Many people have heard about softening hard water and are trying to order a softener for their water treatment. Is this so important and necessary?
The physiological norm of hardness is specified in SanPiN 2.1.4.1116-02 for bottled water and is from 1.5 to 3.5 mmol/l. Household appliances require even softer water to prevent scale formation.
There are two types of hardness:
Carbonate (temporary)- called because it is eliminated by boiling.
Non-carbonate (permanent)- called because when boiling, hardness is not eliminated, but when evaporated, a light white, slightly soluble precipitate such as calcium or magnesium sulfate forms in the form of scale on the walls of the vessel. Salts MgCl2, CaCl2, MgSO4 contained in water with constant hardness cause corrosion of steel structures and accelerate the wear and tear of water heating and heating equipment. When hard water is used for water heating equipment and heating equipment, scale is formed from calcium and magnesium carbonates, gypsum and other salts. The formation of scale makes it difficult to heat water and causes an increase in electricity and fuel consumption.
In hard water, meat, vegetables, and cereals do not cook well, and tea does not brew well. When washing fabrics (as when washing your hair), the insoluble compounds formed are deposited on the surface of the threads and gradually destroy the fibers.
Water softening is the process of removing hardness cations from it, i.e. calcium and magnesium.
Thermal method is based on heating water to a temperature above its boiling point, distilling it or freezing it in order to eliminate calcium carbonate and magnesium carbonate. Due to the use of this method, the residual hardness of water is no more than 0.7 mmol/l. Therefore, the thermal method is used for technical needs, in particular when using water used to feed low-pressure boilers, as well as in combination with reagent methods.
When softening water reagent methods they use reagents that, when interacting with calcium and magnesium, form poorly soluble compounds with their subsequent separation in illuminators, thin-layer sedimentation tanks and lighting filters. Lime, soda ash, sodium and barium hydroxides and other substances are used as precipitating reagents. The choice of reagents depends on the quality of the source water and the conditions of its further use. When using reagent methods, the residual hardness of water will be up to 0.7 mg/l. In accordance with the recommendations of the “Building Codes and Rules” (SN and P), reagent methods are mainly used to soften surface water, when water clarification is also required.
Water softening based on different diffusion rates of these substances through a semi-permeable membrane, separating concentrated and dilute solutions. Water softening by dialysis is carried out in membrane devices with nitro- and cellulose acetate film membranes. As a result of using this method, the residual water hardness will be up to 0.01 mg/l and lower. The negative side of the dialysis method is the high cost of membrane devices.
Magnetic water treatment- Commonly used to combat scale formation. The essence of the method is that when water crosses magnetic lines of force, scale formers are released not on the heating surface, but in the mass of water. The resulting loose sediments (sludge) are removed by blowing.
Received the greatest practical application ion exchange method water softening. The essence of the ion exchange method lies in the ability of ion exchange materials (ion exchangers) to absorb positive or negative ions from water in exchange for an equivalent amount of ion exchanger ions. Depending on the composition, there are mineral and organic cation exchangers, which, in turn, are divided into substances of natural and artificial origin. In water treatment technology, organic cation exchangers of artificial origin, so-called ion exchange resins, are widely used. The quality of ion exchange resins is characterized by their physical properties, chemical and thermal resistance, working capacity, etc. In water softening installations, it uses ion exchange resins based on the use of a cation exchange resin in the Na-form and an anion exchange resin in the Cl-form, i.e. uses the sodium-chlorine ionization method. This method consists of the following stages: sodium cationization and chlorine cationization. At the sodium cationization stage, calcium and magnesium ions, which give water hardness, are replaced by sodium ions.
As a result, the treated water is softened, and calcium and magnesium form an insoluble polymer. When sodium-cationized water is passed through a chlorine-anoion, exchange reactions of anions contained in Na-cationized water for chlorine ions occur and the alkalinity of the treated water decreases. To restore the properties of the ion exchange resin (regeneration), a solution of table salt is used. Thus, deep softening of water is achieved (up to 0.03 ... 0.05 mmol/l). When using the sodium-chlorine ionization method, only one reagent is consumed - table salt, no corrosion protection of equipment, pipelines and special fittings is required, the amount of equipment is reduced, and control of the operation and operation of the water softening unit is simplified. The result is increased reliability and reduced cost of the water softener. Just drink this softened one all the time
It is necessary to know the degree of hardness of the water used. Many aspects of our life depend on the hardness of drinking water: how much washing powder to use, whether measures are needed to soften hard water, how long aquarium fish will live in water, whether it is necessary to introduce polyphosphates in reverse osmosis, etc.
There are many ways to determine hardness:
- by the amount of detergent foam formed;
- by district;
- by the amount of scale on the heating elements;
- according to the taste properties of water;
- using reagents and special devices
What is hardness?
The main cations present in water are calcium, magnesium, manganese, iron, strontium. The last three cations have little effect on water hardness. There are also trivalent cation of aluminum and iron, which at a certain pH form limestone plaque.
Hardness can be of different types:
- overall hardness– total content of magnesium and calcium ions;
- carbonate hardness– content of hydrocarbonates and carbonates at a pH greater than 8.3. They are easy to remove by boiling: during heating they decompose into carbonic acid and sediment;
- non-carbonate hardness– calcium and magnesium salts of strong acids; cannot be removed by boiling.
