Technical Field of the Invention

The invention relates to a technically and economically advantageous hard water softening system that provides the recovery of salt-containing regeneration wastewater with a low carbon footprint and to the operating method thereof. In particular, the invention relates to a closed-circuit ion resin regeneration system for water softening and to the operating method thereof.

State of the Art

Water is the source of life for all living things in nature and contains minerals or ions such as sodium, potassium, calcium, magnesium, and phosphate. Water is classified as hard water and soft water according to the type and ratio of minerals or ions in it. Water containing high levels of dissolved calcium and magnesium ions is called hard water. The ions it contains cause the structure of the water to be hard and its taste bitter. Soft water, on the other hand, is water with a higher sodium content and a small concentration of calcium and magnesium [1],

Minerals or ions that cause hardness in hard water containing high dissolved calcium and magnesium ions lose their solubility rapidly over time or by heating the water, and these minerals or ions begin to stick to the surfaces where the water passes. Due to the adhesion of minerals or ions to the surface, the inside of the water pipes is quickly filled, and the water pressure and flow are reduced. In addition, hard waters have a higher boiling point due to their high ion concentration and therefore more energy is consumed in heating these waters. Increasing calcification on the surfaces where the water is heated increases the electricity consumption as it causes insulation. The calcification in the heating system causes an increase in fuel consumption. Due to said negativities, calcium and magnesium ions that cause hardness in the water are not desired in the processes that use water in the majority of industries. For this reason, hard water is subjected to water softening process to remove the hardness [2],

Water softening is the removal of calcium, magnesium and some other metal cations in hard water during water treatment. The resulting soft water requires less soap for the same cleaning effort, as soap is not wasted mopping up calcium ions. Soft water also extends the life of the plumbing by reducing or eliminating lime build-up in pipes and fittings. Water softening is usually accomplished using lime softening or ion exchange resins but is increasingly done using nanofiltration or reverse osmosis membranes in water filter systems. In the softening process, reverse osmosis, cationic ion resin and lime/soda softening methods are used to remove hardness ions originating from calcium and magnesium [3].

The reverse osmosis method, one of the water softening processes, is a water purification process that acts as the cell membrane used to remove unwanted molecules and particles larger than drinking water. The reverse osmosis process can also be defined as the process of filtering unfiltered water by applying pressure through a semi-permeable membrane. It is a completely natural method. Membrane filter is used in Reverse Osmosis technology. The membrane has pores large enough to accept water molecules for passage, and hardness ions such as calcium ions (Ca ²⁺ ) and magnesium ions (Mg ²⁺ ) cannot pass through these pores. Since high-cost membranes and special equipment are used in the reverse osmosis method, the investment cost of this method is high. It produces water close to pure water at high pressure (40-55 bar), but due to the high pressure applied, the carbon footprint and operating cost are very high due to the energy cost. Since reverse osmosis is a filtration process, 40-50% of the treated water comes out as concentrated waste. Since there is water loss at the same rate with concentrated waste, it is a method that should not be preferred except in mandatory cases [4],

Another water softening process is the cationic resin method. In the cationic resin method, the sodium ion taken from saline water is replaced by the calcium and magnesium ions in the water during backwashing performed by the water softening systems. Thus, the limeforming ions are removed. When the resin reaches saturation, it is washed with salt and used again. Cationic resin method is the hardness removal method with the lowest investment cost compared to other methods. The operating cost is very low and water loss is very low compared to reverse osmosis. Water loss is due to the formation of 6-15% saline regeneration wastewater. While salty wastewater causes environmental effects such as barrenness in soils and changes in water habitat in fresh waters, salt loss along with 6-15% water loss causes significant financial loss in industries [5].

Another water softening process is the lime/soda method. In the lime/soda method, hard water is first treated with a lime, calcium oxide (CaO) or calcium hydroxide (Ca(OH)2) lime, and then with caustic soda. In this method, the hardness in the water is removed by precipitation as calcium carbonate or magnesium hydroxide. Lime is added either as calcium hydroxide or calcium oxide, and soda as sodium carbonate or sodium hydroxide. There is a very high consumption of chemicals in the lime/soda method. It requires very large mixing and settling tanks in the treatment of all waters. It needs a very high area compared to other water softening methods. Since the pH value is brought to around 11 with chemicals in the softening process, very high acid is consumed when the pH is lowered again. There is also a very high amount of sludge [6].

