Methods for removing heavy metals from wastewater
Due to the growing trend of development of various industries, alarming amounts of toxic heavy metals are introduced into the environment along with the effluents of these industries, which endangers public health, especially in developing countries. Today, heavy metals are considered as priority environmental pollutants and have become one of the most serious environmental problems. In the following, we will examine the methods of removing heavy metals from industrial effluents and we will also examine the importance of their removal
The importance of removing heavy metals from water and wastewater
Heavy metal ions have lethal effects on all forms of life and enter the food chain through waste disposal in water channels. Due to non-biodegradability, heavy metal ions are accumulated and their amount increases along the food chain. Among various metal ions, lead, mercury, cadmium and chromium are at the top of the toxicity list. Among the most important characteristics of heavy metal toxicity, the following can be mentioned
The toxicity of these metals can remain in nature for a long time
Some heavy metals can even increase their toxicity in a certain environment over time, such as mercury
The accumulation and biological intensification of heavy metals through the food chain can damage natural physiological activities and ultimately endanger human life
Therefore, in order to protect people and the environment, these heavy metals should be removed from industrial wastewater
Investigating the methods of removing heavy metals from water and wastewater
The methods of removing metals from wastewater basically include physical, chemical and biological processes. In the following, the most common methods of removing heavy metals from industrial wastewater are examined
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Chemical sedimentation method
Chemical precipitation is a very effective method that is widely used in industries because it is very simple to operate. In sedimentation processes, chemicals react with heavy metal ions to form insoluble precipitates. The sediments formed can be separated from water by sedimentation and filtration processes. Wastewater treated in this way will be suitable for disposal in the environment or reuse
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Ion exchange method
Ion exchange is an effective method for removing heavy metals from wastewater. The meaning of this method is the ability to transfer cationic or anionic ions through the solids around the ions. In this method, sodium ion can replace ions that cause water hardness. The ion exchange method is able to purify heavy metal solutions with high purity and is also able to perform continuous operations. Therefore, pre-treatment processes will be needed to reduce the concentration of suspended solids for maximum efficiency
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Membrane purification technology
Membrane purification processes are designed for physical or physico-chemical separations. The development of water and wastewater treatment processes based on membrane technology is presented in the following table
| Process | Purpose |
|---|---|
| Microfiltration | Removal of suspended solids including microorganisms |
| Ultrafiltration | Removal of soluble solute molecules and colloidal suspended particles |
| Nanofiltration | Removal (selective) of polyvalent ions and some charged molecules |
| reverse osmosis | Removal of inorganic ionic impurities |
| Electrodialysis | Extraction (selective) of ions from water or concentration of these ions in the wastewater stream |
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Reverse osmosis method
Reverse osmosis is one of the main methods of membrane purification. This method is designed based on the osmotic properties of the human body. In this way, whenever two liquids with different salt concentration are separated by a semi-permeable membrane, the flow is established from the solution with lower concentration to the more concentrated solution. This law is used to sweeten water. In the reverse osmosis method, only water passes through the membrane and all dissolved and suspended substances are separated. The reverse osmosis system needs a high quality input for effective operation, so the wastewater must be pre-treated before entering the reverse osmosis system to remove suspended solids in the wastewater
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Electrodialysis method
The basis of work in this method is the same as the reverse osmosis method, with the difference that in the electrodialysis method, electric power is used to drive heavy metal ions into the membrane. In this method, the water entering the process must have passed a high degree of purification and must be free of microorganisms
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Coagulation process
Coagulation and flocculation processes facilitate the removal of suspended solids in colloidal particle sizes. The action of coagulation includes the destabilization of colloidal particles by adding coagulants. Adding coagulants to form larger clots can be on the agenda. It can also be useful to use a stirrer to create gentle turbulence in order to gather the formed clots and settle them faster. This method has the ability to remove some heavy metals from wastewater
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Electrocoagulation method to remove heavy metals
In this method, electric current is used to remove heavy metals. This method is based on the plating of metal ions on the surface of a cathode and can recover metals in their elemental state. This method is used in the PECR-V1 advanced purification reactor
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Surface adsorption method
Today, surface absorption is known as an effective and economical method for wastewater treatment from heavy metals. In this method, heavy metals are trapped and accumulated between the solid and liquid phases and at their interface. The absorbent solid in this method is activated carbon. Activated carbon adsorbents are widely used to remove heavy metal pollution. The reason for their good performance in this field is due to the high volume of micropores and mesopores in them and the resulting high specific surface area. The surface adsorption process is flexible in design and operation and in many cases produces a quality purified effluent stream
Advantages and disadvantages of heavy metal removal methods from wastewater
Each of the mentioned methods, in addition to the advantages of application, also have disadvantages, such as being less effective in removing heavy metal ions, consuming a large amount of chemicals, producing toxic sludge in large quantities, high cost, and issues related to Safe disposal of the resulting material. Therefore, the need for new technologies that can reduce the concentration of heavy metals with acceptable costs to the acceptable levels of the environmental standard is felt. Knowing the advantages and disadvantages of these methods is necessary to choose and use these processes optimally. The following table lists the advantages and disadvantages of each of the above methods
| Method | Advantages | Disadvantages |
|---|---|---|
| Chemical deposition | Simple, cheap, most metals are removed | Production of large amounts of sludge, dependence on pH, difficulty of separation, need for chemicals |
| ion exchange | No sludge produced, high quality effluent, metal recovery possible | Sensitive to particles, high operating cost, metal deposition, fewer metals can be removed |
| Membrane processes | High quality effluent, less solid waste is produced, less chemicals are consumed | Membrane fouling, limited membrane useful life, high cost, high pressure, low flow rate |
| Coagulation and clotting | Produces very fine particles of sediment, dewatering | A lot of chemicals are needed, sludge production, high cost |
| Electrocoagulation | The ability to select the metal, not using chemicals, the ability to obtain pure metals | A large surface area of the electrode is required |
| Surface adsorption with activated carbon | Most metals can be removed, high efficiency (more than 99%) | High price of activated carbon, lack of regeneration |
Choosing the right process for wastewater treatment to reach environmental standard levels requires experience and expertise. With more than 20 years of successful experience in the treatment of industrial and sanitary wastewaters, PASENCO Company accompanies you in this important matter. In order to communicate with the technical experts of this collection, you can communicate through the communication methods mentioned on this page