There are several units of water hardness: mol/m 3, mg-eq/l, dH, d⁰, f⁰, ppm CaCO 3.
Why is water hard? Alkaline earth metal ions are found in all mineralized waters. They are taken from deposits of dolomite, gypsum and limestone. Water sources can have hardness in different ranges. There are several rigidity systems. Abroad they approach it more “harshly”. For example, in our country water is considered soft with a hardness of 0-4 mEq/l, and in the USA - 0-1.5 mEq/l; very hard water in Russia - over 12 mg-eq/l, and in the USA - over 6 mg-eq/l.
The hardness of low-mineralized waters is 80% due to calcium ions. With increasing mineralization, the proportion of calcium ions sharply decreases, and magnesium ions increases.
Most often, surface waters have less hardness than groundwater. The hardness also depends on the season: when the snow melts, it decreases.
The hardness of drinking water changes its taste. The sensitivity threshold for calcium ion is from 2 to 6 mEq/l, depending on the anions. The water becomes bitter and has a bad effect on the digestion process. WHO does not make any recommendations on water hardness, since there is no accurate evidence of its effect on the human body.
Limiting hardness is necessary for heating devices. For example, in boilers - up to 0.1 mEq/l. Soft water has low alkalinity and causes corrosion of water pipes. Utilities use special treatments to find a compromise between plaque and corrosion.
There are three groups of water softening methods:
- physical;
- chemical;
- psychic.
Reagent methods of water softening
Ion exchange
Chemical methods are based on ion exchange. The filter mass is an ion exchange resin. It consists of long molecules that are collected into yellow balls. Small processes containing sodium ions protrude from the balls.
During filtration, water permeates the entire resin, and its salts replace sodium. Sodium itself is carried away by water. Due to the difference in ion charges, 2 times more salts are washed out than deposited. Over time, the salts are replaced and the resin stops working. Each resin has its own operating period.
The ion exchange resin can be in cartridges or poured into a long barrel - a column. Cartridges are small in size and are used only to reduce the hardness of drinking water. Ideal for softening water at home. An ion exchange column is used to soften water in an apartment or small industry. In addition to the high cost, the column must be periodically loaded with recovered filter mass.
If there are no sodium ions left in the resin of the cartridge, then it is simply replaced with a new one, and the old one is thrown away. When using an ion exchange column, the resin is restored in a special tank with brine. To do this, dissolve the tableting salt. The saline solution regenerates the resin's ability to exchange ions.
The downside is the added ability of water to remove iron. It clogs the resin and renders it completely unusable. You should do a water analysis on time!
Use of other chemicals
There are a number of less popular, but effective ways to soften water:
- soda ash or lime;
- polyphosphates;
- antiscalants – compounds against scale formation.
Softening with lime and soda
Softening water with soda
The method of softening water using lime is called liming. Slaked lime is used. The carbonate content decreases.
A mixture of soda and lime is most effective. To demonstrate how to soften water at home, you can add soda ash to your washing water. Take 1-2 teaspoons per bucket. Stir well and wait for sediment to form. Women in Ancient Greece used a similar method using stove ash.
Water after lime and soda is not suitable for food purposes!
Softening with polyphosphates
Polyphosphates are capable of binding hardness salts. They are large white crystals. Water passes through the filter and dissolves polyphosphates, binding salts.
The disadvantage is the danger of polyphosphates for living organisms, including humans. They are a fertilizer: after entering the reservoir, active growth of algae is observed.
Polyphosphates are also unsuitable for softening drinking water!
Physical method of water softening
Physical methods combat the consequences of high hardness - scale. This is a reagent-free water purification. When using it, there is no reduction in salt concentration, but simply prevents harm to pipes and heating elements. The water becomes soft or, for greater understanding, softened.
The following physical methods are distinguished:
- use of magnetic field;
- using an electric field;
- ultrasonic treatment;
- thermal method;
- use of low-point current pulses.
A magnetic field
Reagent-free water softening using a magnetic field has many nuances. Efficiency is achieved only if certain rules are observed:
- a certain water flow rate;
- selected field strength;
- certain ionic and molecular composition of water;
- temperature of incoming and outgoing water;
- time of processing;
- Atmosphere pressure;
- water pressure, etc.
Changing any parameter requires a complete reconfiguration of the entire system. The response must be immediate. Despite the difficulty of controlling parameters, magnetic water softening is used in boiler rooms.
But softening water at home using a magnetic field is almost impossible. If you want to purchase a magnet for a pipeline, think about how you will select and ensure the necessary parameters.
Using Ultrasound
Ultrasound leads to cavitation - the formation of gas bubbles. The likelihood of a meeting of magnesium and calcium ions increases. Crystallization centers appear not on the surface of the pipes, but in the water column.
When softening hot water with ultrasound, the crystals do not reach the size necessary for sedimentation - scale does not form on the heat exchange surfaces.
Additionally, high-frequency vibrations occur, which prevent the formation of plaque: they repel crystals from the surface.
Bending vibrations are detrimental to the formed layer of scale. It begins to break off into pieces that can clog the channels. Before using ultrasound, surfaces must be descaled.
Electromagnetic pulses
Reagent-free electromagnetic pulse-based water softeners change the way salts crystallize. Dynamic electrical impulses with different characteristics are created. They go along a winding wire on a pipe. The crystals take the form of long shelves, which are difficult to attach to the heat exchange surface.