Patent application CN201911319867A is in the state of the art relates to a zero-emission treatment method and system for sewage wastewater with high salt content and high chemical oxygen demand (COD). Said method comprises the process steps of “Performing biochemical treatment on wastewater; realisation of separation and removal of electrochemical pollution to obtain purified wastewater: carrying out electrochemical treatment on treated wastewater in an electrochemical treatment tank by applying a direct current voltage between a sacrificial anode and a combined anode containing an inert anode and a cathode to remove inorganic ammonia nitrogen impurities”. The method also comprises chemical softening, filtration and separation, reverse osmosis and nanofiltration salt separation. Chemical softening: The second stage purified wastewater is transported to the softening reactor and the wastewater is further softened by adding sodium carbonate (Na2COs) and/or sodium hydroxide (NaOH) to obtain the third stage purified wastewater. In the system or method described in the said patent application no CN201911319867A, there is the same amount of water loss with the concentrated waste, and therefore the carbon footprint is high.

Due to the use of high cost membranes, special equipment, high amounts of chemicals and very large mixing and settling tanks, in the water softening systems or methods in the state of the art, and due to the high investment cost of these systems or methods, the high carbon footprint due to the concentrated waste of these systems or methods, environmental effects such as barrenness in soils, change of water habitat in fresh waters, and sludge formation, it is necessary to develop an environmentally friendly and cost- effective system that minimizes waste generation, has a low carbon footprint for the softening of hard water,

Brief Description and Aims of the Invention

The invention describes a hard water softening system. In said hard water softening system, a closed-circuit ion resin regeneration and water softening method is used. With the invention, a technical and low-cost water softening system that provides the recovery of salt-containing regeneration wastewater with a low carbon footprint and the operation method thereof are provided.

The most important aim of the invention is to provide a hard water softening system that does not harm the environment and has a low carbon footprint. In the invention, the closed-circuit ion resin system and the water softening method with closed circuit ion resin regeneration are explained. In the system that is the subject of the invention, magnesium ions (Mg ⁺² ) and calcium ions (Ca ⁺² ) in the wastewater containing salt released during the water softening process react with sodium carbonate (Na2COs) to form calcium carbonate (CaCOs) and magnesium carbonate (MgCOs) solids and these solids precipitate. Thus, the ions in the water are eliminated from the media.

Another aim of the invention is to provide a low-cost hard water softening system and a hard water softening method that ensure savings. In the system and its method that are the subject of the invention, only 1 -2% of saline water by volume is treated without pH balancing, that is, without the use of additional chemicals. A low-cost hard water softening method is provided by not using any additional chemicals. In addition, approximately 1 - 1 .7 tons of salt is used to treat 1000 m ³ of water in the industry and these salty waters are released to the environment. This also means a high financial loss for the water sent along with the salt. With the invention, closed circuit use of salt is ensured, and the saline water formed as a result of ion resin regeneration is completely prevented from being discharged to the environment.

Another aim of the invention is to provide a hard water softening system and method that minimizes sludge formation. The sludge formed in the invention is only the calcium and magnesium in the water to be purified. In the method that is the subject of the invention, only 1 -2% of saline water by volume is treated without pH balancing, that is, without the use of additional chemicals. In the method that is the subject of the invention, sludge formation is minimised by means of the fact that no additional chemicals are added from the outside.

Another aim of the invention is to provide a hard water softening method that prevents inorganic blockages in wastewater recycling systems. Since the hardening ions are completely disposed of as sludge at the source with the invention, the control of blockages caused by hardness ions in the recycling of wastewater is fully ensured.

With the invention, a hard water softening system that do not harm the environment, have a low carbon footprint, have a low cost, minimize sludge formation, and prevent inorganic blockages in waste water recycling systems and the operation method thereof are provided.

Description of Drawings

Figure 1. Water softening system with closed circuit ion resin.

