Water softening comes down to a decrease in the concentration of calcium and magnesium salts in it. Water softening must be done to power boiler installations, and the water hardness for boilers is medium and low pressure should be no more than 0.3 mEq/l.

Water softening is also required for industries such as textile, paper, and chemical industries, where the water should have a hardness of no more than 0.7 -1.0 mEq/l.

Softening water for domestic and drinking purposes is also advisable, especially if it exceeds 7 mg-eq/l.

Water softening can be carried out using various methods; they can be divided into the following groups:

Thermal method of water softening

When water is heated to a boil, calcium and magnesium bicarbonates are converted into carbonates according to the following schemes:

Ca (HCO 3) 2 = CaCO 3 ↓+ CO 2 + H 2 O;

Mg(HCO 3) 2 = MgCO 3 + CO 2 + H 2 O.

These reversible processes can be shifted almost entirely to the right by boiling water, since at high temperatures the solubility of carbon dioxide decreases.

However, carbonate hardness cannot be completely eliminated, since calcium carbonate, although slightly (about 9.95 mg/l at 15 °C), is soluble in water. The solubility of MgCO 3 is quite high (110 mg/l), therefore, during prolonged boiling, it hydrolyzes to form slightly soluble (8 mg/l) magnesium hydroxide:

MgCO 3 + H 2 O ═ Mg (OH) 2 ↓ + CO 2 .

This method can be used to soften water containing predominantly carbonate hardness and used to feed low and medium pressure boilers.

Flaws: only temporary (carbonate) hardness decreases; large energy costs are required - in industry this method of water treatment is used only in the presence of cheap heat sources (at thermal power plants, for example).

Reagent water softening

The most common of the reagent methods is soda-lime softening method. Its essence boils down to obtaining, instead of Ca and Mg salts dissolved in water, insoluble salts CaCO 3 and Mg(OH) 2, which precipitate.

Both reagents - soda Na 2 CO 3 and lime Ca (OH) 2 - are introduced into the water to be softened simultaneously or alternately.

Salts of carbonate, temporary hardness are removed with lime, non-carbonate, permanent hardness - with soda.

Chemical reactions when removing carbonate hardness proceed as follows:

Ca(HCO 3) 2 + Ca(OH) 2 = 2CaCO 3 + 2H 2 O

Magnesium oxide hydrate Mg(OH)2 coagulates and precipitates. To eliminate non-carbonate hardness, Na2CO3 is added to the water being softened.

Chemical reactions when removing non-carbonate hardness are as follows:

Na 2 CO 3 + CaSO 4 = CaCO 3 + Na 2 SO 4;

Na 2 CO 3 + CaCl 2 = CaCO 3 + 2NaCl.

As a result of the reaction, calcium carbonate is obtained, which precipitates. Reagents used in water treatment are introduced into the water in the following places:

a) chlorine (during preliminary chlorination) - into the suction pipelines pumping station first rise or into water pipelines supplying water to the treatment station;

b) coagulant - into the pipeline in front of the mixer or into the mixer;

c) lime for alkalization during coagulation - simultaneously with the coagulant;

d) activated carbon to remove odors and tastes in water up to 5 mg/l - before filters. For large doses, coal should be introduced at the first lift pumping station or simultaneously with the coagulant into the mixer of the water treatment plant, but not earlier than 10 minutes after the introduction of chlorine;

e) chlorine and ammonia for water disinfection are introduced before treatment facilities and into filtered water. If phenols are present in water, ammonia should be introduced during both preliminary and final chlorination.

TO special types water purification and treatment include desalination, desalination, iron removal, removal of dissolved gases from water and stabilization.

This method is usually used only in some industries for the preliminary purification of process water. The technology is not applicable in ordinary household use.

Water softening with barium salts.

This method is similar to the lime-soda method, but has the advantage that the products formed during the reaction are insoluble in water. With this method, the content of salts that cause water hardness is reduced, and softening is much more complete. In addition, the insolubility of BaCO 3 does not require strict dosages; the process can proceed automatically.

The reactions that occur during softening with barium compounds can be represented by the following diagrams:

1) CaSO 4 + Ba (OH) 2 ® Ca (OH) 2 + BaSO 4 ↓;

2) MgSO 4 + Ba (OH) 2 ® Mg (OH) 2 ↓ + BaS0 4 ↓;

3) Ca (HCO 3) 2 + Ba (OH) 2 ® CaCO 3 ↓ + BaCO 3 ↓ + 2H 2 O;

4) Mg (HC0 3) 2 + 2Ba (OH) 2 ® 2BaCO 3 ↓ + Mg (OH) 2 ↓ + 2H 2 O;

5) BaCO 3 + CaSO 4 ® BaSO 4 ↓ + CaCO 3 ↓;

6) Ca (OH) 2 + Ca (HCO 3) 2 ® 2CaCO 3 ↓ + 2H 2 O.

When softening with barium salts, the reactions do not lead to the replacement of one salt with another, but to their complete removal from the water; This is the advantage of softening with barium salts. The disadvantages of this method include the high cost of barium salts and the slow reaction with barium carbonate BaCO 3 .

Reagent water treatment It is used only at large water treatment plants, since it is associated with a number of specific problems: disposal of solid sludge, specially equipped storage facilities for reagents, the need for precise dosage of chemicals and their correct supply to the source water.

Ion exchange water softening

Substances capable of sorption exchange of ions with an electrolyte solution are called ion exchangers.

Ionites- These are solid granular substances that swell in water, but are not soluble in it. According to the composition of the main skeleton, which binds together ionogenic groups, Ion exchange sorbents are divided into:

• mineral
• organic.

Ion exchangers used in water purification are of natural and artificial origin. An example of the former can be glauconite and humus coals, and an example of the latter can be sulfonated coals and synthetic ion-exchange resins.

Ion exchange resins- these are networked, three-dimensional polymers that do not dissolve in water, but swell in it to a limited extent and contain ionic groups, i.e. groups capable of exchanging ions. The number and length of bridges connecting linear polymer chains determine the “density” of the network, which has a strong influence on the properties of ion exchangers.

Ionites are divided into cation exchangers And anion exchangers. Substances that exchange cations are called cation exchangers, and those that exchange anions are called anion exchangers.

Cation exchangers dissociate into small, mobile and capable of ion exchange cations (for example, H +) and a high molecular weight anion (R m -1), and anion exchangers give small, easily moving anions (for example, OH -) and a high molecular weight cation (R n +).

Conventionally, their dissociation can be represented in the following form:

Н m R = mH + + R m – ; R(OH) n = R n + + nOH – ,

where m and n are the number of mobile ions in the cation exchanger and anion exchanger.

Of the cation exchange resins, the most widely used are resins formed by the polycondensation of phenols and formaldehyde, as well as polymers - products of copolymerization of styrene with diene hydrocarbons.

From resin anion exchangers Amino-formaldehyde anion exchangers and polystyrene anion exchangers, products of addition from the main groups to polystyrene copolymers, are more often used.

All ion exchangers can have the same or different ionogenic groups. Cation exchangers with mixed functional groups are found in the following combination:

1. sulfonic and hydroxyphenolic;
2. sulfonic acid and carboxyl;
3. phosphoric acid and hydroxyphenol residues;
4. arsenic acid and hydroxyphenolic;
5. carboxyl and oxyphenolic.

According to the degree of dissociation, ion exchangers are divided into:

1. strongly acidic
2. weakly acidic;
3. strongly basic
4. weakly basic.

Strong acid cation exchangers react with salts dissolved in water in neutral and acidic environments.

Weak acid cation exchangers, containing carboxyl or oxyphenol groups, exchange their proton in neutral solutions only with cation exchangers of salts of weak acids, and the completeness of the exchange increases with increasing pH of the medium.

Strong anion exchangers react with salt solutions in a neutral and even slightly alkaline environment.

Weak base anion exchangers enter into an exchange reaction only in acidic environments, and the completeness of the exchange of the hydroxyl group of the anion exchanger for the anion of the dissolved electrolyte increases with increasing acidity of the environment. The strength of ionic groups is greatly influenced by other functional groups directly associated with them.

Consequently, most cation exchangers are polymeric polyfunctional acids, which include groups – COOH, –SO 3 H, –OH, –SH, SiOOH, etc.