During the processing, carbon dioxide is released, which fights existing limescale and forms a protective film on metal surfaces.
Thermal softening
This is the first time someone hears about this method. But in fact, everyone has been using it since childhood. This is the boiling of water that is familiar to us.
Everyone has noticed that after boiling water, a precipitate of hardness salts forms. Coffee or tea is made from softer water than tap water.
How long does it take to boil? It's simple: with increasing temperature and its impact, hardness salts become less soluble and precipitate more. During the heating process, carbon dioxide is released. The faster it evaporates, the more limestone plaque forms. A tightly closed lid prevents the release of carbon dioxide, and in an open container the liquid quickly evaporates.
When using heat softening, leave the lid of the container slightly open. It is also necessary to ensure the maximum area of salt deposition to accelerate the softening of drinking water.
With a hardness of up to 4 mEq/l, thermal softening is not necessary: the salts will settle more slowly than the water evaporates. The remaining water will have an increased concentration of many impurities.
Water softening– the process of reducing rigidity. Water hardness is due to the presence of calcium and magnesium salts. To reduce water hardness, the following methods are used: reagent; cationite; electrodialysis; membrane technologies.
Reagent softening methods water is based on the conversion of calcium and magnesium ions into poorly soluble and easily removed compounds using chemicals. Of the reagent softening methods, the most common is the lime-soda method. Its essence is the conversion of Ca 2+ and Mg2+ salts into poorly soluble compounds CaCO 3 and Mg(OH) 2, which precipitate. With the lime-soda method, the process is carried out in two stages. Initially, organic impurities and a significant part of carbonate hardness are removed from water using aluminum or iron salts with lime. After this, soda is introduced. Deeper water softening can be achieved by heating it.
The soda-sodium method is used to soften water whose carbonate hardness is slightly higher than non-carbonate hardness.
The barium method of water softening is used in combination with other methods. First, barium containing reagents (Ba(OH) 2, BaCO 3, BaAl 2 O 4) is introduced to eliminate sulfate hardness, then after clarification, the water is treated with lime and soda for additional softening. Due to the high cost of reagents, this method is used very rarely.
Phosphating is used for additional softening of water after reagent softening using the lime-soda method, which allows obtaining a residual hardness of 0.02−0.03 mEq/l. Such deep purification allows in some cases not to resort to cation exchange softening. Phosphate softening is usually carried out by heating water to 105−150 ◦ C. Due to the high cost of trisodium phosphate, the phosphate method is used to soften water that has been previously softened with lime and soda.
Cationite method is based on the ability of ion-exchange materials to exchange calcium and magnesium cations present in water for exchangeable sodium or hydrogen cations. Organic cation exchangers of artificial origin are used as cation exchangers. The cation exchange method allows you to achieve deep softening of water.
The N-cation exchange method is used to soften water with a suspended content of no more than 8 mg/l and a color value of no more than 30 degrees. Water hardness decreases with single-stage Na-cationization to 0.05....1, with two-stage - to 0.01 mg - eq/l. The process of Na-cationization is described by the following exchange reactions:
2Na[K] + Ca (HCO 3) ↔ Ca[K] + 2NaHCO 3,
where [K] is the insoluble polymer matrix.
After the working exchange capacity of the cation exchanger is depleted, it loses its ability to soften water and must be regenerated. The process of water softening using cation exchanger filters consists of the following sequential operations: filtering water through a layer of cation exchanger until the maximum permissible hardness in the filtrate is reached; loosening the cation exchanger layer with an ascending flow of softened water; lowering the water cushion to avoid dilution of the regenerating solution; regeneration of the cation exchanger by filtering the appropriate solution; Washing the cation resin with unsoftened water.
The combination of processes has found the greatest practical application
H – Na – cationization, as a result of which the required alkalinity or acidity of water can be achieved. The process of H – Na cationization can be carried out according to the following schemes: parallel H – Na cationization, sequential H – Na – cationization and joint H – Na – cationization.
Electrodialysis is a method for separating solutes that differ significantly in molecular weight. It is based on different rates of diffusion of these substances through a semi-permeable membrane separating concentrated and dilute solutions. Dialysis is carried out in membrane devices with nitro- and cellulose acetate film membranes.
Desalination and desalination of water. Existing methods of desalination and desalination of water are divided into two groups: with and without changes in the aggregative state of water. The first group of methods includes distillation, freezing, and gas hydrate method; to the second group - ion exchange, electrodialysis, reverse osmosis, hyperfiltration.
Distillation method based on the ability of water to evaporate when heated and disintegrate into fresh steam and salty brine. When salt water is heated to a temperature higher than its boiling point, the water begins to boil. The resulting steam at a pressure of less than 50 kg/cm 2 is practically incapable of dissolving the salts contained in the desalinated water, so when it condenses, fresh water is obtained.
Ion exchange method desalination and desalination is based on sequential filtration of water through H - cation-exchange and OH - - anion-exchange filters. Water containing NaCl is desalted according to the following schemes:
H[K] + NaCl ↔ Na[K] +HCl.
OH[A] +HCl ↔ Cl[A] + H 2 O
Ion exchange units are supplied with water containing salts up to 3.0 g/l, sulfates and chlorides - up to 5 mg/l, suspended substances - no more than 8 mg/l and having a color of no higher than 30 degrees and permanganate oxidation up to 7 mgO 2 / l.
