abstract
With the continuous increase of industrial production and domestic sewage discharge, the hardness ions in wastewater have a serious impact on the environment and subsequent treatment processes. This article systematically expounds the basic principles of wastewater dewatering, analyzes in detail the technical characteristics, application scope, and existing problems of various dewatering methods such as chemical precipitation, ion exchange, membrane separation, and adsorption, and looks forward to the development trend of wastewater dewatering technology in the future, aiming to provide theoretical reference for the optimization and technological innovation of wastewater dewatering processes.
1、 Introduction
The hardness of wastewater is mainly caused by the presence of calcium and magnesium ions (in the form of salts such as calcium carbonate, magnesium carbonate, calcium sulfate, magnesium sulfate, etc.) in the water. In industrial production, high hardness wastewater can cause equipment scaling, reduce heat exchange efficiency, increase energy consumption, and even lead to equipment failure; In the process of domestic sewage treatment and reuse, hard water can affect the washing effect and reduce the comfort of domestic water. In addition, the discharge of wastewater containing a large amount of hardness ions may also have adverse effects on the ecological balance of natural water bodies. Therefore, effectively removing hardness ions from wastewater is of great significance for the stable operation of industrial production, the recycling of water resources, and the protection of the ecological environment.
2、 Principle of Wastewater Dehydration
The hardness in wastewater is usually divided into temporary hardness and permanent hardness. Temporary hardness is mainly composed of bicarbonate salts of calcium and magnesium, which can be decomposed into calcium carbonate and magnesium hydroxide precipitates and removed by heating; The permanent hardness is composed of sulfates, chlorides, etc. of calcium and magnesium, which need to be removed through chemical, physical, or physicochemical methods. The basic principles of wastewater dewatering are mainly based on precipitation reaction, ion exchange, selective membrane permeation, adsorption, etc. Precipitation reaction is the use of certain chemical agents to react with calcium and magnesium ions to generate insoluble precipitates, thereby separating them from wastewater; Ion exchange method is the use of ion exchange resin to exchange calcium and magnesium ions in water and fix them on the resin; The membrane separation method is based on the difference in membrane retention ability for different ions to achieve the separation of hardness ions from water; The adsorption principle is to remove calcium and magnesium ions by adsorption through the active sites on the surface of the adsorbent.
3、 Methods for removing hardness from wastewater
（1） Chemical precipitation method
1. Lime soda ash method
The lime soda ash method is one of the most commonly used chemical precipitation methods for removing hardness. This method involves adding lime (Ca (OH) ₂) and soda ash (Na ₂ CO3) to wastewater. Lime first reacts with bicarbonate ions in water to form calcium carbonate precipitate, while magnesium ions are converted into magnesium hydroxide precipitate. Soda ash further reacts with calcium ions in water to form calcium carbonate precipitate. The reaction process is as follows:
Ca(HCO_{3})_{2}+Ca(OH)_{2}rightarrow 2CaCO_{3}downarrow +2H_ {2}O
Mg(HCO_{3})_{2}+2Ca(OH)_{2}rightarrow 2CaCO_{3}downarrow +Mg(OH)_{2}downarrow +2H_ {2}O
CaSO_{4}+Na_ {2}CO_ {3}rightarrow CaCO_{3}downarrow +Na_ {2}SO_ {4}
The advantages of this method are low treatment cost, wide range of chemical sources, and significant treatment effect on high concentration hardness wastewater. But the disadvantages are also quite obvious, such as the production of a large amount of sludge and the high cost of sludge treatment; The reaction process requires precise control of pH value and dosage of reagents, otherwise it will affect the hardness removal effect; The treated wastewater may retain a certain amount of sodium carbonate, leading to an increase in water alkalinity.
1. Phosphate precipitation method
The phosphate precipitation method involves adding phosphate to wastewater to react with calcium and magnesium ions to form insoluble calcium and magnesium phosphate precipitates. For example, sodium tripolyphosphate (Na ₅ P ∝ O ₁₀) reacts with calcium ions to form insoluble calcium phosphate precipitates. This method has high hardness removal efficiency and good treatment effect on low concentration hardness wastewater. However, this method introduces a large amount of phosphorus element, which may lead to eutrophication of water bodies, and the relatively high price of phosphate agents increases treatment costs.
（2） Ion exchange method
The ion exchange method utilizes exchangeable ions on ion exchange resins to exchange with calcium and magnesium ions in wastewater. Strong acidic cation exchange resin (such as sulfonic acid resin) is a commonly used ion exchanger, and its exchange process is as follows:
2R - SO_ {3}H + Ca^{2 + }rightarrow (R - SO_{3})_ {2}Ca + 2H^{+}
2R - SO_ {3}H + Mg^{2 + }rightarrow (R - SO_{3})_ {2}Mg + 2H^{+}
When the exchangeable ions on the resin are saturated with calcium and magnesium ions, it is necessary to use acids (such as hydrochloric acid and sulfuric acid) for regeneration to restore the resin's exchange capacity. The advantages of ion exchange method are good hardness removal effect, stable effluent quality, and the ability to meet higher water quality requirements; The equipment occupies a relatively small area and is relatively easy to operate. However, this method has the disadvantages of high resin prices and a large amount of acidic and alkaline wastewater generated during the regeneration process, which needs to be properly treated, otherwise it will cause secondary pollution; Resin has high requirements for the quality of incoming water, and impurities such as suspended solids and organic matter in wastewater can easily cause resin blockage and poisoning, affecting its service life and other issues.