Anion exchangers are high-molecular compounds containing a huge number of basic groups, such as –NH 2, –NH 3 OH, –NHR, –NR 2, etc. The composition of the same ion exchanger may include ionogenic groups with varying degrees of acidity and alkalinity.

For filtering purposes, they try to obtain the resin in the form of spherical particles by suspension polymerization or mixing of the molten, yet “uncrosslinked” resin in an inert solvent, followed by cooling. Ion exchangers (in such a loose form) create favorable conditions for the movement of the filtered liquid.

The exchange process is based on a chemical reaction occurring on the external and internal surfaces of the ion exchangers. The exchange of ions occurs in strictly equivalent quantities.

Exchange reactions in solution occur almost instantly, but ion exchange processes with ion exchangers occurring in a heterogeneous environment have a quite measurable speed. In fact, the observed rate is determined by the diffusion rate, the slowest step in ion exchange. In this case, the ion exchange rate decreases with increasing grain size of the ion exchanger.

The exchange of ions in solutions occurs selectively. As the absolute concentration of the solution decreases, multivalent ions are adsorbed better than monovalent ions, and at high concentrations a monovalent ion is adsorbed. For example, when softening water, Ca 2+ and Mg 2+ ions are selectively absorbed, while Na+ ions are practically not adsorbed. When treated with a concentrated NaCl solution, divalent metal ions are displaced from the cation exchanger by sodium ions. This is used when regenerating a cation exchange filter.

The main technological characteristic of ion exchangers is their exchange capacity, which is determined by the number of ions extracted from water by 1 g of air-dried ion exchanger.

In water purification practice, H- and Na-cation exchangers are often used. Depending on the cation, this process is called H-cationization and Na-cationization.

With H-cationization, the acidity of the water increases, and with Na-cationization, the alkalinity of the filtrate increases if the source water contains carbonate hardness.

It should be noted that the rate of ion exchange during cationization depends on many factors, for example, the valence of the ions, their charge, the amount of hydration, and the effective radius of the ion. Based on the rate at which ions enter the cation exchanger, they are arranged in the following descending row: Fe 3 +>Al 3 +>Ca 2 +>Mg 2 +>Ba 2 +>NH 4 + >K + >Na+. This pattern can be changed by increasing the concentration of ions during the regeneration of cation exchange filters when treating them with a concentrated solution of sodium chloride.

A cation exchanger filter is a steel cylindrical tank with a diameter of 1 to 3 m, in which a layer of cation exchanger is placed on a drainage device. The height of the filter layer is 2...4 m. Filtration speed is from 4 to 25 m/h. The filters are designed for operating pressures up to 6 atm.

The cation exchanger filter works in the following stages:

• filtering through a prepared filter until the exchange capacity of the cation exchanger is saturated;
• loosening of the cation exchanger by an ascending flow;
• regeneration of the filter with NaCl solution (with Na-cationization);
• washing the load from excess amounts of regenerating substance.

Load regeneration lasts from one and a half to two hours.

Na-cationization ensures water softening to 0.05 mEq/l. In practice, two-stage Na-cationization is used. First-stage filters perform rough softening of water, reducing hardness by approximately 75%. The remaining hardness is removed by repeated filtration through second-stage filters. The bulk of calcium and magnesium ions are retained by the first-stage filters; the second-stage filters bear a slight load in terms of hardness and their operating cycle lasts up to 150¼200 hours. The residual hardness of water after two-stage Na-cationization is 0.01¼0.02 mEq/l. This method of water softening leads to saving salt on the regeneration of first-stage filters. For this purpose, wash water from second-stage filters is used. In addition, two-stage Na-cationization simplifies the operation of the installation by lengthening the filter cycle and does not require constant maintenance of the filtrate.

During cationization the following processes occur:

2NaR + Ca (HCO3) 2 ═ CaR 2 + 2NaHCO 3 ;

2NaR + Mg (HCO 3) 2 ═ MgR 2 + 2NaHCO 3 ;

2NaR + CaSO 4 ═ CaR 2 + Na 2 SO 4 ;

2NaR + MgCl 2 ═ MR 2 + 2NaCl.

When filtering water containing non-carbonate hardness, salts of strong acids and strong bases are obtained. These salts are not subject to hydrolysis even at high temperatures. But when carbonate hardness is removed, sodium bicarbonate is formed, which hydrolyzes at high temperatures to form a strong alkali:

NaHCO 3 + H 2 O ═ NaOH + H 2 CO 3.

To reduce the alkalinity of water, it is filtered sequentially through Na- and then H-cation exchangers, or the flow is divided into two parts, one of them is passed through Na-cation exchanger, and the second through H-cation exchanger, and then the filtrates are mixed.

Disadvantages of the ion exchange method of water treatment:

• relatively high consumption of reagents (especially for parallel-flow sodium cation exchanger filters);
• the increase in operating costs is proportional to the salt content of the source water and, if necessary, reduce the desalting limit of the treated water;
• depending on the quality of the source water, pretreatment is required - sometimes very complex;
• treatment of wastewater and difficulties with its discharge are necessary.

Reagent-free water treatment

Ultrasonic installations

- do a good job of removing scale, but to achieve efficiency the unit must be operated at high power. It means high level sound exposure, which entails the possibility of damage to the protected equipment (in areas of seam welding and rolling), as well as an increased danger to personnel.

Water softening in devices with permanent magnets.

In comparison with other common methods (ion exchange, baromembrane), magnetic water treatment is distinguished by its simplicity, low cost, safety, environmental friendliness, and low operating costs.

According to SNiP 11-35-76 “Boiler installations”, it is advisable to carry out magnetic treatment of water for heating equipment and hot water boilers if the content of iron ions Fe 2+ and Fe 3+ in the water does not exceed 0.3 mg/l, oxygen - 3 mg/l, constant hardness (CaSO 4, CaCl 2, MgSO 4 , MgCl 2) - 50 mg/l, carbonate hardness (Ca(HCO 3) 2, Mg(HCO 3) 2) is not higher than 9 mEq/l, and the water heating temperature should not exceed 95 0 C.

To power steam boilers - steel, allowing intra-boiler water treatment, and cast iron sectional - the use of magnetic water treatment technology is possible if the carbonate hardness of the water does not exceed 10 mEq/l, the content of Fe 2+ and Fe 3+ in the water is 0.3 mg/l, when water comes from a water supply or a surface source.

A number of industries are establishing more stringent regulations for process water, up to deep softening (0.035-0.05 mEq/l): for water tube boilers (15-25 ati) - 0.15 mEq/l; fire tube boilers (5-15 ati) - 0.35 mEq/l; boilers high pressure(50-100 ati) - 0.035 mEq/l.

Flaws– it is necessary to mechanically clean the magnet poles from deposits of ferromagnetic particles once every 5–7 days; Magnetized water retains its properties for less than a day ( this phenomenon of loss of magnetic properties is called relaxation, or the “water habituation” effect).

Therefore, in systems where water is present for many hours and days (circulating water supply systems, circulation circuits of boilers and heating systems, etc.), it is necessary to provide recirculation systems where at least 10% of the water in the system is directed, and this part of the water is constantly magnetize

Electromagnetic water softening

The basis of the device is an electronic microprocessor unit that generates an output aperiodic audio frequency signal (1–10 kHz). The signal is supplied to emitters wound on the pipeline with the liquid being processed in a certain order, and creates a pulsating dynamic electromagnetic field.

The mechanism of influence on the treated water is physical (reagent-free) in nature. Calcium and bicarbonate salts in aqueous solution exist in the form of positively and negatively charged ions. This implies the possibility of effectively influencing them with the help of electromagnetic field. If a coil is wound around a pipeline with a flowing liquid and a certain dynamic electromagnetic field is induced in it, calcium bicarbonate ions, electrostatically bound to water molecules, are released. The positive and negative ions released in this way are combined as a result of mutual attraction, and aragonite crystals (a highly dispersed suspension) are formed in the water, which do not form scale.

Since carbon dioxide is a by-product in the formation of aragonite crystals, water treated in this way has the properties of rainwater, i.e. is capable of dissolving existing hard carbonate deposits in the pipeline.