In accordance with the required depth of water desalination, one-, two- and three-stage installations are used.
In single-stage ion-exchange resin installations, water is sequentially passed through a group of filters with a strongly acidic H - cation exchanger, and then through a group of filters with a weakly basic anion exchanger: free carbon dioxide is removed in a degasser, which is installed after the cation-exchange or anion exchanger filters. Each group must have at least two filters.
Ion exchange installations with a two-stage water desalination scheme consist of first-stage H-cation-exchange and anion-exchange filters (with a weakly basic anion exchanger), a degasser for removing free carbon dioxide, and second-stage H-cation-exchange and anion-exchange filters (with a strong-base anion exchanger). Anion exchange filters of the first stage retain the anions of strong acids, and the anions of weak acids (organic acids and silicic acid) of the second stage.
In installations with a three-stage scheme, at the third stage a filter with a mixed loading of cation exchanger and anion exchanger is used, or H - cation exchanger filters of the third stage, followed by third-stage anion exchanger filters with a strong base anion exchanger.
Electrodialysis is the process of removing solute ions from a solution by selectively transferring them through membranes selective for these ions in a direct electric current field.
When a constant electric field is applied to a solution of ionized substances (electrolytes), a directed movement of ions of dissolved salts, as well as H + and OH - ions occurs. Moreover, cations move to the cathode, and anions move to the anode. If the solution is divided into sections using special membranes that are permeable only to cations or only to anions, then the cations, moving towards the cathode, will freely pass through the cation exchange membrane. It is practically impermeable to anions. Anions, having passed through the anion exchange membrane, will move towards the anode. Thus, the solution will be divided into demineralized water located between the membranes and concentrated brines - alkaline catholyte and acidic anolyte.
Currently, multi-chamber flat-dimensional devices are used for water desalination.
The scope of application of electrodialysis is limited to the salt content of solutions of 0.5 - 10 g/l, since at lower concentrations the conductivity of solutions decreases and the efficiency of energy use decreases, and at higher concentrations the process becomes economically unprofitable due to a significant increase in energy costs, since the energy expended is proportional to the amount of removed ions.
Water desalination hyperfiltration consists of filtering salt water through special semi-permeable membranes that allow water to pass through and retain the ions of salts dissolved in it. In this case, it is necessary to create excess pressure to filter water through the membrane.
Deferrization of water. In natural water, especially in water from underground sources, dissolved iron and often manganese are found in large quantities. The norm for iron in drinking water according to SanPiN 2.1.4.1074 - 01 is 0.3 mg/l and 0.1 mg/l for manganese.
Iron is found in water in the form:
Ferrous iron – in the form of dissolved Fe 2+ ions;
Trivalent;
Organic iron (in the form of soluble complexes with natural organic acids (humates));
Bacterial iron is a waste product of iron bacteria (iron is in the shell).
Groundwater contains mainly dissolved divalent iron in the form of Fe 2+ ions. Ferric iron appears after contact of such water with air and in worn-out water distribution systems when water comes into contact with the surface of pipes.
In surface waters, iron is in the trivalent state and is also part of organic complexes and iron bacteria. If only trivalent iron is present in the water in the form of suspension, then simple settling or filtration is sufficient.
To remove ferrous iron and manganese, they are first converted into an insoluble form by oxidizing them with atmospheric oxygen, chlorine, ozone or potassium permanganate, followed by filtration through a mechanical filter with sand, anthracite or gravel loading. The process of oxidation and formation of flakes is quite long.
2 Fe 2+ +O 2 +2H + =2 Fe 3+ +2OH -
Fe 3+ +OH -= Fe(OH) 3 ↓.
Fundamentally new products that have appeared recently are catalytic charges, which allow deferrization and demanganization to be carried out with high efficiency. Such loadings include Birm, pyrolusite, magnetite, Manganese Greensand, MZ−10 and MTM. These natural materials contain manganese permanganate and filtration through these loads oxidizes iron and manganese, converting them into insoluble hydroxide, which is deposited on the load. The film of manganese oxides is consumed for the oxidation of iron and manganese, and therefore it must be restored. To do this, the load is periodically treated with a solution of potassium permanganate or it is dosed into the water using a proportional dosing system before it enters the filter.
Fluoridation and defluoridation of water. A lack of fluoride in water, as well as its excess, has a negative impact on human health. The optimal fluorine content in water is 0.7 - 1.5 mg/l.
Defluoridation of water is carried out using the following methods: reagent, filtration through fluorine-selective materials, which include: activated aluminum oxide; phosphate-containing sorbents; magnesium sorbents (magnesium oxyfluorides); activated carbons; aluminum-modified materials.
In the reagent method of water defluoridation, the following reagents are used: aluminum sulfate, aluminum polyoxychlorides.
