Method and effect of using activated carbon in organic wastewater treatment

Due to the discharge of a large amount of sewage, many rivers, lakes and other waters in China have been seriously polluted. Water pollution prevention and control has become one of the most urgent environmental problems in China. There are various methods for the treatment of water pollution. The adsorption method uses a porous solid adsorbent to transfer the pollutants in the wastewater to the solid adsorbent by using the substance transfer at the same liquid phase interface, thereby making it from the wastewater. The method of separation and removal. A porous solid substance having an adsorption capacity is called an adsorbent. According to the adsorption force on the surface of the adsorbent, it can be divided into physical adsorption, chemical adsorption and ion exchange adsorption. The adsorption process that occurs in wastewater treatment is often a combination of several adsorptions. The adsorbents commonly used in wastewater are activated carbon, sulfonated coal, zeolite, and the like. This paper discusses the application of activated carbon in wastewater treatment.

The methods for treating DMF wastewater include: activated carbon adsorption-dichloromethane regeneration method, chemical hydrolysis method and biochemical method. Both chemical hydrolysis and biochemical methods only destroy DMF without recovering DMF, and the treatment cost is high, especially for treating higher concentration of DMF wastewater. For high-density DMF (nearly 100g/L) tannery wastewater, the factory currently uses direct rectification to separate DMF from water, and the recovered DMF is recycled for production. However, the energy consumption of the method is relatively high. When the concentration of DMF in the wastewater is low (for example, less than 50 g/L), the recovery cost will be greatly increased.

The common characteristics of high-concentration organic wastewater are complex and variable composition, high chroma, BOD/COD generally <0.15, poor biodegradability and difficult treatment. It is a heavily polluted wastewater. The high-concentration organic wastewater often mentioned in the literature includes: Papermaking waste liquid (black liquor, middle water and white water), printing and dyeing wastewater, coal chemical industry wastewater (coking wastewater, gas washing wastewater), oily wastewater (oil production wastewater, metal processing wastewater, oil refining wastewater), pharmaceutical wastewater, food industry wastewater ( MSG production wastewater, brewing industrial wastewater, saccharin production wastewater, etc., as well as daily chemical, rubber, synthetic fiber industrial wastewater.

1. Application of activated carbon in physical and chemical separation technology:

The physicochemical separation technology is used for high-concentration organic wastewater for the following purposes: recovery of useful components (such as recovery of pulp and acid lignin from papermaking waste liquid, recovery of petroleum hydrocarbons from oil production wastewater, recovery of yeast protein from MSG production wastewater, Recovering raw materials from synthetic fiber production wastewater, etc.; removing most of the suspended solids in wastewater, reducing the organic load of subsequent biochemical treatment processes; removing chemical substances (such as antibiotics, polycyclic aromatic hydrocarbons, and phenols) that are harmful to biochemical processes in wastewater. , cyanogen, etc.; remove harmful substances from the water, discharge or return to production.

This technical method is aimed at separation, and the focus is not on the degradation of pollutants. Common separation methods are: mechanical filtration (including mechanical grid, sieve, centrifuge, filter press and other separation methods, mostly used in the primary treatment of wastewater, generally at the front end of the process); flocculation sedimentation or flocculation air flotation Neutralization precipitation (including acid precipitation and alkali precipitation); adsorption filtration (filter material with ceramsite or porous ceramic plate, quartz sand, anthracite, coke, synthetic fiber, walnut shell, fly ash, various types of activated carbon, etc. It is used to remove colloidal or dissolved organic matter, and is often used in primary and tertiary treatment processes, as well as gas/liquid phase absorption and separation techniques using various industrial waste gases. Activated carbon is used for the physical and chemical separation of pollutants in high-concentration organic wastewater, and has the following usages.

1.1. Used as a flocculation/adsorption separating agent in the primary treatment process:

The most typical application technology is the PACT process:

Adding appropriate amount of powdered activated carbon to petrochemical, printing and dyeing, coking industrial wastewater, can remove non-degradable COD, color and odor in wastewater, avoid aeration tank foaming phenomenon; make coagulated flocs or bioflocs grow rapidly Precipitation; removal of heavy metal ions or their complexes in wastewater.