Under the influence of an electromagnetic field, a certain amount of hydrogen peroxide appears in water, which, when in contact with a steel surface inside the pipeline, forms a chemically stable film of Fe 3 0 4 on it, which protects the surface from corrosion. Hydrogen peroxide also has a significant antiseptic and antibacterial effect - it destroys about 99% of aquatic bacteria. The resulting hydrogen peroxide molecules, however, have a very short life cycle and are quickly converted into the form of oxygen and hydrogen, so drinking water treated in this way does not have any harmful effects. side effects on human health.

Today, this is the most environmentally friendly and economically feasible method of softening hard water.

Reagent-free water softening. Water softener Rapresol

Reagent-free water treatment using water softeners Rapresol effectively replaces the costly method of chemical water treatment, bringing significant savings to the enterprise.

Operating costs are reduced (reagents, regeneration, disposal, personnel, etc.), which ensures the greatest economic effect and rapid payback of the device with very high functional efficiency. The system is characterized by ease of installation and minimal operating costs.

Electromagnetic water softening technology is one of the recommended energy-saving technologies (RD 34.20.145-92) and allows not only to increase the operating life of heat exchange equipment between its forced stops for cleaning, but also to achieve real cost and energy savings.

Feasibility studies (feasibility studies) and calculation of payback periods for Rapresol devices:

• for organizations,
• for enterprises,

Combined water treatment methods

Installation of Rapresol water softener before installing ion exchange softening allows to significantly increase the inter-regeneration service life of filters and throughput filters

• The Rapresol device binds calcium ions into an insoluble state before ion exchange purification;
• qualitatively activated (the absorption capacity of ion exchangers increases) and ion exchange reactions are accelerated several times;
• the concentration of dissolved calcium ions in water before ion exchange is significantly reduced;
• Due to a decrease in the concentration of calcium bicarbonates, much more purified water can be obtained in one filter cycle.

Achieved economic effect:

• water consumption for washing resin during the regeneration process is reduced, and the influence of “slips” of untreated water is minimized.
• The time between repairs of boilers and heat exchangers increases by 2-3 times (the scale formed from residual hardness will be loose and can be easily removed by conventional blowing after 500-1000 hours of operation).
• Reagent washing of equipment and contamination are completely eliminated environment;
• reliable anti-scale and anti-corrosion cleaning and protection of both the heating unit and all pipelines is ensured;
• the internal surface of equipment and networks is strengthened;
• the heat transfer of the boiler and the thermal conductivity of the pipe lines increases;
• fuel is saved;

In addition, costs are reduced tenfold:

• salts and other regeneration reagents;
• water for loosening, regeneration and cleaning of filters;
• electricity consumed by pumps for pumping reagents.
• the discharge of rinsing salt-containing waters is reduced;

Excess iron, magnesium and calcium salts increase water hardness.

This negatively affects the operation of household appliances and equipment, the condition of hair, nails and skin, and provokes the development of chronic diseases of the gastrointestinal tract and cardiovascular system.

How to safely soften hard water using simple and affordable methods?

Signs of increased stiffness

What is water hardness? This is an indicator that determines the level of magnesium and calcium salts that are part of the chemical composition of the liquid. Units of measurement are mol/cub.m and mg.eq./liter.

Hard water is a common phenomenon that is caused by groundwater saturated with salts of chemical elements. In addition, such a liquid may contain chloride and phosphate compounds, as well as various organic pollutants.

To determine the hardness of water with your own hands, it is recommended to use a special device - a conductometer, designed to measure the electrical conductivity parameter of a liquid. A high indicator indicates an increased concentration of metal salts in water.

During the boiling process, chemical salts form a sedimentary mass, but most of the compounds enter the human body and settle on the walls of instruments, machinery and equipment.

What kind of water will be considered hard? The main signs of increased salt concentration are as follows:

• Detergents do not foam well;
• after boiling, scale and white deposits form;
• after washing clothes and dishes, characteristic stains remain;
• hard liquid acquires an unpleasant bitter taste;
• water has Negative influence on the performance characteristics of fabrics;
• An increased concentration of salts leads to diseases of the excretory system, as well as sagging and dry skin.

Types of Hard Water

According to the degree of hardness (in degrees), water is:

• Soft (from 0 to 2 degrees). It is common in areas with a large number of swamps and peat bogs. This category also includes clean melt water.
• Medium (from 2 to 7 degrees). This type of liquid is common in almost any area. As a rule, private households are provided with water of medium hardness.
• Hard (from 7.1 to 11 degrees). Found in areas with an abundance of chemical salts and pollutants. Has a negative effect on the human body.
• Super hard (from 11 degrees). Natural water is made hard by the proximity of caves and mines, so it is not used for drinking.

Based on the concentration of chemicals, water hardness can be:

• Constant. It is determined by the presence of aggressive components and metal salts that are resistant to decomposition during the boiling process. To remove them, special filter systems are used.
• Temporary. It is caused by the temporary presence of calcium and magnesium salts, the heating of which leads to disintegration and the formation of a sedimentary mass. This means that such compounds can be removed by conventional heat treatment.

Many consumers are interested in the answer to a fairly common question - how to soften water at home? Are there effective ways water softening solutions that can be easily implemented in practice?

• heat treatment;
• freezing;
• reagent effect;
• filtration.

Removing hardness by heat treatment (boiling)

The easiest way to soften water at home is heat treatment, i.e. boiling. Exposure to high temperatures leads to the destruction of ionic bonds between chemical elements and the formation of sediment. Further, soft water can be used for drinking and household purposes.

Boiling water is carried out as follows:

• hard water is poured into a container and brought to a boil;
• After boiling, the water cools to room temperature and pour into a clean container.

A more complex option involves boiling water for an hour and letting it sit for 24 hours.

Boiling removes metal salts and vapors carbon dioxide, chloride compounds and mechanical impurities.

Despite its popularity and simplicity, heat treatment has some disadvantages:

• boiling leads to rapid formation limescale, which is difficult to remove;
• boiled water is not suitable for watering indoor plants;
• prolonged use of liquid after heat treatment can lead to deterioration of the gastrointestinal tract;
• water changes its organoleptic characteristics.

Freezing is a simple and effective way

You can reduce the hardness of water by regular freezing or freezing. This method involves exposure to low temperature conditions on salts of chemical elements to form crystals. In this case, water softening occurs gradually, without changing the structure of the liquid.

Freezing is performed as follows:

• the container is filled with water and loaded into the freezer;
• after freezing 75% of the liquid, the remainder, which contains all the harmful elements, is drained;
• The melted liquid becomes potable, which means it can be used for cooking, watering flowers and washing delicate fabrics.

The only drawback of this method is the difficulty of preparing a large volume of melt water.

Treatment with chemical and food reagents

Softening hard water with reagents is an effective way to combat metal salts. The effect of chemicals on impurities in water leads to the formation of sediment. The following reagents are used for these purposes:

• Baking soda. It helps reduce acidity and salt concentration. Softening water with soda occurs as follows: for washing, use 2 tsp. for 11 liters, for cooking - 1 tsp. for 3 liters.
• Soda ash (caustic). Used to soften liquids intended for household and household needs - 2 tsp. for 11 liters. This liquid cannot be used for food purposes.
• Citric and acetic acid, lemon juice. Natural food reagents that help soften and oxidize water. Used to remove scale from dishes and when rinsing hair. The optimal concentration is 1 tbsp per 2 liters of water. l. acetic acid, 1 tsp. citric acid or lemon juice.
• Synthetic reagents in tablet and powder form. Increased hardness can be eliminated with special chemicals designed for dishwashing or washing equipment.

To the disadvantages this method can be attributed:

• the need to maintain the exact dosage of each reagent;
• maintaining storage conditions for special products - caustic soda and synthetic softeners at home in accordance with the manufacturers' recommendations. The exception is food reagents - soda, vinegar and citric acid.

Reducing hardness with filter systems

How to make water soft if it is obtained from a well or a well built next to the house?

• Jug type filters. This is the most popular way to purify and soften tap or well water. This is the name of the filter, which looks like a jug equipped with a carbon cartridge for cleaning. The small volume of the container allows you to filter from 1 to 4 liters of water in one cycle. Hard water purified with a pitcher filter acquires not only softness, but also a specific taste. The frequency of cartridge replacement is every 2 months.
• Ion exchange units. Such filter systems are represented by two containers equipped with special filters based on ion exchange resins and saline solution. First, hard water enters the reservoir with resins, and then enters the container with brine solution. Why does the liquid lose its hardness in this case? Because it is saturated with sodium, which gradually displaces magnesium and calcium salts.
• . This is the most effective way to clean and soften liquid. The installation is equipped with a special membrane filter that creates operating pressure inside the chamber. Thanks to this, hard water is completely purified from foreign impurities, which means it becomes soft.