Water deodorization. The odors and tastes of water are caused by the presence in it of microorganisms, some inorganic (hydrogen sulfide and iron) and organic substances. Sometimes the organoleptic properties of water deteriorate due to an overdose of reagents or due to improper operation of water treatment facilities. There are no universal deodorization methods, but the use of some of them in combination provides the required degree of cleaning. If substances that cause unpleasant tastes and odors are in a suspended and colloidal state, then their coagulation gives good results. Tastes and odors caused by inorganic substances that are in a dissolved state are removed by degassing, deferrization, and desalting. Odors and tastes caused by organic substances are very persistent. They are extracted by oxidation and sorption. To eliminate odors and tastes caused by microorganisms in water, oxidation is used, followed by sorption of substances. Odors and tastes of natural water can be eliminated together with chlorination or ozonation, as well as oxidation with potassium permanganate. The action of oxidizing agents is effective only against a limited number of contaminants. The disadvantage of the oxidative method is the need to dose the oxidizing agent.
Water preparation in circulating cooling systems. The circulating systems of industrial enterprises are provided with cooling water, which is pumped from an artificial cooler, where the water transfers heat to the air. In recirculating systems, water is cooled in cooling towers, spray ponds, and cooling ponds.
Water circulating in the recirculating cooling system is subjected to physical and chemical influences: evaporation, heating, cooling, aeration, repeated contact with the cooled surface, as a result of which its composition changes. Especially often, the normal operation of circulation systems is disrupted as a result of the appearance of scale, biological fouling, and corrosion of metal elements of the systems on the walls of heat exchangers. Deposits on the walls of apparatus and pipes also cause an increase in pressure losses when water moves through them, deterioration of heat transfer conditions and a decrease in cooling water flow rates, which leads to a decrease in the cooling effect and disruption of the technological operating conditions of heat exchangers. Water losses due to evaporation and splashing are compensated by additional water from the source.
Water loss due to evaporation Q 1 is determined by the formula:
Q 1 =k 1 ∆tQ o ,
where k 1 is a coefficient depending on the air temperature; ∆t – temperature difference before and after cooling; Q o – flow rate of cooled water, m 3 /h.
Water losses from the system due to splashing Q 2 depend on the type, design and size of the cooler and are determined by the formula:
where k 2 is the coefficient of water loss due to splashing.
The need to treat cooling water to combat scale deposits arises in circulating water supply systems. The main compound found in scale in cooling systems is calcium carbonate CaCO 3 . To prevent the formation of calcium carbonate, the following water treatment methods are used:
1. Refreshment of circulating water, i.e. continuous addition of fresh water with lower carbonate hardness to the system and discharge (blowdown) of part of the waste water.
2. Introduction of phosphates into the additional water, which inhibit the crystallization process of CaCO 3 .
3. Acidification of water. In this case, the carbonate hardness of fresh water turns into non-carbonate water, the salts of which do not precipitate, which leads to a decrease in pH and an increase in the concentration of free carbon dioxide CO 2.
4. Softening water in order to reduce the content of Ca 2+ and Mg 2+ ions, which in the form of insoluble salts are removed from the water by settling during liming or as a result of cationization.
5. Recarbonization of circulating water – compensation of losses of equilibrium carbon dioxide.
6. Magnetic acoustic treatment of water.
To combat the development of biological fouling in circulating systems, water treatment with chlorine and copper sulfate is most widely used.
Cooling systems of heat exchangers are subject to processes of electrochemical and biological corrosion. Preventing the corrosive effects of water can be achieved in one of the following ways:
1. Application of protective coatings to metal surfaces washed with water.
2. Removal of correlating agents (oxygen, hydrogen sulfide, free carbon dioxide) from water.
3. Applying a carbonate, silicate or phosphate film to the internal surfaces of pipes.
Softening water means removing calcium and magnesium from it. The total hardness of water supplied by water pipes for household and drinking needs should not exceed 7 mEq/dm3, and in special cases, in agreement with the sanitary and epidemiological service authorities, no more than 10 mEq/dm3. The hardness level of steam generator feed water can reach 0.05 mEq/dm3. Depending on the quality of the source water and the desired effect of reducing hardness, reagent, thermochemical, ion exchange softening methods or various combinations of them are used.
Reagent softening. Reagent methods are based on the ability of Ca2+ and Mg2+ cations to form insoluble and slightly soluble compounds when treating water with reagents. The most commonly used reagents are lime and soda.
Decarbonization of water only by liming is used in cases where a simultaneous reduction in water hardness and alkalinity is required.
Lime, together with soda, is used to soften water, which contains calcium and magnesium in combination with anions of strong acids.
The theoretical limit of water softening is determined by the solubility of calcium carbonate and magnesium hydroxide. The solubility of calcium carbonate in a monosolution at a temperature of 0°C is 0.15 mEq/dm3, and at a temperature of 80°C - 0.03 mEq/dm3; for magnesium hydroxide - 0.4 and 0.2 mEq/dm3, respectively.
Both CaCO3 and Mg(OH)2 have the ability to form supersaturated solutions, which only very slowly approach an equilibrium state even when in contact with the solid phase of the resulting precipitate. In practice, it is not advisable to keep water in water softeners for a long time until an equilibrium state occurs. Therefore, water softened by liming (if the hardness is all carbonate) or the lime-soda method usually has a residual hardness of at least 0.5-1 mEq/dm3.
