Chemical wastewater treatment

2022-07-21 11:00
[brief introduction]
Chemical industry includes organic chemical industry and inorganic chemical industry. There are many kinds of chemical products with complex components. The wastewater discharged by chemical plants is called chemical wastewater. There are many kinds of chemical wastewater, most of which are highly toxic, difficult to purify, have a certain accumulation effect in organisms, have obvious oxygen consuming properties in water bodies, and are easy to deteriorate the water quality.
Main types
Inorganic chemical wastewater includes the industry of preparing basic chemical raw materials such as acid, alkali and salt from inorganic minerals. This kind of production is mainly cooling water. The discharged wastewater contains acid, alkali, a large number of salts and suspended solids, and sometimes sulfide and toxic substances. Organic chemical wastewater has a variety of components, including synthetic rubber, synthetic plastics, man-made fibers, synthetic dyes, paints, coatings, pharmaceutical and other wastewater discharged in the process, with strong oxygen consumption, strong toxicity, and because most of them are artificial organic compounds, they are highly polluting and not easy to decompose.
Chemical wastewater refers to the wastewater produced in the production process of chemical plants, such as the oily wastewater from the production of ethylene, polyethylene, rubber, polyester, methanol, glycol, oil tank farm, air separation air compression station and other devices. After biochemical treatment, it can generally meet the national secondary discharge standard. Now, due to the shortage of water resources, it is necessary to recycle the water that meets the discharge standard after further in-depth treatment to meet the requirements of industrial water replenishment. Since the impurities in the water are mainly suspended particles and fine wool fibers, the microporous filtration technology is used to remove the impurities based on the principle of mechanical filtration. PLC or time relay controls the working condition of the filter equipment to realize automatic backwashing and automatic operation. The lifting pump provides the water head required by the filter, and the outlet water is directly introduced into the production system.
1、 Physicochemical method
As a pretreatment or single treatment method of coating wastewater, physical-chemical method has a very significant effect in removing suspended solids, pigments, pigments and other substances, as well as solid substances and heavy metals in wastewater. Physicochemical treatment commonly includes air flotation, coagulation sedimentation, adsorption, membrane separation and other methods, of which air flotation and coagulation sedimentation are mainly used for pretreatment, and adsorption and membrane separation are used for advanced treatment.
❂ 1. Air flotation method
Air floatation method is to produce enough tiny bubbles in wastewater. The solid, liquid and gas three-phase pollutants can form a suspended state. Under the action of surface tension and buoyancy, tiny bubbles adhere to the pollutant particles to be removed, and the density of the adhesive is less than that of water, so that the pollutants in the water can be separated and removed.
Paint wastewater contains a considerable proportion of volatile components and oils. Air flotation can be used for paint wastewater treatment, that is, highly dispersed microbubbles are used as carriers to adhere the suspended solids in the wastewater, so that their density is less than that of water and float to the water surface, or the flocculation of flocculants is used to realize solid-liquid separation. The application of air flotation method can make the removal rate of suspended solids in wastewater reach 65%, and the removal rates of CODcr and BOD5 are more than 50%, which can be used as the primary treatment after homogenization.
❂ 2. Coagulation method
Coagulation is a method often used in industrial wastewater treatment. By adding chemicals, colloidal particles and micro suspended solids in water are gathered to play a separation role. Coagulation and flocculation are collectively called coagulation. It is used for the treatment of coating wastewater, mainly to remove fine suspended solids and colloidal particles in wastewater, and reduce the turbidity and pigment of wastewater. It can not only be used as a treatment method independently, but also often used in conjunction with other methods.
Quguangyuan et al. Used alkaline aluminum chloride as flocculant and polyacrylamide as coagulant aid to treat the wastewater entering the regulating tank by coagulation air flotation, and achieved satisfactory results. Han Yunhua et al. Used water-soluble carboxymethyl chitosan as flocculant to conduct flocculation test on the treatment of coating wastewater. The study found that the dosage of this flocculant was only 1/20 of the dosage of Chemical Flocculant, and it was very little affected by pH value and temperature. At the same time, the floc formed rapidly, which was non-toxic, non secondary pollution, easy to use and so on. In the process of intermittent production of water-soluble coatings, a large amount of cleaning wastewater is produced, accounting for about 65% of the total wastewater discharged. In view of the large change in the concentration of solid particles in cleaning wastewater, jewel et al. Studied the relationship between its concentration and the dosage of coagulant, which can avoid the toxicity and inefficiency caused by excessive dosing, or the production of a large amount of wastewater caused by insufficient dosing.
