Areas that have no or limited surface water or groundwater may choose to desalinate seawater or brackish water to obtain drinking water. Reverse osmosis is the most common method of desalination, Sea Water Reverse Osmosis (SWRO) is a reverse osmosis desalination membrane process that has been commercially used since the early 1970s. Because no heating or phase changes are needed, energy requirements are low in comparison to other processes of desalination, though still much higher than other forms of water supply (including reverse osmosis treatment of wastewater).
The typical single pass SWRO system consists of the following components:
- High-pressure pump
- Membrane assembly
- Remineralization and pH adjustment
Deionized water, also known as demineralized water (DI water or de-ionized water; can also be spelled deionised water), is water that has had its mineral ions removed, such as cations from sodium, calcium, iron, copper and anions such as chloride and bromide. Deionization is a physical process which uses specially-manufactured ion exchange resins which bind to and filter out the mineral salts from water. Because the majority of water impurities are dissolved salts, deionization produces a high purity water that is generally similar to distilled water, and this process is quick and without scale buildup. However, deionization does not significantly remove uncharged organic molecules, viruses or bacteria, except by incidental trapping in the resin. Specially made strong base anion resins can remove Gram-negative bacteria. Deionization can be done continuously and inexpensively using electrodeionization.
Deionization does not remove the hydroxide or hydronium ions from water. These are the products of the self-ionization of water to equilibrium, so removing them would lead to the removal of the water itself.
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Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic ‘ion exchangers’.
Ion exchange resin beads.
Typical ion exchangers are ion exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay, and soil humus. Ion exchangers are either cation exchangers that exchange positively charged ions (cations) or anion exchangers that exchange negatively charged ions (anions). There are also amphoteric exchangers that are able to exchange both cations and anions simultaneously. However, the simultaneous exchange of cations and anions can be more efficiently performed in mixed beds that contain a mixture of anion and cation exchange resins, or passing the treated solution through several different ion exchange materials.
Ion exchangers can be unselective or have binding preferences for certain ions or classes of ions, depending on their chemical structure. This can be dependent on the size of the ions, their charge, or their structure. Typical examples of ions that can bind to ion exchangers are:
H+ (proton) and OH- (hydroxide)
Single charged monoatomic ions like Na+, K+, or Cl-
Double charged monoatomic ions like Ca2+ or Mg2+
Polyatomic inorganic ions like SO42- or PO43-
Organic bases, usually molecules containing the amino functional group -NR2H+
Organic acids, often molecules containing -COO- (carboxylic acid) functional groups
Biomolecules which can be ionized: amino acids, peptides, proteins, etc.
Ion exchange is a reversible process and the ion exchanger can be regenerated or loaded with desirable ions by washing with an excess of these ions.
Ion exchange is widely used in the food & beverage, hydrometallurgical, metals finishing, chemical & petrochemical, pharmaceutical, sugar & sweeteners, ground & potable water, nuclear, softening & industrial water, semiconductor, power, and a host of other industries.
Most typical example of application is preparation of high purity water for power engineering, electronic and nuclear industries; i.e. polymeric or mineralic insoluble ion exchangers are widely used for water softening, water purification, water decontamination, etc.
Ion exchange is a method widely used in household (laundry detergents and water filters) to produce soft water. This is accomplished by exchanging calcium Ca2+ and magnesium Mg2+ cations against Na+ or H+ cations (see water softening).
Industrial and analytical ion exchange chromatography is another area to be mentioned. Ion exchange chromatography is a chromatographical method that is widely used for chemical analysis and separation of ions. For example, in biochemistry it is widely used to separate charged molecules such as proteins. An important area of the application is extraction and purification of biologically produced substances such as proteins and amino acids (e.g. DNA and RNA).
Ion-exchange processes are used to separate and purify metals, including separating uranium from plutonium and other actinides, including thorium, and lanthanum,neodymium, ytterbium, samarium, lutetium, from each other and the other lanthanides. There are two series of rare earth metals, the lanthanides and the actinides, both of which families all have very similar chemical and physical properties. Ion-exchange used to be the only practical way to separate them in large quantities, until the advent of solvent extraction techniques which can be scaled up enormously.
A very important case is the PUREX process (plutonium-uranium extraction process) which is used to separate the plutonium and the uranium from the spent fuel products from a nuclear reactor, and to be able to dispose of the waste products. Then, the plutonium and uranium are available for making nuclear-energy materials, such as new reactor fuel and nuclear weapons.
The ion-exchange process is also used to separate other sets of very similar chemical elements, such as zirconium and hafnium, which incidentally is also very important for the nuclear industry. Zirconium is practically transparent to free neutrons, used in building reactors, but hafnium is a very strong absorber of neutrons, used in reactor control rods.
Ion exchangers are used in nuclear reprocessing and the treatment of radioactive waste.
Ion exchange resins in the form of thin membranes are used in chloralkali process, fuel cells and vanadium redox batteries. Ion exchange can also be used to remove hardness from water by exchanging calcium and magnesium ions for hydrogen and chlorine ions in an ion exchange column.
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Hydrogen Sulfide Removal
The presence of hydrogen sulfide in home drinking water supplies is not a health hazard, but is a common nuisance contaminant whose distinctive “rotten egg” odor makes water treatment desirable. Several treatment methods are available, and often hydrogen sulfide can be treated and removed using the same process and equipment used for iron and manganese removal.
Sources of Hydrogen Sulfide
Hydrogen sulfide is a gas formed by the decay of organic matter such as plant material. It is most commonly found in groundwater characterized by relatively low concentrations of dissolved oxygen and by a pH less than 6.0 (relatively acidic). In higher pH waters other forms of sulfur may be present (sulfide or bisulfide). Surface waters are typically less likely to contain hydrogen sulfide since flowing waters are aerated naturally, which promotes an oxidation reaction. The hydrogen sulfide either escapes as a gas or is precipitated as a solid.
