lift the cake. Then, a forward flush is used to transport the dirt out of the module
  160. 160. Air/water flush • An air/water flush can be used to clean the membrane wall from adhering fouling. • The air/water flush is actually a forward flush with a combination of air and water. • The air is used to create a turbulent flow in the membrane under process conditions where no turbulence is attained with the water flow. • The cleaning efficiency depends on the kind of two-phase flow obtained in the membranes. • If the water/air ratio is high, only small air bubbles are present in the water and the turbulence is only slightly enhanced. • When the water/air ratio is too small, the air flows through the middle of the membrane and the cleaning effect is low. • The best cleaning is obtained with bullet-like air bubbles.
  161. 161. Air/water flush
  162. 162. Chemical cleaning • If forward flush, back flush and air flush are not enough to clean the membrane, a chemical cleaning (often called enhanced back flush or chemical back flush) can also decrease the clean water resistance. • This kind of cleaning means that the module is soaked with a solution of hypochloric acid, hydrogen chloride, hydrogen peroxide. • After the soaking, a backwash or a forward flush removes the dissolved dirt. • The main drawback of chemical cleaning is that the membranes age because of the chemicals, and the lifetime of the membranes, therefore will be shortened. • Also, the chemicals are a cost factor and with a chemical cleaning a chemical waste stream should be discharged. • Besides the periodical chemical cleaning which is part of the automated process control of the installation, a more intensive chemical cleaning might also be necessary. • The so-called “cleaning in place” (CIP) can last from a few hours to several days and is typically not automated. • If the CIP is not able to clean the membranes, they are replaced by new ones.
  163. 163. Practice / Module design Tubular membranes Capillary membranes Plate membrane Cushion membranes Spiral Wound Flat Sheet Membrane
  164. 164. Choosing a module design
  165. 165. Membrane Processes / Electro-dialysis • What is a Membrane? • The membrane can be defined essentially as a barrier, which separates two phases and restricts transport of various chemicals in a selective manner. • Membrane Separation Technology • A membrane separation system separates an influent stream into two effluent streams known as the permeate and the concentrate. • The permeate is the portion of the fluid that has passed through the semi- permeable membrane. • The concentrate stream contains the constituents that have been rejected by the membrane.
  166. 166. Membrane Processes The main membrane processes are • Dialysis • Electro-dialysis • Reverse osmosis Driving forces that cause mass transfer of solutes are: • Difference in concentration (dialysis) • Difference in electric potential (electro-dialysis) • Difference in pressure (reverse osmosis)
  167. 167. Dialysis Theory • Dialysis depends on separating solutes of different ionic or molecular size in a solution by means of a selectively permeable membrane. • The driving force for dialysis is the difference in the solute concentration across the membrane The mass transfer of solute through the membrane is given by: • M = mass transferred per unit time (gram/hour) • K = mass transfer coefficient [gram/(hr-cm2)(gram/cm3)] • A = membrane area (cm2) • ΔC = difference in concentration of solute passing through the membrane (gram/cm3) • In environmental engineering, Dialysis is not used to an appreciable extent. • In industrial applications, Dialysis can be used to recover Sodium Hydroxide from textile wastewater. • Dialysis is limited to small flows due to small mass transfer coefficient (K)
  168. 168. Electrodialysis (ED) • ED is used to transport salt ions from one solution through ion exchange membranes to another solution under the influence of an applied electric potential difference. • The cell consists of a feed (diluate) compartment and a concentrate (brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes • multiple electrodialysis cells are arranged into a configuration called an electrodialysis stack, with alternating anion and cation exchange membranes forming the multiple electrodialysis cells • dissolved species are moved away from the feed stream rather than the reverse
  169. 169. Method In an electrodialysis stack, the diluate (D) feed stream, brine or concentrate (C) stream, and electrode (E) stream are allowed to flow through the appropriate cell compartments formed by the ion exchange membranes
  170. 170. Anode and Cathode Reactions At the cathode 2e- + 2 H2O → H2 (g) + 2 OH- At the anode H2O → 2 H+ + ½ O2 (g) + 2e- or 2 Cl- → Cl2 (g) + 2e- • Small amounts of hydrogen gas are generated at the cathode and small amounts of either oxygen or chlorine gas (depending on composition of the E stream and end ion exchange membrane arrangement) at the anode. • These gases are combined to maintain a neutral pH and discharged or recirculated to a separate E tank. • hydrogen gas may be collected for use in energy production. • ED systems can be operated as continuous or batch production processes • Continuous process, feed is passed through a sufficient number of stacks placed in series to produce the final desired product quality. • Batch processes, the diluate and/or concentrate streams are re-circulated through the ED systems until the final product or concentrate quality is achieved
  171. 171. Electrodialysis Reversal • An improvement to ED, referred to as Electrodialysis Reversal (EDR) • EDR utilizes the same concept of ED except that there is periodic automatic reversal of polarity and cell function to reverse the flow of ions across the membrane. • This returns anions across the anionic membranes and helps break up scale formed on the concentrating face of the membranes and minimizes membrane fouling.
  172. 172. Application Some applications of electrodialysis include: • Large scale brackish and seawater desalination and salt production or Small and medium scale drinking water production • Water reuse (e.g., industrial laundry wastewater, produced water from oil/gas production, cooling tower makeup & blowdown, metals industry fluids. • Pre-demineralization (e.g., boiler makeup & pretreatment, ultrapure water pretreatment, process water desalination, power generation, semiconductor, chemical manufacturing, food and beverage) • Food processing • Agricultural water • Glycol desalting (e.g., antifreeze / engine- coolants, capacitor electrolyte fluids, oil and gas dehydration, conditioning and processing solutions, industrial heat transfer fluids, secondary coolants from heating, venting, and air conditioning. • Glycerin Purification
  173. 173. Example Treatment Train • The conventional EDR treatment train typically includes raw water pumps, debris screens, rapid mix, slow mix flocculator, basin or clarifier, gravity filters, EDR membranes, chlorine disinfection, and clearwell storage. • Microfiltration (MF) could be used in place of flocculation, sedimentation, and filtration.
  174. 174. Benefits • ED and EDR can operate with minimal fouling or scaling, or chemical addition. • Low pressure requirements. • ED and EDR facilities are quieter than RO. • Long membrane life expectancy. • Unaffected by non-ionic sealants such as silica1. • Low chemical usage for pretreatment1. • Ability to treat feed water with higher SDI, TOC and silica concentrations, and more turbidity than RO. • Can operate with up to 0.5 ppm of free chlorine in the feed water to control the biological matter in the feed water.
  175. 175. Maintenance • ED membranes are durable, can run under a wide range of pH conditions (pH 2 – 11), and endure high temperatures during cleaning. • They can be removed from the unit and scrubbed if necessary. • If operated properly, membranes have an average life of 12 to 15 years. • Solids can be flushed out by turning the power off and letting water circulate through the stack. • The ED stack must be disassembled, mechanically cleaned, and reassembled at regular intervals. • They can also be cleaned using a 5% hydrochloric acid solution

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