❂ 3. Adsorption method
Adsorption method is to use the surface of porous solid adsorbent to adsorb one or more pollutants in wastewater to achieve the purpose of wastewater purification. The unit operation of adsorption method usually includes three steps. The first is to make the waste water contact with the solid adsorbent, and the pollutants in the waste water are adsorbed by the adsorbent; The second step is to separate the adsorbent adsorbed with pollutants from the wastewater; Regenerate or update the adsorbent. According to the way of contact and separation, the adsorption operation can be divided into static intermittent adsorption method and dynamic continuous adsorption method.
After coarse adsorption and flocculation sedimentation of the coating wastewater stock solution, Liu Shicai and others treated it with composite ratio of activated carbon, and combined static adsorption with dynamic adsorption to make the coating wastewater meet the industrial wastewater discharge standard after treatment.
❂ 4. Extraction method
Extraction method is to use a specific solvent to fully mix and contact with the wastewater, so that some pollutants dissolved in the wastewater are redistributed and transferred into the solvent, and then the solvent and the extracted wastewater are separated, so as to achieve the purpose of purifying the wastewater.
For high concentration coating wastewater, Wang Jufang selects xylene extraction and sulfuric acid acidification to demulsify. The removal rate of CODCr in the wastewater is 85% - 95%, and the pretreatment effect is very significant; And most of the organic matter in the wastewater is recovered, and the extractant can also be reused. The pretreated organic wastewater is combined with other wastewater to be treated by coke adsorption, air flotation, electrolysis and oxidation pond in turn, and the purification effect is better.
❂ 5. Membrane separation method
The application of membrane separation technology in large-scale wastewater treatment and reuse is a new technology that has been accepted and developed in recent years. According to the Research Report of American business communication company, the average annual growth rate of membrane technology used in wastewater treatment will reach 6.8% by 2006.
At present, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (or) technologies are mainly used in the treatment of coating wastewater at home and abroad.
(1) Microfiltration
Microfiltration and ultrafiltration are mostly used in the pretreatment of reverse osmosis, both of which are liquid-phase separation processes driven by static pressure difference. Generally, microfiltration can intercept particles between 0.1 ~ 1 micron. The microfiltration membrane allows macromolecular organics and soluble solids (inorganic salts) to pass through, but it can block the penetration of suspended solids, bacteria, some viruses and large-scale colloids.
(2) Ultrafiltration
Ultrafiltration is suitable for separation with molecular weight greater than 500 and diameter 0.005 ~ 10 μ M macromolecule and colloid, which can be used to retain the pigment of paint. The paint in the electrophoretic paint wastewater accounts for 10% - 50% of the total paint used. If it is discharged directly without treatment, it will not only waste resources, but also cause serious environmental pollution. The electrophoretic paint in wastewater can be recovered by ultrafiltration, and the remaining water can be reused as cleaning water; At the same time, harmful inorganic salts can also pass through the ultrafiltration membrane, so as to improve the specific resistance of electrophoretic paint and improve the quality of electrophoretic paint.
At present, microfiltration and ultrafiltration are mostly used in the pretreatment of reverse osmosis. It is expected that the application will grow rapidly in the next few years. Microfiltration and ultrafiltration in wastewater reuse have accounted for more than 1/5 of the total production capacity of its equipment.
(3) Reverse osmosis
When the diluent and concentrated solution are separated by a semi permeable membrane, an external pressure is applied in the direction of the concentrated solution, and the water molecules of the concentrated solution will penetrate into the diluent side. This phenomenon is called reverse osmosis. At present, reverse osmosis technology is used to treat industrial wastewater, and the treated water and the components of the intercepted concentrate can be reused on site.