The depth of softening depends on the presence in the treated water of an excess of precipitated ions and precipitating reagents. So, at 40°C, the salt content of water is up to 800 mg/dm3, the presence of Ca2+ ions in it in an amount of 0.7-1.0; 1-3 and > 3 mEq/dm3, residual carbonate hardness in the absence of crystallization retarders usually does not exceed 0.5-0.8; 0.6-0.7 and 0.5-0.6 mg-eq/dm3, respectively, and< 1,2; Щгидр < 0,4 и Жо6щ < 1,0 мг-экв/дм3. При солесодержании 800-2000 мг/дм3 Щ0бЩ = 2,0-2,2 мг-экв/дм3, Щгидр < 0,5-0,8 мг-экв/дм3 и Жобщ < 2,0 мг-экв/дм3. Здесь в подстрочнике «общ» и «гидр» обозначают соответственно «общая» и «гидратная».
It should be noted that water softened by liming or the lime-soda method is usually supersaturated with calcium carbonate and has a very high pH. Therefore, to increase the accuracy of reagent dosing, it is necessary, in addition to automatic control in proportion to the flow rate of the treated water, to adjust the dose also according to pH. It is also possible to adjust the dose depending on the electrical conductivity of the treated water, if the content of SO^, SG and NO3 is stable and low. With small fluctuations in the dosage of lime, Mg2+ plays a buffering role: with an increase in the dosage of lime, the amount of Mg2+ transferred to the sediment increases (thereby worsening its properties), while maintaining the alkalinity of the softened water at an approximately constant level.
The softening process is controlled by the pH value, which should be > 10 due to the need to remove Mg2+ from water, or, less accurately, by the value of hydrate alkalinity, calculated based on titration of water samples with acid in the presence of phenolphthalein and methyl orange indicators.
It should be noted that the process of reagent water softening can be monitored by its electrical conductivity. When lime is added to water and bicarbonates transform into carbonates that precipitate, the electrical conductivity of the treated water changes. In accordance with the conductometric titration curve, at the moment of complete neutralization of carbonate hardness salts, the electrical conductivity reaches a minimum value. With a further increase in reagent additions, the electrical conductivity increases due to the excess of the reagent. Thus, the optimal dose of lime milk introduced into softened water is characterized by the minimum value of the water’s electrical conductivity.
With increasing water temperature, chemical reactions and crystallization of CaCO3 and Mg(OH)2 sediments accelerate. Temperature fluctuations worsen deposition conditions.
Coagulation improves the precipitation of CaCO3 + Mg(OH)2. Due to the high pH of the softener, only iron-based coagulants and sodium aluminate are used. For 1 mole of FeS04, 4 mg of 02 is required in water.
Air entering the clarifier leads to sedimentation and removal of sediment with the softened water. The supersaturation of water with air can be determined by determining the oxygen content in the water after the air separator using an iodometric method and comparing the results obtained with the tabulated ones for given temperatures.
Thermochemical softening consists of heating water above 100°C and using lime and soda, less often sodium hydroxide and soda. As a result of thermochemical softening, calcium hardness can be reduced to 0.2 mEq/dm3, and magnesium hardness to 0.1 mEq/dm3. The thermochemical method is often combined with phosphate softening of water. Di- or trisodium phosphate is used as phosphate reagents. As a result of phosphate softening, it is possible to obtain water with a residual hardness of 0.04-0.05 mEq/dm3.
Sulfate hardness is removed with barium carbonate, hydroxide or barium aluminate.
Appropriate analytical controls are required to ensure that the water softening processes described above are carried out correctly. Recommended tests and frequency of their performance are given in table. 1.7.
The following rules can serve as a useful guide to ensure a good softening effect: 1) hydrate alkalinity should exceed magnesian hardness by approximately 0.4 mEq/dm3 in an unheated process and by 0.2 mEq/dm3 in a heated process; 2) carbonate alkalinity should exceed calcium hardness by approximately 1.2 mEq/dm3 in an unheated process and by approximately 0.8 mEq/dm3 in a heated process.
Since some poorly soluble salts may precipitate during long-term storage, and NaOH turns into Na2C03, you should not use data from average samples of softened water.
Also, due to the presence of leaks of the CaCO3 and Mg(OH)2 suspension into softened water, it must be additionally filtered through crushed anthracite. In this case, quartz sand is an undesirable material due to the fact that it can enrich the water with silicic acid compounds.
Ionite softening. It is carried out mainly using Na+-, H+- and NHj-forms.
In the process of water softening by Na-cation, the calcium and magnesium content in water can be reduced to very small values. The total alkalinity will not change, the dry residue increases slightly as a result of replacing one calcium ion in water, having a molecular weight of 40.08, with two sodium ions (mass 2 x 22.99 = 45.98).
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When filtering through a cation exchange resin in the H-form, all cations of dissolved salts (including cations of hardness salts) will be sorbed on its grains; an equivalent amount of H+ ions will pass into the water; salts dissolved in water will be converted into the corresponding acids. The acidity of water passing through an H-cation exchanger filter, which is loaded with a strong basic cation exchanger, will be equal to the sum of the concentrations of strong acid salts in the source water.
Regeneration of H-cation exchanger filters with acid in an amount insufficient to completely displace hardness cations from the cation exchanger (“hungry” regeneration), allows in the operating cycle to reduce the alkalinity of water to 0.4-0.5 mEq/dm3, without reducing its non-carbonate hardness .
If the presence of sodium and potassium carbonates is not allowed in softened water, but the presence of ammonium ions is allowed, then instead of H-Na cationization, NH4-Na-Ka ionization can be used.