Kunshan Huacheng Weaving & Dyeing Co., Ltd. uses powdered activated carbon and high-efficiency flocculant to treat high-color printing and dyeing wastewater, and quickly separate active organic dyes in wastewater, and achieved good treatment results. The wastewater of basic fuchsin dye production is adsorbed by high-efficiency flocculation and flue ash, and then adsorbed and filtered by granular activated carbon. The effluent water quality can reach the first-class discharge standard of GB8978-88 and is used for production. Using the first-stage treatment process to achieve three-stage treatment effect, this is a promising dyeing wastewater treatment technology, in which the role of activated carbon is indispensable. The primary treatment effect of several activated carbon and enhanced coagulation technology on the printing and dyeing wastewater of carpet factory was studied in detail. The results show that the wastewater treated by coagulation sedimentation, sand filtration and granular activated carbon can be reused for production. In addition, the activated carbon is better when it is regenerated by V2O5-Ca(ClO)2 catalytic oxidation. A treatment process for oily emulsified wastewater from small and medium-sized metal products factory is proposed. After the wastewater is oxidized, degreased and flocculated by NaClO, it is treated with suitable activated carbon adsorption, and the effluent can be discharged directly. In the process of pharmaceutical diclofenac production, a variety of high-concentration organic wastewater were discharged. A low-investment wastewater treatment process was proposed. The wastewater was acid-dissolved, slag adsorption, lime milk coagulation, and then powdered activated carbon equivalent to 3%wt of wastewater. After adsorption treatment at 60 ° C for 1 hour, the wastewater can reach the secondary discharge standard. Waste carbon is incorporated into coal for use as a boiler heat source. The technical feasibility of using PACT process for wastewater from organophosphorus pesticide production was studied. When 0.2% powdered activated carbon was added to the bio-enhanced primary treatment system, the treatment effect was better. The waste carbon can be regenerated by acid and alkali, and the recovery rate of adsorption performance is 80%.

When edible sugar is produced using sugar cane molasses as a raw material, the waste liquid discharged from the crude distillation tower contains various difficult-to-biodegrade pigment substances. The types and formation mechanism of pigment materials were studied in detail, and various decolorization materials were studied. Finally, the best decolorization material available was powdered activated carbon. When the dosage is 10% (W/V), adsorption at pH 4 ~ 5, 60 ° C for 30 ~ 40min, can nearly 100% decolorization of wastewater, while removing 56% of COD. The waste charcoal is better when it is regenerated with alkali or methanol solution.

The acidic phenol-containing wastewater discharged from the salicylic acid production process is also a kind of refractory organic wastewater. The phenol is extracted and dephenolized by using kerosene and N-503 extractant, and then the residual phenol is removed by adsorption of granular activated carbon, and the concentration of residual phenol in the effluent meets the discharge requirement. The butyl acrylate production wastewater contains various soluble compounds such as acrylic acid, phenols, benzenesulfonic acid and NaOH, which are extremely biodegradable, and the conventional treatment process cannot achieve the effluent standard.

In the traditional primary treatment process, a three-stage activated carbon fiber adsorption column is added to make the effluent COD meet the standard. The saturated ACF is regenerated with superheated steam at 200 to 500 °C. Tris(2,4-dibromophenyl)phosphate is a high-efficiency flame retardant and mildew-proof rodenticide. The phenol-containing wastewater discharged during the production process can be recycled by acid precipitation to produce raw materials 2,4-two. After bromophenol, it is filtered and adsorbed by three-stage granular activated carbon, and the effluent can reach the first-level discharge standard. Activated carbon is regenerated using superheated steam at 150 °C. In the car body electrophoresis phosphating process, the automobile manufacturer discharges a large amount of production wastewater containing surfactants, emulsified oil, polymer resin, ethers, alcohols, and amine organic compounds, and its short-term COD peak can reach 2~ 30,000, difficult to biochemical degradation, after the conventional primary treatment (flocculation, air flotation, sand filtration), with powdered activated carbon for enhanced adsorption treatment, so that the toxicity of water is reduced, the effluent can enter the biochemical process for routine biological treatment.

1.2, activated carbon used as a microbial carrier in the secondary biochemical treatment process:

The secondary treatment of wastewater, ie biochemical degradation, is a biochemical reaction process that decomposes COD and BOD of wastewater and is absorbed and digested by microorganisms. The biochemical treatment of high-concentration organic wastewater is rarely used due to frequent fluctuations in water quality and serious uneven water volume. Various forms of bioreactors that can resist high-load water quality and water impact are studies in recent years. Hotspots, carrier performance are important factors in determining the performance of such reactors. Materials which can be used as a biological carrier include various synthetic plastic products, synthetic resin materials, waste rubber particles, coke particles, activated carbon, and the like. When activated carbon is used as a biological carrier, it only acts as a carrier and adsorbs and enriches pollutants, so this usage is also attributed to the field of physical and chemical separation. The anaerobic composite biofilm reactor (20% and 10% of the total volume of the reactor) was filled with fixed elastic filler and powdered activated carbon, and the domesticated strain was inoculated. At the same time, a certain kind of suspended filler and columnar activated carbon were used to fill the aerobic composite bed. The bioreactor (30% and 10% of the total volume of the reactor, respectively) was inoculated with aerobic flora. The A/O biochemical treatment process consisting of these two reactors has achieved satisfactory results in the treatment of high-concentration organic wastewater discharged from fiberboard production. The anaerobic treatment process of papermaking black liquor was carried out by using powdered activated carbon and dendritic gum as anaerobic carrier, which proved that powdered charcoal has a special effect of accelerating the maturity of granular sludge. In the design of ramie textile printing and dyeing wastewater treatment process, activated carbon, plastic ring, petroleum coke and diatom ball are selected as the biofilm carrier of the three-phase biological fluidized bed reactor, so that the amount of activated sludge per unit volume is 40-50 mg/L. The sewage treatment capacity is 20 to 60 times that of the biological turntable, and the COD removal load is as high as 2.5 kg/m3 of the carrier.