Reverse osmosis is generally used as the terminal treatment of industrial wastewater, which has a high interception rate of inorganic salts, organics, heavy metal ions, etc. the effluent quality is excellent, and can be reused as cooling water or process water for recycling. It not only saves the use of fresh water, saves production costs, but also reduces the discharge of sewage, which is of great significance to environmental protection and sustainable development.
(4) Ultrafiltration reverse osmosis coupling
Ultrafiltration technology can effectively remove most suspended solids, colloids and some organic substances attached to suspended solids in wastewater. Combined with reverse osmosis technology, i.e. using ultrafiltration as pretreatment, the reverse osmosis influent water quality can be better controlled, so as to reduce the cleaning frequency of reverse osmosis membrane and simplify the pretreatment operation. At present, 98% of car bodies in the world use electrophoretic paint as primer. In order to improve the recovery rate of electrophoretic paint and water, many foreign manufacturers have adopted ultrafiltration / reverse osmosis coupling technology.
The membrane must be cleaned regularly to maintain a certain membrane flux and prolong the service life of the membrane. The cleaning method is generally determined according to the nature of the membrane and the nature of the treated liquid. Generally, it is similar to reverse osmosis, that is, it is first washed by water, and then cleaned by different chemical detergents according to the situation. For example, ionic solubilizers can be used for electric coating materials, and "bridge bond" solvents can be used for water-soluble organic coatings.
2、 Biological method
Biological treatment is to use the metabolism of microorganisms to convert toxic and harmful organic substances into less toxic substances, and the cost is economical and reasonable. However, coating wastewater is generally high in concentration and contains a large number of toxic organics, which is difficult to be directly biodegradable; Therefore, at present, the combination of physicochemical method and biological method is mostly used at home and abroad, that is, the physicochemical method is used to fully homogenize the water quality and quantity first, so as to reduce the impact load on the subsequent biological treatment.
❂ 1. Activated sludge process
In the process of treating coating wastewater by activated sludge method, the coating wastewater and deionized water are combined for chemical flocculation and sedimentation, and then settled in the upflow contact sedimentation tank, and then enter the aeration tank. The original high concentration coating wastewater is lack of nitrogen and phosphorus necessary for microbial growth. It is mixed with domestic sewage to provide the required nutrients. At the same time, a certain amount of special nutrients are added according to the actual needs, which helps to stimulate the growth of degrading bacteria, improve the operation performance of activated sludge treatment system and improve its biodegradation efficiency. The mixed sewage is treated in the activated sludge tank and precipitated in the secondary sedimentation tank. In order to achieve the effect of three-stage filtration, the wastewater from the secondary sedimentation tank is filtered again and the water quality is analyzed. After four months of experimental treatment, the BOD of the original coating wastewater decreased from 15000 mg/l to 21 mg/l, and the total removal rate reached 99 3%。
Wang Fangyuan and others used the oil separation air flotation circulating activated sludge process filtration process to treat coating wastewater. The indicators of the effluent basically met the design requirements, and the total removal rate of COD was about 98%. In view of the difficulty of physicochemical separation of water-soluble solvents in coating wastewater, coagulation filtration can be selected as the pretreatment process, and then flow into the membrane bioreactor with good treatment effect and high degree of automation. After activated sludge aeration biochemical treatment, the COD and SS of the effluent have a good removal effect.
❂ 2. Biofilm method
Biofilm method is to make microbial groups adhere to the surface of other objects in a film shape, which can be purified by contacting with wastewater. Biological rotary table method and biological contact oxidation method with higher biochemical efficiency than activated sludge method are selected to treat comprehensive coating wastewater. Because these two biofilm processes are more effective in treating wastewater with small water volume, high toxicity and many refractory substances.