Harmless sulfur bacteria are also found in many private water supplies and distribution systems. These bacteria feed off natural sulfur compounds in water, producing hydrogen sulfide as a result. Sulfur bacteria are not a risk to human health, but their presence in drinking water can be a source of unpleasant tastes and odors.
Sometimes hydrogen sulfide may only be present in the household hot water. This condition is caused by a biochemical reaction between sulphates in the water, sulphate-reducing bacteria, a magnesium rod in the hot water heater or organic matter in the water. If the odor problem in the water heater is caused by heat-loving sulphate-reducing bacteria, disinfect the water heater with chlorine bleach or hydrogen peroxide. Sometimes the reaction with the magnesium rod is the cause of odor problems. The purpose of the magnesium rod is to prevent corrosion of the water heater. Removing the magnesium rod will often prevent the odor problem, but will void the warranty and lead to the possible earlier deterioration of the tank. If corrosion is a concern, the magnesium rod can be replaced with a zinc or aluminum rod.
Common Treatment Options
Most methods for treating sulfur water rely on the oxidation of hydrogen sulfide gas into elemental sulfur, a solid. Oxidation is the process by which soluble or dissolved contaminants are converted to soluble byproducts or insoluble products that can be filtered. This process changes the chemical and physical properties of the reactants. Hydrogen sulfide can be oxidized by several methods. If concentrations exceed 6.0 mg/l, chemical oxidation such as chlorination is recommended. If concentrations do not exceed 6.0 mg/l and water pH is above 6.8, an oxidizing filter such as manganese greensand can be used.
Continuous chlorination is a widely used and effective method for oxidizing hydrogen sulfide, especially if the water pH is 6.0-8.0. Chlorine is usually administered as sodium hypochlorite, which reacts with sulfide, hydrogen sulfide, and bisulfide to form compounds that do not cause foul taste or odors in drinking water.
The amount of hypochlorite to be used depends on the concentration of hydrogen sulfide in the water supply, however a recommended dosage is 2.0 mg/l chlorine for every 1.0 mg/l hydrogen sulfide. Chlorine should be added into the system ahead of the mixing tank, and sufficient storage must be provided to allow the water to be in contact with the chlorine for twenty minutes. Treated water may have lingering tastes or odors caused by the formation of certain harmless by-products or residual chlorine. After the required contact time, therefore, the water should be passed through an activated carbon filter to remove final suspended sulfur or excess chlorine.
Chlorination systems are available as a pellet-drop unit or a liquid-chemical feed. The pellet-drop system automatically dispenses a measured amount of chlorine down the well casing or into the retention tank during the pumping cycle. The chemical feed system features a liquid feeder connected to the well pump.
Another common treatment for sulfur water is aeration. Hydrogen sulfide is physically removed by agitating the water via bubbling or cascading and then separating or “stripping” the hydrogen sulfide in a container. The undesired hydrogen sulfide is removed as a volatile gas by venting it into a waste pipe or to the outdoors. Aeration is most effective when hydrogen sulfide concentrations 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. Most household water supplies contain less than 10 mg/l of sulfur, in which case an aeration tank about the same size as the filter tank (10″ x 54″) works fine. When sulfur levels exceed 10 mg/l, larger aeration tanks, repressurization systems, chlorination systems, or a combination may be needed.
Aeration is not always practical for home water treatment, especially if hydrogen sulfide concentrations exceed 10 mg/l, because it requires very acidic conditions (pH 4.0-5.0), long contact times for the air and water to mix, and usually large space requirements. In addition, treated water may need to be repressurized for distribution within the house and objectionable odors must be removed by venting the gas outside.
Manganese Greensand Filter
Manganese greensand is another common treatment method for removing sulfur from drinking water. It is usually recommended for water that contains less than 6.0 mg/l hydrogen sulfide. A manganese greensand filter has a special coating that oxidizes hydrogen sulfide gas to solid sulfur particles, which are filtered. When all of the manganese oxide is consumed, the greensand is regenerated with potassium permanganate. Potassium permanganate is a purple oxidizing chemical that is added to the untreated water to maintain the manganese greensand filter. To work properly, the greensand must be regenerated at periodic intervals based on water usage and contaminant concentration. When greensand is used to treat high concentrations of hydrogen sulfide, more frequent regeneration is required.
Catalytic Carbon provides another alternative to chemical treatment. Essentially, catalytic carbon is activated carbon with a modified carbon surface. Activated carbon is typically associated with adsorption, a physical process in which dissolved molecules adhere to the surface of the carbon filter. When used alone, activated carbon filtration removes very small amounts of hydrogen sulfide, generally concentrations below 0.3 mg/l. Activated carbon, however, has a limited capacity to adsorb hydrogen sulfide. Once the filter is saturated, the activated carbon must be replaced, not regenerated. As a result, activated carbon is not effective for removing moderate or high concentrations of hydrogen sulfide in drinking water.
Catalytic carbon retains all of the adsorptive properties of conventional activated carbon, but it combines them with the ability to promote or catalyze chemical reactions. During the treatment process, catalytic carbon first adsorbs sulfides onto the carbon surface. Then, in the presence of dissolved oxygen, it oxidizes the sulfides and converts them to nonobjectionable compounds. In this capacity, catalytic carbon is similar to manganese greensand and chlorination systems that remove sulfides through oxidation. It differs in that it maintains consistent catalytic activity (oxidation) that can be controlled and enhanced to treat sulfur water without use of chemical additives.