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Common methods of hazardous waste disposal
Origin：Hefan Hit： Time：2019-09-19
Hazardous waste treatment methods can be divided into physical, physical, chemical and biological methods. Many of these methods are common to chemical production.
For solid waste (waste residue), common physical treatment processes include compaction, crushing and separation.
For liquid waste (waste liquid), common physical treatment processes include sedimentation, air flotation, centrifugation, filtration, distillation and so on, while blow-off, microfiltration, ultrafiltration, nanofiltration and other processes are seldom used.
Physicochemical treatment processes commonly used for waste residue include heat treatment (incineration, pyrolysis), solidification/stabilization.
Physicochemical treatment processes commonly used for waste liquor include coagulation, chemical precipitation, acid-base neutralization, redox, adsorption and desorption, ion exchange, incineration, etc., while replacement, electrolysis, extraction, electrodialysis, reverse osmosis, photolysis and other processes are seldom used.
Biological methods are only applicable to organic wastes, including composting and anaerobic fermentation, and activated sludge and anaerobic digestion.
Hazardous waste disposal refers to the incineration of hazardous waste and the use of other methods to change its physical, chemical and biological characteristics, so as to reduce the amount of waste generated, reduce the volume of solid hazardous waste, reduce or eliminate its hazardous components, or eventually place hazardous waste in conformity with the requirements of environmental protection regulations. Activities where places or facilities are no longer retrieved.
There are two main ways to dispose of hazardous wastes: geological disposal and marine disposal. Marine disposal includes deep-sea abandonment and incineration. Land disposal includes land tillage, permanent storage or storage, land landfill, deep well filling and deep formation disposal, among which land landfill disposal technology is the most widely used. Marine disposal has been banned by international conventions, but geological disposal is still the most commonly used waste disposal method in the world.
landfill
Landfill is one of the methods for the final disposal of hazardous wastes. This method includes site selection, landfill design, construction and landfill operation, environmental protection and monitoring, site utilization, etc. Its essence is to lay hazardous waste into a thin layer with a certain thickness, compact it and cover the soil. This treatment technology has been widely used at home and abroad. Landfill method is usually divided into sanitary land landfill and safe land landfill.
1. Sanitary Land Landfill
Sanitary land landfill is a method to dispose of general solid waste without endangering public health and environmental safety. It is mainly used to dispose of municipal garbage.
2. Safe Land Landfill
Safe land landfill is an improved sanitary landfill method, also known as safe chemical land landfill. Safe land landfill is mainly used to dispose of hazardous wastes. Therefore, the site construction technology requirements are more stringent. For example, the permeability coefficient of lining should be less than 10-8 cm/s, the leachate should be collected and treated, the surface runoff should be controlled, and the control and treatment of the generated gas should also be considered. In addition, there is a method of land landfill treatment, that is, shallow burial method. This method is mainly used to dispose of low-level radioactive waste.
Compared with other disposal methods, the main advantages of the land landfill method are as follows: it is a complete and final disposal method, and it is the most economical if there is suitable land for use; it is not limited by the type of waste and suitable for disposing of large amounts of waste; the land after landfill can be reused as parking lot and amusement park. Golf courses, etc. The disadvantage is that the landfill must be far away from the residential area; the recovered landfill will need constant maintenance due to settlement; the hazardous wastes buried underground may produce flammable, explosive or toxic gases through decomposition, which need to be controlled and treated.
Incineration method
Incineration is a comprehensive process of high temperature decomposition and deep oxidation. By incineration, combustible hazardous waste can be oxidized and decomposed to reduce volume, remove toxicity, and recover energy and by-products.
The incineration process of hazardous waste is complex. Because the physical and chemical properties of hazardous wastes are complex, the composition, calorific value, shape and combustion state of the same batch of hazardous wastes will change greatly with time and combustion area. At the same time, the composition of exhaust gas and the properties of waste residue will also change after combustion. Therefore, hazardous waste incineration equipment must be adaptable, flexible and have the ability to automatically adjust operation parameters to a certain extent.
Generally speaking, almost all organic hazardous wastes can be treated by incineration, and incineration is the best way to treat them. For some special organic hazardous wastes, incineration is only suitable, such as some toxic intermediate by-products in petrochemical industry.
The advantage of incineration method is that it can reduce the volume of flammable hazardous waste rapidly and substantially. For example, in some newly designed incinerators, the volume of waste after incineration is only 5% or less of the original volume. Some hazardous wastes can destroy their composition and structure or kill pathogenic bacteria by incineration treatment, so as to achieve the purpose of detoxification and elimination.
In addition, thermal energy can be provided by incineration. Disadvantages of incineration:
First, the incineration of hazardous wastes will produce a large number of acidic gases, organic components and slag which are not fully burned. If they are discharged directly into the environment, they will inevitably lead to secondary pollution.
Secondly, the cost of investment and operation management is high. In order to reduce secondary pollution, the incineration process must be equipped with pollution control facilities and complex test instruments, which further increases the cost of treatment.