You can solve the problem of increased water hardness on your own; it is enough to apply effective methods in practice or introduce a unique proprietary technique.

Water softening by dialysis

Magnetic water treatment

Literature

Theoretical foundations of water softening, classification of methods

Water softening refers to the process of removing hardness cations from it, i.e. calcium and magnesium. In accordance with GOST 2874-82 "Drinking water", water hardness should not exceed 7 mEq/l. Selected species production facilities require deep softening of process water, i.e. up to 0.05.0.01 mEq/l. Typically used water sources have a hardness that meets drinking water standards and do not require softening. Water softening is carried out mainly during its preparation for technical purposes. Thus, the hardness of water for feeding drum boilers should not exceed 0.005 mEq/l. Water softening is carried out using the following methods: thermal, based on heating water, its distillation or freezing; reagents, in which the ions present in water Ca ( II ) And Mg ( II ) bind with various reagents into practically insoluble compounds; ion exchange, based on filtering softened water through special materials that exchange the ions included in their composition Na ( I) or H (1) into Ca (II) ions and Mg ( II ), contained in dialysis water; combined, representing various combinations of the listed methods.

The choice of water softening method is determined by its quality, the required depth of softening and technical and economic considerations. In accordance with the recommendations of SNiP when softening groundwater, ion exchange methods should be used; when softening surface waters When water clarification is also required, the lime or lime-soda method is used, and when water is deeply softened, subsequent cationization is used. The main characteristics and conditions for using water softening methods are given in table. 20.1.

softening water dialysis thermal

To obtain water for domestic and drinking needs, usually only a certain part of it is softened, followed by mixing with source water, while the amount of softened water Qy determined by the formula

where is J o. And. - total hardness of source water, mEq/l; F 0. s. - total hardness of water entering the network, mEq/l; F 0. u. - hardness of softened water, mEq/l.

Water softening methods

Index	thermal	reagent	ion exchange	dialysis
Process characteristics	The water is heated to a temperature above 100°C, which removes carbonate and non-carbonate hardness (in the form of calcium carbonate, hydroxy, magnesium and gypsum)	Lime is added to the water, which eliminates carbonate and magnesium hardness, as well as soda, which eliminates non-carbonate hardness.	The water to be softened is passed through cation exchanger filters	Source water is filtered through a semi-permeable membrane
Purpose of the method	Elimination of carbonate hardness from water used to feed low and medium pressure boilers	Shallow softening while simultaneously clarifying water from suspended solids	Deep softening of water containing a small amount of suspended solids	Deep water softening
Water consumption for own needs	-	No more than 10%	Up to 30% or more in proportion to the hardness of the source water	10
Conditions for effective use: source water turbidity, mg/l	Up to 50	Up to 500	No more than 8	Up to 2.0
Water hardness, mEq/l	Carbonate hardness with a predominance of Ca (HC03) 2, non-carbonate hardness in the form of gypsum	5.30	Not higher than 15	Up to 10.0
Residual water hardness, mEq/l	Carbonate hardness up to 0.035, CaS04 up to 0.70	Up to 0.70	0.03.0.05 prn single-stage and up to 0.01 with two-stage cationization	0.01 and below
Water temperature, °C	Up to 270	Up to 90	Up to 30 (glauconite), up to 60 (sulfonite)	Up to 60

Thermal method of water softening

The thermal method of water softening is advisable to use when using carbonate waters used to feed low-pressure boilers, as well as in combination with reagent methods of water softening. It is based on a shift in the carbon dioxide equilibrium when it is heated towards the formation of calcium carbonate, which is described by the reaction

Ca (HC0 3) 2 -> CaCO 3 + C0 2 + H 2 0.

The equilibrium is shifted due to a decrease in the solubility of carbon (IV) monoxide caused by an increase in temperature and pressure. Boiling can completely remove carbon (IV) monoxide and thereby significantly reduce calcium carbonate hardness. However, it is not possible to completely eliminate this hardness, since calcium carbonate, although slightly (13 mg/l at a temperature of 18°C), is still soluble in water.

If magnesium bicarbonate is present in water, the process of its precipitation occurs as follows: first, relatively highly soluble (110 mg/l at a temperature of 18 ° C) magnesium carbonate is formed

Mg (HCO 3) → MgC0 3 + C0 2 + H 2 0,

which hydrolyzes during prolonged boiling, resulting in a slightly soluble precipitate (8.4 mg/l). magnesium hydroxide

MgC0 3 +H 2 0 → Mg (0H) 2 +C0 2 .

Consequently, when water is boiled, the hardness caused by calcium and magnesium bicarbonates decreases. When water is boiled, hardness, determined by calcium sulfate, also decreases, the solubility of which drops to 0.65 g/l.

In Fig. 1 shows a thermal softener designed by Kopyev, characterized by the relative simplicity of the device and reliable operation. The treated water, preheated in the apparatus, enters through the ejector onto the socket of the film heater and is sprayed over vertically placed pipes, and flows down through them towards the hot steam. Then, together with the blowdown water from the boilers, it enters the clarifier with suspended sediment through the central supply pipe through the perforated bottom.

The carbon dioxide and oxygen released from the water along with excess steam are discharged into the atmosphere. Calcium and magnesium salts formed during the heating of water are retained in the suspended layer. Having passed through the suspended layer, the softened water enters the collection tank and is discharged outside the apparatus.

The residence time of water in the thermal softener is 30.45 minutes, the speed of its upward movement in the suspended layer is 7.10 m/h, and in the holes of the false bottom 0.1-0.25 m/s.

Rice. 1. Thermal softener designed by Kopyev.

15 - discharge of drainage water; 12 - central supply pipe; 13 - false perforated bottoms; 11 - suspended layer; 14 - sludge discharge; 9 - collection of softened water; 1, 10 2 - boiler blowing; 3 - ejector; 4 - evaporation; 5 - film heater; 6 - steam release; 7 - ring perforated pipeline for water drainage to the ejector; 8 - inclined separating partitions

Reagent methods of water softening

Water softening using reagent methods is based on treating it with reagents that form poorly soluble compounds with calcium and magnesium: Mg (OH) 2, CaC0 3, Ca 3 (P0 4) 2, Mg 3 (P0 4) 2 and others, followed by their separation in clarifiers , thin-layer sedimentation tanks and clarification filters. Lime, soda ash, sodium and barium hydroxides and other substances are used as reagents.

Water softening by liming used for high carbonate and low non-carbonate hardness, as well as in cases where it is not necessary to remove non-carbonate hardness salts from water. Lime is used as a reagent, which is introduced in the form of a solution or suspension (milk) into preheated treated water. When dissolved, lime enriches the water with OH - and Ca 2+ ions, which leads to the binding of free carbon monoxide (IV) dissolved in water with the formation of carbonate ions and the transition of hydrocarbonate ions into carbonate ones:

C0 2 + 20H - → CO 3 + H 2 0, HCO 3 - + OH - → CO 3 - + H 2 O.

An increase in the concentration of CO 3 2 - ions in the treated water and the presence of Ca 2+ ions in it, taking into account those introduced with lime, leads to an increase in the solubility product and the precipitation of poorly soluble calcium carbonate:

Ca 2+ + C0 3 - → CaC0 3.

If there is an excess of lime, magnesium hydroxide also precipitates.

Mg 2+ + 20H - → Mg (OH) 2

To accelerate the removal of dispersed and colloidal impurities and reduce the alkalinity of water, coagulation of these impurities with iron (II) sulfate is used simultaneously with liming, i.e. FeS0 4 *7 H 2 0. The residual hardness of softened water during decarbonization can be obtained 0.4-0.8 mg-eq/l more than non-carbonate hardness, and the alkalinity is 0.8-1.2 mg-eq/l. The dose of lime is determined by the ratio of the concentration of calcium ions in water and carbonate hardness: a) at the ratio [Ca 2+ ] /20<Ж к,

b) with the ratio [Ca 2+ ] /20 > J c,

where [CO 2 ] is the concentration of free carbon monoxide (IV) in water, mg/l; [Ca 2+ ] - concentration of calcium ions, mg/l; Fc - carbonate hardness of water, mEq/l; D k - dose of coagulant (FeS0 4 or FeCl 3 in terms of anhydrous products), mg/l; e k- equivalent mass of the active substance of the coagulant, mg/mg-eq (for FeS0 4 e k = 76, for FeCl 3 e k = 54); 0.5 and 0.3 - excess lime to ensure greater completeness of the reaction, mEq/l.