Description of the References in the Figures

1. Hard water tank

2. Pump 3. Cationic sodium resin

4. Saline wastewater tank

5. Salt recovery/settlement tank

6. Saline water/regenerant tank

7. Production facility

8. Sludge (CaCOs, MgCOs) tank

Detailed Description of the Invention

The invention relates to a hard water softening system that do not harm the environment, have a low carbon footprint, have a low cost, minimize sludge formation, and prevent inorganic blockages in waste water recycling systems and the operation method thereof. In particular, the invention relates to a closed-circuit ion resin regeneration system for water softening and to the operating method thereof.

Water softening system with closed circuit ion resin regeneration that is the subject of the invention comprises;

• hard water tank (1 ) comprising the hard water to be softened,

• the pump (2) positioned between the hard water tank (1 ) and the cationic sodium resin (3), which ensures that the hard water desired to be softened is drawn from the hard water tank (1 ) to the cationic sodium resin (3),

• cationic sodium resin (3) for the removal of magnesium (Mg) and calcium (Ca) ions in hard water,

• Saline wastewater tank (4) for the saline wastewater formed after the water softening process to fill in,

• Salt recovery/settlement tank (5) to ensure the recovery of the salt contained in the salt-water solution,

• Saline water/regenerant tank (6).

The operation method of the closed-circuit ion resin regeneration and water softening system, which is the subject of the invention, comprises the process steps of softening the hard water with the cationic sodium resin method and then treating the salt-water solution used during the regeneration process. The capacity of the cationic sodium resin (3), the classical capacity of which is 1000 milliequivalents/liter (meq/l) at 25°C, is filled as a result of the ion exchange reaction during the water softening process. Cationic sodium resin (3), the capacity Of which is full, is regenerated with a salt-water solution containing 10-15% sodium chloride (NaCI) salt. The pH value of said salt-water solution is brought above 10 with a base. After the regeneration process, the regeneration salt-water solution is first taken to the saline wastewater tank (4) and then to the salt recovery/settlement tank (5). Sodium carbonate (Na2CO3) equivalent (in meq/L) to the hardness ions found in the salt-water solution taken into the salt recovery/settlement tank (5) is added. As a result of the addition of Na2COs, magnesium ions (Mg ⁺² ) and calcium ions (Ca ⁺² ) enter into a displacement reaction with Na2COs, forming calcium carbonate (CaCOs) and magnesium carbonate (MgCOs) solids and these solids precipitate. Equivalent to the calcium retained in the resin, that is, 2 moles of sodium ions (Na ⁺¹ ) and 1 moles of sodium carbonate (Na2COs) are given to the salt-water solution for 1 mole of calcium or magnesium hardening ions. Thus, during the regeneration with salt, the calcium ion (Ca ²⁺ ) and magnesium ion (Mg ²⁺ ) ions that pass into the regeneration wastewater are precipitated by passing them to the solid phase as calcium carbonate (CaCOs) and magnesium carbonate (MgCOs) in the salt-water solution. Thus, by adding Na2COs equivalent to Ca ²⁺ and Mg ²⁺ ions only in the salt-water solution, closed-circuit salt-water solution is recovered. Said precipitation reactions are as follows:

