Seven methods for removing hardness from high-salinity wastewater
As is well known, many industries in China discharge wastewater that is high in salt and hardness.
It not only contains high levels of inorganic salt ions such as K ＋ 、Na ＋ 、Ca 2＋ 、Mg 2＋ 、Cl － 、SO 4 2－ etc., but often also contains other high-concentration pollutants, such as organic matter, ammonia nitrogen, suspended solids, and heavy metals.
Therefore, high-salt wastewater must undergo pretreatment to effectively remove hardness. Otherwise, scale-forming ions will precipitate and adhere to the membrane or tube wall during membrane concentration or evaporation concentration, forming hard scale that is difficult to remove. This will, at best, reduce the water production efficiency of the membrane and evaporation, and at worst, block the membrane, pipelines, or equipment.
What is the difference between temporary hardness and permanent hardness?
Hardness is divided into total hardness, temporary hardness, and permanent hardness. Among them,
total hardness refers to the total concentration of calcium and magnesium ions in water. The content of Ca 2+ in water is called calcium hardness; the content of Mg 2+ in water is called magnesium hardness; when the total hardness of water is less than the total alkalinity, the difference between them is called negative hardness.
Temporary hardness, also known as carbonate hardness, mainly consists of Ca(HCO 3)2 、Mg(HCO 3)2 。Because these salts decompose into precipitates and are removed from the water after heating, it is called temporary hardness.
Permanent hardness, also known as non-carbonate hardness, mainly refers to CaSO 4 、MgSO 4 、CaCl 2 、MgCl 2 、Ca(NO 3)2 、Mg(NO 3)2 and other salts. This type of hardness cannot be removed by heating decomposition, hence the name permanent hardness.
Hardness is an important indicator of water quality, and the removal of hardness from water is called water softening. Currently, in addition to traditional methods such as pH method, ion exchange softening method, and scale inhibitor descaling method, many new technologies such as ultrasonic descaling method and high-frequency polarization descaling method have emerged.
Below, we will introduce and compare the characteristics of these methods in detail. Users can find the most reasonable dehardening process based on the characteristics of the raw water and processing requirements.
Ion exchange softening method
The ion exchange softening method is mainly used before membrane treatment. Its principle is to use specific cation exchange resin to replace Ca 2+ 、Mg 2+ in the water, thereby achieving the effect of softening hard water.
It is worth mentioning that ion exchange resins can be recycled.
After the exchange resin adsorbs Ca 2+ 、Mg 2+ , its softening ability will decrease rapidly, and the ion exchange resin will also lose its activity. The ion exchange resin that has lost its activity can quickly restore its softening ability by using a sodium chloride solution, and after restoring its softening ability, it will have the ability to exchange again. This process is also called the regeneration process.
The ion exchange softening method has a simple process, little sludge, simple operation, thorough removal, and can reduce the hardness of water to below 1 mmol/L according to needs.
However, due to the limitation of the specific surface area of the ion exchange resin, saturation will occur and regeneration or renewal is required. If the amount of wastewater treated is large, the cost of regeneration or renewal is unaffordable for many enterprises.
Fluidized bed enhanced crystallization technology
The use of fluidized beds to remove hardness from water began in the 1990s. The basic principle of a fluidized bed is to use gas or liquid to keep solid particles in a suspended state of motion.
A researcher used aeration to increase the pH of wastewater to enhance crystallization. As a result, the removal rates of phosphate, Mg 2+ and Ca 2+ reached 65%, 51%, and 34%, respectively.
The removal of metal ions by the fluidized bed reactor is mainly achieved through adsorption and coprecipitation, but a certain amount of nucleation material needs to be filled in the reactor.
Nowadays, granular calcite (CaCO 3 ) and quartz sand are mainly added to the fluidized bed reactor. Its advantages are not only the effective removal of calcium and magnesium ions, but also the recycling of the produced calcium and magnesium precipitates.
Currently, there are three main process flows for water softening using fluidized bed crystallization reactors , and after comparison, it can be found that:
Process a) has the advantage of low operating costs, but also has the disadvantages of increased reagent consumption due to CO2, resulting in easy breakage of flocs and affecting floc growth, thus increasing effluent turbidity.
Process b) has no interference factors in floc growth, but there are more solid particles carried in the effluent, mainly because CaCO 3 crystallizes on the flocs formed by iron.
Process c) is the most widely used and stable and safe process at present.
pH precipitation dehardening method (double alkali method)
By adding an appropriate amount of Na 2 CO 3 or alkaline reagents such as lime, can produce insoluble precipitates such as calcium carbonate, magnesium carbonate, and calcium hydroxide, thereby removing most calcium and some magnesium ions.
Researchers have used the double alkali method to remove hardness from high-salt wastewater, and have drawn the following four conclusions:
(1) When the double alkali method is used as a pretreatment process for high-salt wastewater, sodium carbonate and Ca 2+ 、Mg 2+ The optimal reaction pH is 11.5.
(2) The addition of appropriate amounts of sodium carbonate, flocculant, and coagulant can not only effectively remove hardness through flocculation and precipitation, but also have a certain removal effect on COD and SiO 2 There is a certain removal effect.