The expression D k / e k is taken with a minus sign if the coagulant is introduced before the lime, and with a plus sign if together or after.

In the absence of experimental data, the dose of the coagulant is found from the expression

D k = 3 (C) 1/3, (20.4)

where C is the amount of suspended matter formed during water softening (in terms of dry matter), mg/l.

In turn, C is determined using the dependence

where M and is the content of suspended solids in the source water, mg/l; m- CaO content in commercial lime, %.

Lime-soda water softening method is described by the following basic reactions:

Using this method, residual hardness can be brought to 0.5.1, and alkalinity from 7 to 0.8.1.2 mEq/l.

Doses of lime D and soda D s (in terms of Na 2 C0 3), mg/l, are determined by the formulas

(20.7)

where is the content of magnesium in water, mg/l; Jn. K. - non-carbonate water hardness, mEq/l.

With the lime-soda method of water softening, the resulting calcium carbonate and magnesium hydroxide can supersaturate solutions and remain in a colloidal dispersed state for a long time. Their transition into coarse sludge takes a long time, especially at low temperatures and the presence of organic impurities in the water, which act as protective colloids. With a large amount of them, water hardness during reagent water softening can be reduced by only 15.20%. In such cases, before softening or during the softening process, organic impurities are removed from the water using oxidizing agents and coagulants. With the lime-soda method, the process is often carried out in two stages. Initially, organic impurities and a significant part of carbonate hardness are removed from the water, using aluminum or iron salts with lime, carrying out the process at optimal conditions coagulation. After this, soda and the rest of the lime are introduced and the water is softened. When removing organic impurities simultaneously with water softening, only iron salts are used as coagulants, since at a high pH value of water necessary to remove magnesium hardness, aluminum salts do not form sorption-active hydroxide. The dose of the coagulant in the absence of experimental data is calculated using formula (20.4). The amount of suspension is determined by the formula

where W o - total water hardness, mEq/l.

Deeper softening of water can be achieved by heating it, adding an excess of precipitating reagent and bringing the softened water into contact with previously formed sediments. When water is heated, the solubility of CaCO 3 and Mg (OH) 2 decreases and softening reactions occur more fully.

From the graph (Fig. 2, a) it is clear that residual hardness, close to theoretically possible, can be obtained only with significant heating of the water. A significant softening effect is observed at 35.40°C; further heating is less effective. Deep softening is carried out at temperatures above 100° C. It is not recommended to add a large excess of the precipitating reagent during decarbonization, since the residual hardness increases due to unreacted lime or if there is magnesium non-carbonate hardness in the water due to its transition to calcium hardness:

MgS0 4 + Ca (OH) 2 = Mg (OH) 2 + CaS0 4

Rice. 2. The influence of temperature (a) and dose of lime (b) on the depth of water softening using the lime-soda and lime method

Ca (0H) 2 + Na 2 C0 3 = CaC0 3 + 2NaOH,

but excess lime leads to wasteful overconsumption of soda, increasing the cost of water softening and increasing hydrate alkalinity. Therefore, excess soda is taken at about 1 mEq/L. Water hardness as a result of contact with previously fallen sediment is reduced by 0.3-0.5 mg-eq/l compared to the process without contact with sediment.

The water softening process should be controlled by adjusting the pH of the softened water. When this is not possible, it is controlled by the value of hydrate alkalinity, which is maintained within 0.1-0.2 mg-eq/l during decarbonization, and 0.3-0.5 mg-eq/l during lime-soda softening.

With the soda-sodium method of softening water, it is treated with soda and sodium hydroxide:

Due to the fact that soda is formed by the reaction of sodium hydroxide with bicarbonate, the dose required to add it to water is significantly reduced. If the concentration of bicarbonates in the water is high and the non-carbonate hardness is low, excess soda may remain in the softened water. Therefore, this method is used only taking into account the relationship between carbonate and non-carbonate hardness.

Soda-sodium method Usually used to soften water whose carbonate hardness is slightly higher than non-carbonate hardness. If the carbonate hardness is approximately equal to the non-carbonate hardness, you don’t need to add soda at all, since the amount required to soften such water is formed as a result of the interaction of bicarbonates with caustic soda. The dose of soda ash increases as the non-carbonate hardness of the water increases.

The soda-regenerative method, based on the renewal of soda during the softening process, is used in water preparation and for feeding low-pressure steam boilers

Ca (HC0 3) 2 + Na 2 C0 3 = CaC0 3 + 2NaHC0 3.

Sodium bicarbonate, entering a boiler with softened water, decomposes under the influence of high temperature

2NaHC0 3 = Na 2 C0 3 + H 2 0 + C0 2.

The resulting soda, together with the excess soda initially introduced into the water softener, is immediately hydrolyzed in the boiler to form sodium hydroxide and carbon monoxide (IV), which enters the water softener with the purge water, where it is used to remove calcium and magnesium bicarbonates from the softened water. The disadvantage of this method is that the formation of a significant amount of CO 2 during the softening process causes corrosion of the metal and an increase in dry residue in the boiler water.

Barium water softening method used in combination with other methods. First, barium containing reagents are introduced into the water (Ba (OH) 2, BaCO 3, BaA1 2 0 4) to eliminate sulfate hardness, then after clarification of the water, it is treated with lime and soda to soften it. The chemistry of the process is described by the reactions:

Due to the high cost of reagents, the barium method is used very rarely. For the preparation of drinking water due to the toxicity of barium reagents, it is unsuitable. The resulting barium sulfate settles very slowly, so large settling tanks or clarifiers are required. To introduce BaCO3, flocculators with mechanical stirrers should be used, since BaCO3 forms a heavy, quickly settling suspension.

The required doses of barium salts, mg/l, can be found using the expressions: barium hydroxide (product of 100% activity) D b =1.8 (SO 4 2-), barium aluminate D b =128Zh 0; barium carbonate D in = 2.07γ (S0 4 2-);

Barium carbonate is used with lime. By exposing barium carbonate to carbon dioxide, barium bicarbonate is obtained, which is dosed into the water to be softened. In this case, the dose of carbon dioxide, mg/l, is determined from the expression: D arc. = 0.46 (SO 4 2-); where (S0 4 2-) is the content of sulfates in the softened water, mg/l; γ=1.15.1.20 - coefficient taking into account the loss of barium carbonate.

Oxalate method of water softening based on the use of sodium oxalate and the low solubility of the resulting calcium oxalate in water (6.8 mg/l at 18° C)

The method is distinguished by its simplicity of technological and hardware design, however, due to the high cost of the reagent, it is used to soften small quantities of water.

Phosphating is used to soften water. After reagent softening using the lime-soda method, the presence of residual hardness (about 2 mEq/l) is inevitable, which can be reduced to 0.02-0.03 mEq/l by phosphate softening. Such deep purification allows in some cases not to resort to cation exchange water softening.

Phosphating also achieves greater stability of water, reduces its corrosive effect on metal pipelines and prevents carbonate deposits on the inner surface of pipe walls.

Hexametaphosphate, sodium tripolyphosphate (orthophosphate), etc. are used as phosphate reagents.

The phosphate method of water softening using tri-sodium phosphate is the most effective reagent method. The chemistry of the water softening process with trisodium phosphate is described by the reactions

As can be seen from the above reactions, the essence of the method is the formation of calcium and magnesium salts of phosphoric acid, which have low solubility in water and therefore precipitate quite completely.

Phosphate softening is usually carried out by heating water to 105.150 ° C, achieving its softening to 0.02.0.03 mEq/l. Due to the high cost of trisodium phosphate, the phosphate method is usually used to soften water previously softened with lime and soda. The dose of anhydrous trisodium phosphate (Df; mg/l) for additional softening can be determined from the expression

D F =54.67 (W OST + 0.18),

where Zhost is the residual hardness of softened water before phosphate softening, mEq/l.