Solidification method
The solidification method is that cement, plastics, water glass, asphalt and other coagulants are mixed with hazardous waste to solidify, so that the harmful substances contained in sludge are sealed in the solidified body and not leached, so as to achieve the goal of stabilization, harmlessness and reduction.
The solidification process can reduce the permeability of wastes and make them into final products with high strain capacity, thus turning hazardous wastes into harmless wastes.
1. Cement solidification method
Cement solidification is a method of treating hazardous wastes by using cement as solidifying agent. The hydration reaction will occur when the cement is mixed with appropriate proportion of water. When the cement loses its fluidity after setting, it will harden gradually. Cement solidification is a method that sludge (mixture of hazardous solid waste and water) is added to cement instead of water to solidify it.
When the harmful sludge is solidified, the hydration reaction of cement and sludge water generates gelatin, which contains the harmful sludge particles and gradually hardens to form cement solidified bodies. It can be considered that the structure of the solidified body is mainly the hydration reactant of cement. This method makes the harmful substances sealed in the solidified body to achieve the goal of stabilization and harmlessness.
Cement solidification method has higher utilization value because of its cheaper cement, simple operation equipment, high strength of solidified body, good long-term stability and resistance to heat and weathering.
The disadvantages of cement solidification method are: the leaching rate of cement solidified body is high, usually 10-4-10-5g/(cm2.d), so coating treatment is needed; because oil, organic acids, metal oxides and so on can hinder the hydration reaction of cement, in order to ensure the quality of solidification, the proportion of cement must be increased, resulting in higher compatibility of solidified body; Wastes need to be pretreated and additives added to increase the cost of treatment.
2. Plastic Curing Method
Plastics are used as coagulants to solidify sludge containing heavy metals and seal them up. At the same time, solidified materials can be used as agricultural or building materials.
Plastic curing technology can be divided into thermoplastic curing and thermosetting plastic curing according to the different plastics (resins) used. Thermoplastics, such as polyethylene and polyvinyl chloride resin, are solid at room temperature and become melt viscous liquids at high temperatures, which incorporate hazardous wastes into them and form Plastic solidified bodies after cooling. Thermosetting plastics include urea-formaldehyde resin and unsaturated polyester. Urea-formaldehyde resin has the characteristics of convenient use, fast curing speed, good curing at room temperature or heating. The curing agent formed with hazardous waste has good water resistance, heat resistance and corrosion resistance. Unsaturated polyester resin has suitable viscosity at room temperature and can be solidified at room temperature and pressure. It is easy to ensure the quality and is suitable for the solidification of hazardous wastes and radioactive wastes.
The characteristics of plastic curing method are: generally, it can be operated at room temperature; only a small amount of catalyst can be added to make the mixture coagulate; the compatibilization ratio and the density of the cured body are small. This method can treat both dry waste residue and sludge slurry, and plastic solid is non-flammable. The main disadvantage is that the aging resistance of plastic curing body is poor. Once the curing body breaks down, the leaching of pollutants will pollute the environment. Therefore, container packaging should be used before disposal, thus increasing the cost of treatment. In addition, harmful smoke released during the mixing process pollutes the surrounding environment.
3. Water glass curing method
Water glass solidification is based on sodium silicate as solidifying agent, inorganic acids (such as sulphuric acid, nitric acid, hydrochloric acid, etc.) as auxiliary agent, which reacts with harmful sludge according to certain proportions and condensation dehydration to form gel body, which will contain harmful sludge, and gradually form water glass solidification body through condensation and hardening. The solidification of sludge by water glass is based on the hardening, binding, inclusion and adsorption properties of water glass.
Sodium silicate curing process is characterized by simple operation, low cost of raw materials, low treatment cost, strong acid resistance, good water permeability and low leaching rate of heavy metals. However, this method is still in the experimental stage.
4. Bituminous solidification method
Asphalt solidification is a saponification reaction between asphalt as solidifying agent and hazardous waste under certain temperature, proportioning, basicity and stirring, so that hazardous waste is evenly contained in asphalt, forming a solidified body.
Compared with cement solidification, the leaching rate of harmful substances is lower. By using asphalt solidification, stable solidified solids can be obtained regardless of the type and nature of sludge. In addition, asphalt can harden immediately after solidification, without 20 to 30 days of maintenance like cement. However, because of the poor thermal conductivity of asphalt, the efficiency of heating and evaporation is not high. If the moisture content in sludge is large, there will be foaming and entrainment of fog during evaporation, which will easily pollute the exhaust gas. For sludge with high moisture content, before asphalt solidification, water content should be reduced to 50%-80% by separation and dehydration. Moreover, asphalt is flammable, and it must be considered that if the asphalt is heated and evaporated, it will cause great danger.
Chemical method
Chemical method is a method that utilizes the chemical properties of hazardous wastes to transform harmful substances into harmless final products by means of acid-base neutralization, oxidation-reduction and precipitation.
Many hazardous wastes can be detoxified by biodegradation, and the detoxified wastes can be accepted by soil and water. At present, biological methods include activated sludge method, gasification pond method and oxidation pond method.