Mg ²⁺ (dissoived) + Na2COs — MgCOs(soiid) + 2Na ⁺

Ca ²⁺ (dissoived) + Na2COs — ► CaCOs(soiid) + 2Na ⁺

Carbonate precipitation is achieved without the use of acid-base or additional processes such as carbonate evaporation. While the precipitated calcium carbonate and magnesium carbonate solids are disposed of as sludge, the purified salt-water solution is reused in the closed circuit. Additional rinsing wastewater as much as the salt-water solution, which decreases with the disposal of the sludge, is taken into the tank and salt is added at an amount of salt that decreases by 1 -3%. The water softening system with closed circuit ion resin regeneration, which is the subject of the invention, is shown in Figure 1 . The working method of the water softening system with closed circuit ion resin regeneration that is the subject of the invention, and does not harm the environment, has a low carbon footprint, has a low cost, minimizes sludge formation, and prevents inorganic blockages in waste water recycling systems comprises the process steps of: i. taking the water taken from the water source into the hard water tank (1 ), ii. drawing the hard water in the water tank (1 ) into the cationic sodium resin with the pump (2), ill. removing the hardness ions present in hard water in cationic sodium resin (3), iv. supplying the soft water obtained after removing the hardness ions to the production processes, v. regenerating the cationic sodium resin (3) with a salt-water solution to restore its ion exchange capacity, vi. transferring the salt-water solution first to the saline wastewater tank (4) and then to the salt recovery/precipitation tank (5) after the cationic sodium resin is regenerated, vii. adding sodium carbonate (Na2COs) equivalent (in meq/L) to the hardness ions present in the salt-water solution taken to the salt recovery/settlement tank (5) and thereby formation of calcium carbonate (CaCOs) and magnesium carbonate (MgCOs) solids by magnesium ions (Mg ⁺² ) and calcium ions (Ca ⁺² ) entering into a displacement reaction with Na2COs, viii. transferring the precipitated calcium carbonate (CaCOs) and magnesium carbonate (MgCOs) solids to the sludge (CaCOs, MgCOs) tank (8), ix. transferring the salt-water solution purified from calcium carbonate (CaCOs) and magnesium carbonate (MgCOs) solids to the saline/regenerant tank (6), x. adding additional rinsing waste water to the saline/regenerant tank (6) as much as the salt-water solution that decreases with the disposal of the sludge, and adding salt equal to the amount of salt that is reduced by 1 -3%, and xi. recovering the obtained salt-water solution to cationic sodium resin. The comparison of the closed-circuit ion resin regeneration and water softening method, which is the subject of the invention, with the reverse osmosis, cation resin and lime/soda methods in the state of the art is shown in Table 1 .

Table 1. Comparison of the closed-circuit ion resin and water softening system of the invention and the operation method of this system with reverse osmosis, cation resin and lime/soda methods in the state of the art

As can be seen from Table 1 , water softening methods in the state of the art include many chemicals such as lime, soda, caustic, acid, sludge conditioning, antiscalant, acid, base washing. In the method that is the subject of the invention, a low-cost method is provided thanks to the fact that it contains only a low amount of soda. In addition, in the method that is the subject of the invention, very low amount of sludge formation is observed in contrast to the state of the art. As it is given in Table 1 , since the closed circuit and waste water-free regeneration process is applied only for 1 -2% saline with this invention, the required area for the treatment system is very low at the same rate. Applying a similar lime/soda treatment for all waters means that similar treatment is 98 times larger. In the similar lime/caustic/soda method applied for all waters, the pH value of all waters must first be increased and then lowered with the use of acid in the same way. In the invention, only 1 -2% saline treatment of water is provided without pH balancing, that is, without the use of additional chemicals. Again, since the lime/soda method is applied to all water to be treated, excessive chemical sludge is produced. The sludge formed by the invention is only the calcium and magnesium in the water to be purified. It was determined that 90% less sludge was formed in the sludge calculation compared to this method. Because the lime formed in the lime/soda method is not only formed by calcium and magnesium in the water, but also by the lime added to all the water from the outside. In the lime/soda method, after the water is softened, carbon dioxide is given to all the treated waters and the pH value of the treated water is provided to evaporate the carbonate ions. The whole water carbonate evaporation and pH balancing step process is not included in the invention. Since the hardening ions are completely disposed of as sludge at the source with the invention, the control of blockages caused by hardness ions in the recycling of wastewater is fully ensured. Table 2. Application results of closed-circuit water softening system.

The method of the invention was used for the treatment of regeneration wastewater discharged as wastewater at a pilot scale. A total of 30 regenerations were made and the regeneration wastewater was reused. No adverse effects were observed in water softened by reuse, and the hardness of softened water in the system was consistently lower than 2 French hardness (Fr). In many facilities such as textiles, softened water hardness is generally requested below 5 Fr hardness. The results obtained are shown in Table 2. According to the results obtained, it is seen that the system provides a salt and water recovery of over 95% when it is operated regularly. The aforementioned French hardness (Fr) is the unit commonly used in hardness classification in our country. It is used to define the concentration of hardness ions in water. 1 Fr degree is equal to 10 mg/lt CaCOs hardness.

REFERENCES

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