Ca 3 (P0 4) 2 and Mg 3 (P0 4) 2 precipitates formed during phosphate softening well adsorb organic colloids and silicic acid from softened water, which makes it possible to identify the feasibility of using this method for preparing feed water for medium and high pressure boilers (58.8-98.0 MPa).

A solution for dosing sodium hexametaphosphate or sodium orthophosphate with a concentration of 0.5-3% is prepared in tanks, the number of which must be at least two. The internal surfaces of the walls and bottom of the tanks must be coated with corrosion-resistant material. The preparation time for a 3% solution is 3 hours with mandatory mixing using a stirrer or bubbling method (using compressed air).

Technological diagrams and structural elements of reagent water softening installations

Reagent water softening technology uses equipment for preparing and dosing reagents, mixers, thin-layer sedimentation tanks or clarifiers, filters and installations for stabilizing water treatment. The diagram of a pressure water softening installation is shown in Fig. 3

Rice. 3. Water softening plant with a vortex reactor.

1 - hopper with contact mass; 2 - ejector; 3, 8 - supply of source water and removal of softened water; 4 - vortex reactor; 5 - input of reagents; 6 - fast clarification filter; 9 - contact mass release; 7 - softened water tank

This installation does not have a flocculation chamber, since flocs of calcium carbonate precipitate are formed in the contact mass. If necessary, the water before the reactors is clarified.

The optimal structure for softening water using lime or lime-soda methods is vortex reactor (pressure or open spiractor) ( rice. 20.4). The reactor is a reinforced concrete or steel body, narrowed downwards (taper angle 5.20°) and filled to approximately half the height with contact mass. The speed of water movement in the lower narrow part of the vortex reactor is 0.8.1 m/s; the speed of the upward flow in the upper part at the level of drainage devices is 4.6 mm/s. Sand or marble chips with a grain size of 0.2-0.3 mm are used as a contact mass at the rate of 10 kg per 1 m3 of reactor volume. With a helical upward flow of water, the contact mass is suspended, grains of sand collide with each other and CaCO 3 intensively crystallizes on their surface; gradually the grains of sand turn into balls of the correct shape. The hydraulic resistance of the contact mass is 0.3 m per 1 m height. When the diameter of the balls increases to 1.5.2 mm, the largest, heaviest contact mass is released from the lower part of the reactor and a fresh one is added. Vortex reactors do not retain magnesium hydroxide sediment, so they should be used in conjunction with filters installed behind them only in cases where the amount of magnesium hydroxide sediment formed corresponds to the dirt holding capacity of the filters.

With a dirt holding capacity of sand filters equal to 1.1.5 kg/m3 and a filter cycle of 8 hours, the permissible amount of magnesium hydroxide is 25.35 g/m3 (the magnesium content in the source water should not exceed 10.15 g/m3). It is possible to use vortex reactors with a higher content of magnesium hydroxide, but after them it is necessary to install clarifiers to separate magnesium hydroxide.

The consumption of fresh contact mass added using an ejector is determined by the formula G = 0.045QZh, where G- amount of added contact mass, kg/day; AND- water hardness removed in the reactor, mEq/l; Q - installation productivity, m 3 / h.

Rice. 4. Vortex reactor.

1,8 - supply of source water and removal of softened water: 5 - samplers; 4 - contact mass; 6 - air release; 7 - hatch for loading contact mass; 3 - input of reagents; 2 - removal of spent contact mass

In technological schemes of reagent water softening with clarifiers, vertical mixers are used instead of vortex reactors (Fig. 5). In clarifiers, a constant temperature should be maintained, not allowing fluctuations of more than 1°C, for an hour, since convection currents arise, sediment resuspension and its removal.

A similar technology is used to soften troubled waters containing a large number of magnesium salts. In this case, the mixers are loaded with contact mass. When using clarifiers designed by E.F. Kurgaev, mixers and floc formation chambers are not provided, since the mixing of reagents with water and the formation of sediment flocs occurs in the clarifiers themselves.

The significant height and small volume of sediment compactors allows them to be used for softening water without heating, as well as for desiliconizing water with caustic magnesite. The distribution of the source water by nozzles causes its rotational movement in the lower part of the apparatus, which increases the stability of the suspended layer during fluctuations in temperature and water supply. Water mixed with reagents passes through horizontal and vertical mixing partitions and enters the zone of sorption separation and regulation of the sediment structure, which is achieved by changing the conditions for selecting sediment along the height of the suspended layer, creating the prerequisites for obtaining its optimal structure, which improves the effect of softening and clarification of water. Clarifiers are designed in the same way as for conventional water clarification.

At flow rates of softened water up to 1000 m 3 /day, a water treatment plant of the “Jet” type can be used. The treated water with reagents added to it enters a thin-layer sedimentation tank, then onto a filter.

The Institute of Mining of the Siberian Branch of the Russian Academy of Sciences has developed a reagent-free electrochemical technology for water softening. Using the phenomenon of alkalization at the anode and acidification at the cathode when passing a direct electric current through a water system, the water discharge reaction can be represented by the following equation:

2Н 2 0 + 2е 1 → 20Н - + Н 2,

where e 1 is a sign indicating the ability of hardness salts to dissociate into Ca (II) and Mg (II) cations.

As a result of this reaction, the concentration of hydroxyl ions increases, which causes the binding of Mg (II) and Ca (II) ions into insoluble compounds. From the anode chamber of a diaphragm electrolyzer (diaphragm made of belting fabric) these ions pass into the cathode chamber due to the potential difference between the electrodes and the presence of an electric field between them.

In Fig. Figure 6 shows a technological diagram of an installation for softening water using an electrochemical method.

The production plant was installed in the district boiler house, the testing of which lasted about two months. The electrochemical treatment regime turned out to be stable; no deposits were observed in the cathode chambers.

The voltage on the supply busbars was 16 V, the total current was 1600 A. The total productivity of the installation was 5 m3/h, the speed of water movement in the anode chambers was 0.31 n-0.42 m/min, in the gap between the diaphragm and the cathode 0.12- 0.18 m/min.

Rice. 5. Installation of lime-soda water softening.1 ,8 - supply of source water and removal of softened water; 2 - ejector; 3 - hopper with contact mass; 5 input of reagents; 6 - clarifier with a layer of suspended sediment; 7 - fast clarification filter; 4 - vortex reactor

Rice. 6. Installation diagram for electrochemical water softening I - rectifier VAKG-3200-18; 2 - diaphragm electrolyzer; 3, 4 - analyte and catalyte; 5 - pump; 6 - pH meter; 7 - clarifier with a layer of suspended sediment; 8 - clarification fast filter; 9 - discharge into the sewer; 10, 11 - removal of softened water and supply of source water; 12 - flow meter; 13 - exhaust hood

It has been established that from water with W o = 14.5-16.7 mg-eq/l, an anolyte with a hardness of 1.1 - 1.5 mg-eq/l at pH = 2.5-3 and a catholyte with a hardness of 0 are obtained .6-1 mEq/l at pH=10.5-11. After mixing the filtered anolyte and catholyte, the softened water indicators were as follows: the total hardness of liquid was 0.8-1.2 mEq/l, pH = 8-8.5. Electricity costs amounted to 3.8 kW*h/m3.

Chemical, X-ray diffraction, IR spectroscopic and spectral analyzes have established that the sediment predominantly contains CaC0 3, Mg (OH) 2 and partially Fe 2 0 3 *H 2 0. This indicates that the binding of Mg (II) ions occurs during counting of hydroxyl ions during the discharge of water molecules at the cathode.

Electrochemical treatment of water before feeding it to cation exchange filters can significantly (15-20 times) increase their operating cycle.

Thermochemical water softening method

Thermochemical softening is used exclusively in the preparation of water for steam boilers, since in this case the heat spent on heating water is used most efficiently. With this method, water softening is usually carried out at water temperatures above 100°C. More intensive softening of water when heated is facilitated by the formation of heavy and large flakes of sediment, its rapid sedimentation due to a decrease in the viscosity of water when heated, and the consumption of lime is also reduced, since free carbon monoxide (IV) is removed by heating before the introduction of reagents. The thermochemical method is used with or without the addition of a coagulant, since the high density of the sediment eliminates the need to weigh it down during sedimentation. In addition to the coagulant, lime and soda with the addition of phosphates are used, and less often sodium hydroxide and soda. Application of hydroxide Using sodium instead of lime somewhat simplifies the technology for preparing and dosing the reagent, but such a replacement is not economically justified due to its high cost.