(3) In the double alkali hardness removal process, the flocculation effect of polyferric sulfate is better than that of polyaluminum chloride, and the optimal dosage is 100mg/L;
(4) After adding an appropriate amount of soda ash, the removal rate will increase over time, and when the pH value is adjusted back, sulfuric acid reagents can further remove most of the magnesium ions.
Effect of pH value on the removal of total hardness
The pH precipitation and dehardening method has the advantages of simple operation, stable removal rate, low operating cost, and low equipment investment. The disadvantages are that the pH value will change greatly during water treatment, the amount of reagents required is high, the amount of sludge produced is large, and the removal rate is not thorough.
Scale inhibitor dehardening method
Scale inhibitors are a type of agent that can disperse insoluble inorganic salts in water, prevent or interfere with the precipitation and scaling of insoluble inorganic salts on metal surfaces, and maintain good heat transfer performance of metal equipment.
The advantages of using descaling agents are simple operation and only a small amount of agent needs to be added to ensure production.
In terms of mechanism of action, the action of scale inhibitors can be divided into three parts: chelation, dispersion, and lattice distortion.
Among them, the process of forming a complex with a ring structure by bonding a central ion and two or more coordinating atoms of the same multidentate ligand that meets certain conditions is called chelation.
Chelation can make scaling cations (such as Ca 2+ , Mg 2+ ) react with chelating agents to form stable chelates, thereby preventing them from contacting scaling anions (such as CO 3 2- , SO 4 2- , PO 4 3- and SiO 3 2- ) contact, greatly reducing the probability of scaling.
Scale inhibitors have good shielding, anti-permeation, and anti-rust properties, good scale inhibition and heat conduction, and excellent resistance to weak acids, strong alkalis, and organic solvents. Generally speaking, common methods for evaluating the performance of scale inhibitors include:
1) Static scale inhibition method; 2) Dynamic simulation method; 3) Critical pH method; 4) Bubbling method; 5) Limiting carbonate hardness; 6) Turbidity determination method; 7) Calcium ion selective electrode potentiometry; 8) Glass electrode method; 9) Constant component technology; 10) Electrochemical method for evaluating scale inhibition performance.
Electrochemical softening method
The principle of electrochemical dehardening is to adsorb charged particles in water on the surface of charged electrodes, so that dissolved salts and charged particles in water are enriched on the electrode surface, and the dissolved salts, colloids, and charged substances in the wastewater to be treated are continuously reduced, thereby achieving the purpose of dehardening.
Voltage or current density is an important parameter that directly affects the effect of electrochemical dehardening and operating cost.
Studies have shown that when the water hardness is high, scale inhibition is first performed at 7V voltage until the hardness is reduced to below 130mg/L, and then descaling is performed at 5V voltage. This method can remove more than 90% of the scaling ions in the water and reduce energy consumption.
Compared with traditional dehardening methods, the electrochemical method has a simple process flow and is energy-saving and environmentally friendly. Specifically, it has the following characteristics:
(1) No reagents need to be added, it is green and environmentally friendly, and no harmful substances are produced.
(2) The relevant electrochemical parameters can be adjusted according to the actual situation, and the applicable range is wide, suitable for various hardness wastewater treatment requirements.
Ultrasonic dehardening method
Ultrasonic descaling and antifouling equipment mainly consists of an ultrasonic generator, an ultrasonic signal transmission cable, an ultrasonic energy transducer, and a pipeline assembly for installing the ultrasonic transducer.
According to the installation method, it can be divided into three types: external, embedded, and built-in.
It mainly uses the "cavitation", "chemical", "shear", and "suppression" effects of ultrasound to treat fluids in a strong sound field, so that the scaling substances in the fluid undergo a series of changes in their physical form and chemical properties under the action of the ultrasonic field, making them disperse, pulverize, loosen, and detach, and not easily adhere to the pipe wall to form scale.
Among them, The factors affecting the ultrasonic cavitation efficiency include the following four aspects:
(1) Ultrasonic parameters: power, frequency, standing wave;
(2) Heat transfer medium parameters: temperature, flow rate, viscosity coefficient, surface tension coefficient, vapor pressure, gas content and type in liquid, concentration and type of alkaline substances in liquid;
(3) Heat exchanger parameters: pipe shape, pipe diameter;
(4) Others: environmental pressure, angle between the transducer center and the pipe, etc.
High-frequency polarization dehardening method
The principle of polarization dehardening is to increase the solubility of calcium and magnesium ions and form electrostatic molecular groups to prevent scale from forming large-particle precipitates.
It is important to note that this process does not remove the multivalent metal ions that cause hardness in water, but rather prevents the formation of scale. Its function is similar to antiscalants used in boilers, and its main characteristic is the minimal or no use of any chemical reagents.
From the principles of high-frequency polarization descaling and ultrasonic descaling, their advantages are that they do not add any reagents and do not increase the environmental burden. However, the application of such technologies in actual production is still limited.
Poor pretreatment effects have become one of the main factors restricting the development of high-salt wastewater treatment technologies at present.
According to the specific conditions and components of the high-salt wastewater being treated, selecting an optimal pretreatment method can not only comprehensively improve the overall efficiency of hardness removal but also fully reflect the economic benefits in the wastewater treatment process, thereby further achieving "zero discharge" of wastewater and effective recycling of resources.