（3） Membrane separation method
1. Reverse osmosis (RO)
Reverse osmosis is a technology that separates solutes and solvents in a solution through a semi permeable membrane under pressure. The pore size of reverse osmosis membrane is very small (about 0.1-1nm), which can effectively intercept calcium and magnesium ions and achieve wastewater hardness removal. In the process of removing hardness, water molecules pass through the reverse osmosis membrane under pressure, while hardness ions are intercepted, resulting in low hardness produced water. The reverse osmosis method has high efficiency in removing hardness, and the hardness of the effluent can be reduced to a very low level. It is suitable for occasions that require extremely high water quality, such as electronic industry water, boiler feedwater, etc. But its disadvantages are high operating pressure and high energy consumption; Membrane components are expensive and easily contaminated by organic matter, microorganisms, and other pollutants in wastewater, requiring regular cleaning and replacement, resulting in high maintenance costs; Meanwhile, the reverse osmosis process generates a certain amount of concentrated water, and the treatment of concentrated water is also a challenge.
2. Nanofiltration (NF)
The pore size of nanofiltration membrane is between reverse osmosis membrane and ultrafiltration membrane (about 1-10nm), and it has a high retention rate for divalent ions (such as calcium and magnesium ions), while the retention rate for monovalent ions (such as sodium and chloride ions) is relatively low. The nanofiltration process can operate at lower pressures, which reduces energy consumption compared to reverse osmosis. Nanofiltration can not only effectively remove hardness ions, but also partially remove organic matter and heavy metal ions. However, nanofiltration membranes also have the problem of easy contamination and require strict pretreatment of the incoming water; Moreover, the service life and hardness removal effect of nanofiltration membranes are greatly affected by factors such as water quality and operating conditions.
（4） Adsorption method
Adsorption method is the use of active sites on the surface of adsorbents to adsorb and remove calcium and magnesium ions. Common adsorbents include activated carbon, zeolite, bentonite, metal oxides, etc. For example, zeolite has a unique pore structure and ion exchange performance, and its exchangeable cations can undergo exchange adsorption with calcium and magnesium ions in wastewater. The adsorption method is simple to operate and has a certain treatment effect on low concentration hardness wastewater. Moreover, some adsorbents can be reused through regeneration. However, the adsorption capacity of the adsorbent is limited, which results in poor treatment efficiency for high concentration hardness wastewater; The regeneration process of adsorbents is relatively complex, and the regeneration effect is unstable, which may affect the service life and hardness removal effect of adsorbents.
（5） Other methods
1. Electrodialysis method
Electrodialysis utilizes the selective permeability and electric field effect of ion exchange membranes to induce directional migration of ions in water, thereby achieving the removal of hardness from wastewater. During electrodialysis, cation exchange membranes only allow cations to pass through, while anion exchange membranes only allow anions to pass through. Under the action of an electric field, calcium and magnesium ions in wastewater migrate to the negative electrode through cation exchange membranes, thereby separating from water. Electrodialysis method for removing hardness does not require the addition of chemical agents and does not produce sludge, making the operation process relatively environmentally friendly. However, this method requires a large investment in equipment, consumes electrical energy during operation, and has high requirements for the quality of incoming water, requiring strict pretreatment to prevent membrane fouling.
2. Microbial method
Microbial method is to use the metabolic activity of microorganisms or the reaction between extracellular polymers of microorganisms and calcium and magnesium ions to achieve the removal of hardness from wastewater. For example, some microorganisms can increase the pH value of the surrounding environment by secreting alkaline substances, which promotes the precipitation of calcium and magnesium ions; Functional groups in microbial extracellular polymers, such as carboxyl and hydroxyl groups, can also undergo complexation and adsorption with calcium and magnesium ions. Microbial methods have the advantages of low treatment cost and environmental friendliness, but the treatment process is slow and greatly affected by microbial growth conditions such as temperature, pH value, dissolved oxygen, etc. Currently, there are still certain limitations in practical applications.
4、 Comparison and selection of different methods for removing hardness
Different methods for removing hardness from wastewater have their own advantages and disadvantages, and their applicability also varies. In practical applications, it is necessary to comprehensively consider factors such as the water quality characteristics of wastewater (such as hardness ion concentration, other pollutant components, etc.), treatment scale, treatment cost, effluent quality requirements, and environmental protection requirements, and select appropriate methods for removing hardness. For high concentration hardness wastewater, chemical precipitation may be a more economical and effective method; For small-scale treatment that requires high water quality, ion exchange or reverse osmosis methods are more suitable; For low concentration hardness wastewater that is sensitive to cost, adsorption or microbial methods may have certain application potential. In addition, in many cases, a combination of multiple methods for removing hardness can be used to fully leverage the advantages of each method, improve the removal effect, and reduce processing costs.
5、 Conclusion and Prospect
The technology of removing hardness from wastewater is of great significance for ensuring industrial production, realizing water resource recycling, and protecting the ecological environment. At present, various methods for removing hardness, such as chemical precipitation, ion exchange, membrane separation, adsorption, etc., have been widely used in practical engineering, but each method has certain limitations. In the future, the development trend of wastewater de hardening technology mainly includes the following aspects: firstly, developing efficient, environmentally friendly, and economical new de hardening agents and adsorption materials to improve de hardening efficiency, reduce treatment costs and secondary pollution; The second is to strengthen the research and development of membrane materials, improve the anti pollution performance, retention rate, and service life of membranes, and reduce the operating costs of membrane separation technology; The third is to conduct in-depth research on the mechanism of microbial hardness removal, optimize microbial treatment processes, and improve their stability and treatment efficiency; The fourth is to explore the joint application process of multiple methods for removing hardness, achieve complementary advantages, and improve the overall treatment effect. Through continuous technological innovation and process optimization, wastewater dewatering technology will play a more important role in the sustainable utilization of water resources and environmental protection in the future.