Water softened by cationization turns out to be more corrosive than the original one, due to the complete absence of calcium bicarbonate in it, which, under certain conditions, can form a protective layer of calcium carbonate on the surface of the metal in contact with water.
When monitoring the quality of filtrate from cation exchange plants, special attention is paid to determining indicators that are in one way or another related to the concept of water hardness and alkalinity: total and carbonate hardness, carbonate and hydrate alkalinity, the content of calcium and magnesium salts, total salt content, pH value, anion content.
During the operation of cation exchangers, it is additionally necessary to periodically check the absorption or removal of organic substances from them by the filtrate.
Water desalination refers to the process of reducing the salts dissolved in it to the required value. A distinction is made between partial and complete desalting. A special case of water desalination is desalination, as a result of which the salt content in purified water does not exceed 1000 mg/dm3 - the maximum permissible concentration of all salts in drinking water.
The most common methods of water desalination include ion exchange, electrodialysis, reverse osmosis and distillation.
Desalting allows you to almost completely remove from water substances that can completely or partially dissociate (for example, salts and silicic acid); Non-electrolytes may remain in the water. Sometimes there is also a slight decrease in color associated with the absorption of acidic organic substances by ion exchangers and membranes. Since desalting removes those substances that conduct electrical substances, an indicator of the quality of treated water is usually its electrical conductivity, expressed in µS/cm. The calculated value of this parameter at 18°C in “ultrapure” water is 0.037 µS/cm. However, under production conditions it is still possible to obtain “ultrapure” water with a specific electrical conductivity of 0.1 - 1.0 µS/cm.
The main criterion for assessing the quality of water treatment and the ion-exchange capacity of filters is often taken to be the electrical conductivity of water, the threshold value of which is established based on experimental data. For example, the electrical conductivity of water after a cation exchanger should be less than 240, after a weakly basic anion exchanger - 50-220 and after a strongly basic anion exchanger< 20 мкСм/см. Превышение этих значений указывает на истощение ионообменных смол до контрольного уровня и на необходимость их регенерации.
Since existing drinking water quality standards mostly regulate the maximum permissible concentrations of macro- and microcomponents of its composition, desalinated water generally meets current regulatory requirements. However, in connection with the ever-expanding involvement of desalinated waters in centralized drinking water supply systems, there is a need for additional standardization of the minimum required concentrations of the most important hygienic quality indicators: calcium content, bicarbonates, total salt content, sodium, potassium, etc. As modern medical and physiological studies show research, insufficient content of hardness salts in desalinated water (less than 1.5 mEq/dm3) can lead to metabolic disorders and cardiovascular diseases in the body of people who drink such soft water for a long time.
Water softening- a process aimed at removing calcium and magnesium cations from it, i.e. reducing its rigidity.
According to the requirements of SANPiN, the hardness of drinking water should not exceed 7 mEq/l, and the requirements for deep softening are set for water participating in heat exchange processes, i.e. up to 0.05...0.01 mEq/l. The hardness of water used to feed drum boilers of thermal power plants should not exceed 0.005 mEq/l, or 5 mcg-eq/l.
Reducing the total concentration of Mg(II), Ca(II) cations and anions, with which under certain conditions they can form dense insoluble deposits on the walls of pipes and apparatus, occurs in water purification and water treatment systems using various methods, the choice of which is determined by the quality of the source water, the requirements to its purification and technical and economic considerations.
Ion exchange method.
This method is based on the ability of certain materials (cation exchangers and anion exchangers) to absorb ions (cations and anions) from water in exchange for an equivalent amount of ions (cations and anions).
The cationization process is the process in which cations are exchanged. In water treatment during softening - with cation exchanger cations for Ca 2+ and Mg 2+ ions from water.
Anionization process - respectively, with anions, mainly during desalting and deep desalting.
Magnetic water treatment.
The use of magnetic water treatment is advisable in the case of high calcium-carbonate hardness.
As water passes through a magnetic field, crystallization centers are formed in it, which enlarge and fall into non-stick sludge, which is removed during blowing. Those. The precipitation occurs not on the walls of the heating surface, but in the volume of water.
The anti-scale effect is influenced by factors such as the qualitative and quantitative composition of water, the speed of movement of the liquid through magnetic field lines, the strength of the magnetic field and the residence time of water in it.
The conditions for successful magnetic treatment of water must be a high content of calcium carbonate and sulfate, and the concentration of free carbon oxide IV must be less than the equilibrium one. The impurities of iron oxides and others contained in water also increase the anti-scale effect.
Magnetic water treatment devices operate both on the basis of permanent magnets and electromagnets. The disadvantage of devices with permanent magnets is that from time to time they have to be cleaned of ferromagnetic impurities. Electromagnets are cleaned of iron oxides by disconnecting them from the network.
The speed of water in a magnetic field during its processing should not exceed 1 m/s. To increase the volume of treated water per unit of time, devices with layer-by-layer magnetic processing are used.
The magnetic processing method has found application in hot water supply heating networks, at thermal power plants, and in heat exchangers.
The choice of this method when solving the problem of water softening should be mainly based on its effectiveness in purifying water of a given quality - used as the main, subsequent stage or as an additional one.
Reverse osmosis.
Currently, the most widely used method in water treatment is reverse osmosis.