The U.S. Navy uses more than 1.1 billion liters of harmful wastewater for paint removal every year, mainly containing dichloromethane and phenol; In addition, there are alkanes, cellulose derivatives, petroleum sulfonates and naphthalene. The wastewater and domestic sewage were mixed in a ratio of 1:1, and the comparative experiments were carried out by activated sludge method and biological rotating disc method respectively. The results showed that this kind of mixed wastewater could be effectively treated no matter whether the flora was in suspension growth or adsorption growth. In addition, the activated sludge process to improve the nutritional environment (such as adding glucose to wastewater) can significantly increase the number of flora in the treatment system, which is 2 orders of magnitude higher than when using only a single carbon source. In the biological turntable system, the number of bacteria in continuous water inflow is much greater than that in intermittent water inflow, which may be attributed to two reasons: first, the concentration of toxic wastewater in the reaction system is higher in intermittent water inflow than that in continuous water inflow; Second, continuous water inflow is more conducive to the adsorption of bacteria. In addition, after a comprehensive analysis of the number and species of bacteria and their ability to biodegrade toxic compounds, it is considered that Pseudomonas and rod-shaped bacteria are the dominant bacteria to degrade this kind of wastewater.
(1) Biological turntable
The biological rotary table method has the advantages of easy film hanging, high organic matter removal efficiency, strong impact resistance, low power consumption and convenient management. In order to improve the treatment effect of biological rotary table method, the following factors must be considered. The turntable structure shall adopt multi-axis and multi-stage turntables, and each stage shall operate independently. In this way, even if one stage stops rotating, it will not affect the normal operation of other turntables. Because of the treatment efficiency of stage rotary table, the water inflow at all levels can be reduced step by step at the same time to improve the treatment effect. The assembly method of the rotary table adopts the radiant biological rotary table with honeycomb tubes. Its advantages are that it can adapt to high concentration wastewater, the hydraulic agitation is larger than the general rotary table, the specific surface area is increased compared with the general rotary table, and the oxygenation effect is good. In order to ensure the expected effect of wastewater treatment, the effluent from the biological rotary table is sent to the biological contact oxidation tower for treatment.
(2) Biological contact oxidation
Biological contact oxidation is a wastewater treatment method that has developed rapidly in recent years. It has the advantages of high treatment load, short residence time, small floor area and no sludge bulking.
In the treatment of coating industrial wastewater, after primary treatment, the treatment process of biological contact oxidation is applied, which has the advantages of short time of microbial membrane domestication, high activity and fast degradation rate of organic substances in the wastewater. The process of homogeneous oil separation, coagulation, air flotation and biological contact oxidation was used to treat the comprehensive coating wastewater, and the effluent reached the national first-class discharge standard. The research of Zuo Hongying and others shows that after pretreatment by flocculation sedimentation and air flotation, and then by secondary biological contact method, the treated water is treated by activated carbon and industrial circulating cooling water processor, and finally meets the design specification requirements of industrial circulating cooling water.
(3) Biological fluidized bed
The biological fluidized bed uses granular materials with particle size less than about 1 mm as the carrier and fills it in the aeration container. The wastewater fluidizes the carrier from bottom to top, and the carrier surface is covered with biofilm. The coating production wastewater is pretreated by primary air flotation and then treated by pure oxygen biological fluidized bed device. It has high efficiency and can be discharged up to the standard.
At present, the moving bed biofilm process (MBBR) and biological aerated filter (BAF) are combined. MBBR has strong impact load resistance, and BAF has the characteristics of integrating biological oxidation and intercepting suspended solids. The two will form a composite biofilm process to treat coating wastewater. After a period of operation, it is found that the treatment effect of the system is good and the operation is stable. When the hydraulic retention time of the single-stage reactor is 5 h and the average influent COD concentration is 800 mg / L, the COD removal rate of the final effluent reaches 93 05%。
(4) SBR process
SBR process is an intermittent activated sludge process. Aerobic bacteria in the device are fully mixed and contacted with sewage. Under the condition of oxygenation, bacteria adsorb organics in water and decompose them into their own components, so as to remove organics from water. Since the aerobic bacteria (activated sludge) will automatically settle to the bottom of the reactor without aeration, the increasing bacteria will be discharged through the sludge discharge system at the bottom of the reactor, separated from the water, and the clarified liquid will be discharged up to the standard.
SBR operation process consists of five parts: water inflow, aeration, sedimentation, drainage and idleness. Because microbial aeration and sedimentation are carried out in the same container, and sludge and water are separated

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