To ensure the removal of non-carbonate hardness in water, soda is added in excess. In Fig. Figure 7 shows the effect of excess soda on the residual calcium and total hardness of water during its thermochemical softening. As can be seen from the graphs, with an excess of soda of 0.8 mg/eq/l, calcium hardness can be reduced to 0.2, and total hardness to 0.23 mg/eq/l. With further addition of soda, the hardness decreases even more. The residual magnesium content in water can be reduced to 0.05-0.1 mEq/L with an excess of lime (hydrate alkalinity) of 0.1 mEq/L. In Fig. Figure 20.8 shows a thermochemical water softening installation.

Lime-dolomite method used for simultaneous softening and desiliconization of water at a temperature of 120 ° C. With this softening method, the alkalinity of water treated with lime or lime and soda (without excess) can be reduced to 0.3 mEq/l with a residual calcium concentration of 1.5 mg -eq/l and up to 0.5 mEq/l with a residual calcium concentration of 0.4 mEq/l. The source water is treated with lime-dolomite milk and clarified in a pressure clarifier. Then it passes through pressure anthracite and Na-cationite filters of the first and second stages.

In clarifiers, the height of the clarification zone is taken to be 1.5 m, the speed of the upward flow during liming is no more than 2 mm/s. The residence time of water in the clarifier is from 0.75 to 1.5 hours, depending on the type of contamination being removed. Iron (III) salt coagulant is recommended to be added in an amount of 0.4 mEq/l.

Rice. 7. The effect of excess soda on residual calcium (a) and total (b) water hardness during thermochemical softening

Rice. 8. Installation of lime-soda water softening with phosphate softening: 1 - discharge of sludge from the storage tank 2,3 - collection of softened water; 4 - input of lime and soda; 5, 11 - supply of source water and removal of softened water; 6 - steam input; 7, 8 - thermoreactor of the first and second stages; 9 - introduction of trisodium phosphate; 10 - clarification fast filter

High temperature water softening method used to almost completely soften it. Thermochemical water softening units are usually more compact. They consist of reagent dispensers, thin-layer sedimentation tank or clarifier heaters and filters. Doses of lime D and soda D s, mg/l, for thermochemical water softening

where C and and C c are, respectively, the content of CaO and Na 2 C0 3 in the technical product, %.

Water softening by dialysis

Dialysis is a method of separating solutes that differ significantly in molecular weight. It is based on different speeds diffusion of these substances through a semi-permeable membrane separating concentrated and dilute solutions. Under the influence of a concentration gradient (according to the law of mass action), solutes diffuse through the membrane at different rates towards the dilute solution. The solvent (water) diffuses in the opposite direction, reducing the rate of solute transport. Dialysis is carried out in membrane devices with nitro- and cellulose acetate film membranes. The effectiveness of a semi-permeable membrane for water softening is determined by the high values ​​of selectivity and water permeability, which it must maintain over a long operating time. The selectivity of the membrane can be expressed as follows:

(Zh i - Zh y) /Zh i (20.11)

where Ж в is the concentration of the initial solution (hardness); W and - hardness of softened water.

In practice, the salt reduction coefficient is often used - the content of C and /C arr. It most fully reflects changes in the operation of the membrane associated with its manufacture or exposure to external factors.

There are several hypothetical models for the action of semipermeable membranes.

Hyperfiltration hypothesis assumes the existence of pores in a semi-permeable membrane that allow associates of water molecules and hydrated salt ions to pass through during dialysis. basis theoretical developments the position emerged that water and salts dissolved in it penetrate through a semi-permeable membrane using diffusion and flows through the pores.

Sorption model permeability is based on the premise that on the surface of the membrane and in its pores a layer of bound water with reduced dissolving ability is adsorbed. Membranes will be semi-permeable if they, at least in the surface layer, have pores that do not exceed twice the thickness of the layer of bound liquid.

Diffusion model is based on the assumption that the components of the system dissolve in the membrane material and diffuse through it. The selectivity of the membrane is explained by the difference in the diffusion coefficients and solubility of the system components in its material.

Electrostatic theory is as follows. When the source water moves in the chamber on one side of the selective (cationite) membrane, and the brine on the other, sodium ions, in the case when the brine is prepared from a solution of table salt, migrate into the membrane and then into the source water, and calcium ions in the opposite direction, i.e. .e. from hard water to brine. Thus, calcium ions are removed from the source water and replaced with non-precipitating sodium ions. At the same time, side processes occur in the chambers that accompany the main dialysis process: osmotic transfer of water, transfer of like ions, diffusion of electrolyte. These processes depend on the quality of the membrane.

The exchange equation between ions contained in the source water and ions in the membrane has the form

Where x, x- other ions contained in the solution and in the membrane.

Equilibrium constant

The exchange equation is written only for the calcium ion, but> in fact it is necessary to take into account the sum of the calcium and magnesium ions. The equilibrium between the brine and the membrane is:

If k1+ k 2 then

where n is an exponent depending on which ions are included in the solution.

From the last expression we can conclude that if the equilibrium ratio of sodium ions in brine and hard source water is, for example, 10, then the hardness in the source water will be approximately 100 times less than in the brine. Area, m2, membrane surface

where M is the amount of substance that has passed through the membrane; ΔC av - the driving force of the process, i.e. the difference in the concentrations of the substance on both sides of the membrane; Kd is the mass transfer coefficient, usually determined experimentally or approximately from the expression

β 1 and β 2 are the corresponding coefficients of the rate of transfer of a substance in a concentrated solution to the membrane and from it in a dilute solution; b - membrane thickness; D- diffusion coefficient of the solute.

Hardness of softened water after dialysis:

where C d and C p are the concentrations of salts at the beginning of the apparatus, respectively, in the dialysate and in the brine, mEq/l; And Q p - productivity of the device for dialysate and brine, respectively, m 3 /h; F d and F r - hardness of the dialysate and brine at the beginning of the apparatus, mEq/l; a is a constant determined by the properties of membranes and solutions;; L- length of the path of the solution in the dialysate and brine chambers of the apparatus, m; υ d - speed of movement of the dialysate in the chamber, m/s.

Experimental testing of equation (20.13) on MCC cation exchange membranes showed good convergence of results. Analysis of formula (20.13) shows that reducing the speed of movement of the dialysate in the chambers of the apparatus increases the softening effect; the decrease in the hardness of softened water is directly proportional to the brine concentration.

Magnetic water treatment

Recently, in domestic and foreign practice, magnetic water treatment has been successfully used to combat scale formation and encrustation. The mechanism of the influence of a magnetic field on water and its admixture has not been fully clarified, there are a number of hypotheses that E.F. Tebenikhin classified into three groups: the first, which unites most of the hypotheses, relates the effect of a magnetic field on salt ions dissolved in water. Under the influence of a magnetic field polarization and deformation of ions occur, accompanied by a decrease in their hydration, increasing the likelihood of their approach, and in ultimately education crystallization centers; the second assumes the action of a magnetic field on colloidal impurities of water; the third group combines ideas about the possible influence of a magnetic field on the structure of water. This the influence, on the one hand, can cause changes in the aggregation of water molecules, and on the other, disrupt the orientation of the nuclear spins of hydrogen in its molecules.

Treatment of water in a magnetic field is common 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. The method is effective in treating waters of the calcium-carbonate class, which make up about 80% of the waters of all reservoirs in our country and cover approximately 85% of its territory.

Water treatment with a magnetic field has been widely used to combat scale formation in steam turbine condensers, in low-pressure and low-capacity steam generators, in heating networks and hot water supply networks and various heat exchangers, where the use of other water treatment methods is not economically feasible. Compared to water softening, the main advantages of magnetic treatment are simplicity, low cost, safety and almost complete absence of operating costs.

Magnetic processing natural waters(both fresh and mineralized) leads to a decrease in the intensity of scale formation on heating surfaces only if they are oversaturated with both calcium carbonate and calcium sulfate at the time of exposure to a magnetic field and provided that the concentration of free carbon monoxide (IV) is less than its equilibrium concentration . The anti-scale effect of E determines the presence of iron oxides and other impurities in water:

where m n and m m are the mass of scale formed on the heating surface during boiling under the same conditions of the same amount of water, respectively untreated and treated with a magnetic field, g.