The essence of the method is that under high pressure, from 10 to 25 atmospheres, water is supplied to the membranes. Membranes, being a selective material in relation to impurities passing through it, allow water molecules to pass through and do not allow ions dissolved in water to pass through.
Thus, at the output after the reverse osmosis installation we receive two streams - the first stream of pure water that has passed through the membrane, the so-called permeate, and the second stream of water with impurities that has not passed through the membrane, called the concentrate.
The permeate is sent to the consumer and makes up from 50 to 80% of the volume of supplied water. Its quantity depends on the properties of the membrane and its condition, the quality of the source water and the desired cleaning result. Most often it is about 70%.
Concentrate, respectively, from 50 to 20%.
When the load on the membrane increases, i.e. increasing the percentage ratio between the passed water and water with impurities, the selectivity of the membrane decreases and reaches a minimum in the absence of a concentrate, i.e. when all the water supplied to the reverse osmosis installation passes through the membrane.
Reverse osmosis membranes are made of a composite polymer material of a special structure, which allows water to pass through at high pressures and not allow ions and other impurities dissolved in it to pass through. As the load on the membrane increases, its service life is reduced, and when critical parameters are reached, at which the released liquid with impurities passes through the membrane completely, it is destroyed. The average service life of the membrane is 5 years.
Over time, the surface of the membrane can become overgrown with microorganisms and become covered with a layer of poorly soluble compounds. To clean reverse osmosis membranes, solutions of acids and alkalis with the addition of biocides are used.
When washing reverse osmosis, we must not forget that a semi-permeable membrane is not a filter. Flushing should be carried out exclusively in the direction of liquid movement. A reverse flow of water solution will cause the membrane to fail.
Reagent methods of water treatment.
Reagent methods of water treatment serve mainly for shallow softening of water by adding reagents and converting hardness salts into poorly soluble compounds with their subsequent precipitation.
Lime, soda, caustic soda, etc. are used as reagents. At the moment, they are used in few places, but for a general understanding of the processes of converting calcium and magnesium into poorly soluble compounds and their further precipitation, let’s consider them.
Reducing scale by liming.
The method is applicable to water with high carbonate and low non-carbonate hardness.
When lime milk is added, the pH of the water increases, which leads to the transition of dissolved carbon dioxide and bicarbonate ion to carbonate ion:
CO 2 + OH - = CO 3 2- + H 2 O,
HCO 3- + OH - = CO 3 2- + H 2 O.
When water is saturated with carbonate ions, calcium precipitates:
Ca 2+ + CO 3 2- = CaCO 3 ↓.
Also, with an increase in pH, magnesium also precipitates:
Mg 2+ + OH - = Mg(OH) 2 ↓.
If the excess of carbonate hardness is insignificant, then soda is dosed along with lime, whose presence reduces non-carbonate hardness:
CaSO 4 + Na 2 CO 3 = CaCO 3 ↓ + Na 2 SO 4.
For more complete precipitation of magnesium and calcium cations, it is recommended to heat the water to a temperature of 30 - 40 degrees. As it increases, the solubility of CaCO 3 and Mg(OH) 2 decreases. This makes it possible to reduce water hardness to 1 mEq/L or less.
Soda-sodium method of water softening.
Adding soda is necessary if the non-carbonate hardness is greater than the carbonate hardness. If these parameters are equal, adding soda may not be necessary at all.
Calcium and magnesium bicarbonates react with alkali to form poorly soluble calcium and magnesium compounds, soda, water and carbon dioxide:
Ca(HCO 3) 2 + 2NaOH = CaCO 3 ↓ + Na 2 CO 3 + 2H 2 O,
Mg(HCO 3) 2 + 2NaOH = Mg(OH) 2 ↓ + Na 2 CO 3 + H 2 O + CO 2.
The carbon dioxide formed as a result of the reaction of magnesium bicarbonate with alkali reacts again with the alkali to form soda and water:
CO 2 + NaOH = Na 2 CO 3 + H 2 O.
Non-carbonate hardness.
Calcium sulfate and chloride react with soda formed in the reactions of carbonate hardness and alkali and added soda to form calcium carbonate, which does not stick under alkaline conditions:
CaCl 2 + Na 2 CO 3 = CaCO 3 ↓ + 2NaCl,
CaSO 4 + Na 2 CO 3 = CaCO 3 ↓ + Na 2 SO 4
Magnesium sulfate and chloride react with alkali to form precipitated magnesium hydroxide:
MgSO 4 + 2NaOH = Mg(OH) 2 ↓ + Na 2 SO 4,
MgCl 2 + 2NaOH = Mg(OH) 2 ↓ + 2NaCl.
Due to the fact that in the reactions of bicarbonate with alkali, soda is formed, which subsequently reacts with non-carbonate hardness, its amount must be correlated in the ratio of carbonate and non-carbonate hardness: if they are equal, soda can not be added, under the condition J to > Jn, an excess of soda is formed, with the reverse ratio of J to
Combined methods.
The combination of various methods of water treatment in order to reduce its hardness sometimes gives quite high results. This is usually due to high requirements for the quality of water and steam.
An example would be the combination of reverse osmosis with sodium cationization. The main hardness of water is reduced using cation exchanger filters; reverse osmosis is used to desalt it.
In another case, magnetic water treatment can serve as an additional purification stage - the installation is located after the softening system on the hot water circulation pipeline.