The anti-scale effect depends on the composition of the water, the strength of the magnetic field, the speed of water movement and the duration of its stay in the magnetic field and other factors. In practice, magnetic devices with permanent steel or ferrite-barium magnets and electromagnets are used (Fig. 9). Devices with permanent magnets They are structurally simpler and do not require power from the mains. In devices with an electromagnet, coils of wire are wound around a core (core), creating a magnetic field.

The magnetic device is mounted to pipelines in a vertical or horizontal position using adapter couplings. The speed of water movement in the gap should not exceed 1 m/s. The process of operation of the devices may be accompanied by contamination of the passage gap with mechanical, mainly ferromagnetic impurities. Therefore, devices with permanent magnets must be periodically disassembled and cleaned. Iron oxides are removed from devices with electromagnetic devices by disconnecting them from the network.

The results of MGSU research (G.I. Nikoladze, V.B. Vikulina) showed that for water with a carbonate hardness of 6.7 mcg-eq/l, oxidability of 5.6 mg02/l and salt content of 385.420 mg/l, the optimal magnetic field strength was (10.12.8) * 19 4 A/m, which corresponds to a current strength of 7.8 A.

The installation diagram for magnetic treatment of additional feed water of heating steam boilers is shown in Fig. 20.10.

Recently, devices with external magnetizing coils have become widespread. For magnetization large masses Apparatuses have been created for layer-by-layer processing of water.

In addition to preventing scale formation, magnetic treatment , according to P.P. Strokacha can be used to intensify the process of coagulation and crystallization, accelerate the dissolution of reagents, increase the efficiency of using ion exchange resins, and improve the bactericidal effect of disinfectants.

Rice. 9. Electromagnetic device for anti-scale treatment of water SKV VTI: 1,8 - supply of source and removal of magnetized water; 2 - net; 3 - working gap for passage of magnetized water; 4 - casing; 5 - magnetizing coil; 6 - core; 7 - frame; 9 - lid; 10 – terminals

When designing magnetic devices for water treatment, the following data is specified: the type of device, its performance, the magnetic field induction in the working gap or the corresponding magnetic field strength, the speed of water in the working gap, the time it takes water to pass through the active zone of the device, the type and its voltage for the electromagnetic device or magnetic alloy and magnet dimensions for permanent magnet devices.

Rice. 10. Layout of a magnetic installation for treating boiler water without preliminary purification.

1,8 - source and make-up water; 2 - electromagnetic devices; 3, 4 - stage I and II heaters; 5 - deaerator; 6 - intermediate tank; 7 - charging pump

Literature

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2. Stroyizdat, 1994

3. Alferova L.A., Nechaev A.P. Closed water systems of industrial enterprises, complexes and districts. M., 1984.

4. Ayukaev R.I., Meltser V.Z. Production and application of filter materials for water purification. L., 1985.

5. Weitzer Yu.M., Miits D.M. High-molecular flocculants in water purification processes. M., 1984.

6. Egorov A.I. Hydraulics of pressure tubular systems in water treatment plants. M., 1984.

7. Zhurba M.G. Water purification using granular filters. Lvov, 1980.

In one filter, the following are simultaneously removed from water: mechanical impurities, dissolved, colloidal and organic iron, manganese, natural organic compounds(humic and fulvic acids and their salts), hardness salts and heavy metals.

Price: from 32,900 rub.

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According to statistics, 90% of water heating and plumbing equipment breaks down due to hard water. Scale forms, pipelines become clogged, water heaters lose power, Appliances fails. High hardness is also dangerous for people. Sand and stones form in the organs, blood vessels and the heart suffer, the skin becomes dry, and dermatitis occurs. So that there are no accidents at home and health does not deteriorate, they produce water softening using filters.

Hardness is a property of water that depends on the content of calcium salts (Ca) in dissolved form and, in lower concentrations, silicon (Si lat. Silicium), magnesium (Mg).

• Carbonate
• Non-carbonate
• General

Carbonate is temporary. Easily removed by boiling. Determined by the presence of calcium and magnesium bicarbonates in the liquid. Chemical formula - Ca(HCO3)2; Mg(HCO3)2. Forms scale in hot water supply pipelines, in a kettle, on water heating elements of boilers and boilers.

Non-carbonate constant. Cannot be removed by boiling. It is caused by the presence of salts, which differ in properties from carbonate ones. These are mainly chlorides (CaCl2, MgCl2), sulfates (CaSO4, MgSO4).

Total hardness is the sum of the 1st and 2nd hardness. The final indicator of the content of all magnesium and calcium ions and compounds present in the liquid. Since 2014, updated standards have appeared, according to which this parameter is measured in degrees of hardness - °F = 1 mEq per liter. According to the total hardness of water:

• Hard - more than 10°F
• Medium hardness - 2-10
• Soft - up to 2

In Europe, the concentration standard is 2.5; in the Russian Federation - 7.

In well water “hard” chemical compounds come from soluble rocks, which consist of dolomite, lime, and gypsum. If the region is rich in these minerals, they will definitely be in the water. Needed water softener filter.

Why do you soften your home water?

“Hard” salts gradually accumulate in the body. Vessels become clogged. The heart suffers. Stones appear in the kidneys and other organs and cavities of the body. Urolithiasis occurs. Drinking water with high hardness causes serious harm to health. Besides:

• Scale formed on heaters and inside heating radiators reduces heat transfer
• Detergents produce little foam. Consumption of household chemicals increases by 60%
• Dishes take longer to prepare. Meat remains tough after cooking
• 1 millimeter of scale increases power consumption by 10%
• Leads to overheating of heating elements. Causes 90% of water heater failures

Hard water makes things worse appearance. The skin dries out and peels. Dermatitis, acne, and redness appear. The hair is not washed, looks untidy, and becomes unruly. Plaque forms on the teeth.

Hard water is dangerous for newborns. Increases the likelihood of eczema and atopic dermatitis with constant use and bathing. Symptoms appear as early as 3 months. Eczema causes autoimmune allergies and then food allergies and asthma.

The best protection is to buy and install water softening filter. Water treatment and purification devices soften well and make household water suitable for drinking and household use.

Strong magnets are also used in water treatment. The liquid is passed through a powerful magnetic field. As a result, the water changes its physical characteristics, dissolved impurities lose their ability to form salts, and, consequently, scale. In addition, magnetized water destroys and removes already deposited layers of scale. The technology is effective at low contents of calcium, silicon, and magnesium ions.

The liquid is exposed to a highly charged electric field using special membranes. Hardness ions and some other substances are removed. The technology is used for desalination of sea water on an industrial scale, in the production of table salt and for the preparation of water in thermal power plants.

Made using reagents. Use slaked lime Ca(OH)2, sodium orthophosphate Na3PO4 or soda ash Na2CO3. When interacting with the reagent, hardness salts become insoluble, settle to the bottom and are easily filtered out. This technology is justified when purifying large volumes of liquid. During application, a number of specific technological problems arise. An accurate dosage of the chemical reagent is needed.

The technology refers to reagent softening methods. For water purification, granular filter beds are used, mainly ion exchange resins, which are loaded into water softening filters. When interacting with resin granules, ions of “hard” compounds, as well as iron and manganese, are captured from the liquid. Depending on the type of filter material, the ion exchange process produces sodium, potassium or hydrogen ions. With a properly selected load, it is possible to reduce hardness to 0.1-0.01°F even with ultra-high mineralization.

Advantages of ion exchange filters:

• Price is 20-50% lower
• Universal. Suitable for cottages, country houses, city apartments. They are placed on wells, wells, and cut into city water supply pipelines
• Productive. Removes hardness, iron, excess minerals, manganese, organic compounds and other contaminants with one can
• Eliminates ultra-high concentrations of iron - up to 30 mg

Over time, the resins become clogged with contaminants held by chemical bonds and no longer soften the water. However, the ion exchange reaction is reversible. If you pass a solution of table salt through the resin, the impurities will separate, and the sodium contained in the salt will occupy the resulting voids. The separated pollutants are washed into the drain. The updated resin again efficiently cleans and softens water.