1.3. Application of activated carbon in the third-stage deep treatment of wastewater:

High-concentration organic wastewater can be discharged to the standard through primary physicochemical and secondary biochemical treatment. The purpose of adding three-stage advanced treatment is to make the effluent meet the requirements for water quality for production reuse. The advanced treatment and reuse technology of wastewater is the focus of China's water strategy in the future. Three-stage advanced treatment methods for wastewater include membrane filtration, electrodialysis, ion exchange, filtration (filter materials such as sand and ceramsite), adsorption of activated carbon, etc., which are mostly physicochemical methods. Activated carbon has two uses therein, one is ordinary adsorbent. The other is the O3/biochar method (activated carbon as a biofilm carrier). The industrial water consumption of Qilu Petrochemical Rubber Factory is seriously insufficient. Yu Zuiting et al. physicochemical and biochemical treatment of a large amount of high-concentration wastewater discharged from the production process of the plant, followed by secondary coagulation sedimentation, disinfection, sand filtration, activated carbon adsorption, reverse osmosis, etc. The series of advanced treatment research shows that the fresh water quality obtained after the advanced treatment is better than the local groundwater and can be used as the production process water.

The wastewater produced by the Chinese medicine factory is also a high-concentration organic wastewater with complex and variable water quality. After the biochemical treatment (hydrolysis acidification, aeration oxidation), the wastewater is difficult to meet the standard. A Chinese medicine factory in Xi'an uses a granular activated carbon adsorption tower as the final water quality control equipment. In order to ensure the discharge of heavy oil, the steam flooding will produce a large amount of oily wastewater. The reuse of oil production wastewater is an important measure for water saving in oil fields. The quality of recycled water must meet the water supply standard of SYJ10027-93 boiler. The research proposes that the first-stage treatment of dissolved air flotation, secondary treatment of aerobic aeration, and then three-stage treatment by coagulation sedimentation, sand filtration and activated carbon adsorption can meet the water quality requirements of boilers. After the wastewater from the Guangzhou Petrochemical Complex was treated by oil separation, flotation and oxidation ditch, the effluent COD and NH4+-N could not reach the discharge standard. Pang Jinyu et al., the biofilm reactor was used to replace the oxidation ditch for secondary biochemical treatment, and the wastewater was re-treated. After treatment by activated carbon adsorption tower, the effluent can reach the secondary emission standard. The printing and dyeing wastewater discharged by Shandong Tenghua Dyeing Co., Ltd. is subjected to aerobic aeration, effluent from the secondary settling tank and then treated by fiber filtration and two-stage activated carbon adsorption tower. The water quality meets the technical requirements for production reuse, and a sugar paper factory in Nanning (with sugar cane) The slag is the raw material papermaking. The middle section wastewater is treated by coagulation, biochemical, O3/biological activated carbon, and the effluent meets the drinking water hygiene standards.

2. Activated carbon is used in the chemical degradation reaction unit of organic wastewater:

In order to reduce the biochemical treatment load of high-concentration organic wastewater, various biochemical pretreatment reactors based on the principle of electrochemical reaction have emerged in recent years. The reactors currently have the following types: 1. Oxidation flocculation composite reactor (OFR), with electrode plates and catalysts, air as oxidant, requiring electrical energy; 2. Iron-carbon micro-electrolysis reactor, air as oxidant, no The external energy supply is required, but the PH value of the wastewater should be within a reasonable range. 3. The particle group electrode electrolysis reactor consumes electric energy and the air acts as an oxidant.

The principle of electrochemical treatment (sometimes called electrolysis) for the treatment of highly concentrated organic wastewater is to achieve the degradation and flocculation effect of organic matter by the combined effects of redox, electro-accumulation, catalysis, coagulation and adsorption filtration of electrochemical reactions. The continuously generated ·OH radicals and new ecological H in the reactor can break the macromolecule and improve the biodegradability of the wastewater. In addition, since the current electrolytic reactor is generally filled with acid-activated iron filings or other iron-containing materials. The new ecological Fe2+ ion generated by the electrochemical reaction is a viable coagulant, and also a part of the organic matter in the wastewater is flocculated and precipitated. Iron-carbon micro-electrolysis reactor, also known as internal electrolysis reactor, has many research reports because it does not consume electricity. This type of reactor is mainly made up of a certain particle size of machined scrap iron scraps, which is used as the main filler after acid boiling. The coke, graphite, fly ash and the like are used as the bed layer loosening agent and the cathode of the internal electrolysis cell. When the amount of carbon is high, the activated iron filings can be directly used as a filler for the internal electrolysis reactor. In theory, activated carbon replaces coke, graphite, etc. as auxiliary fillers and cathode materials for internal electrolysis reactors, but it has not been reported so far.

Tao Longqi et al proposed that in order to accelerate the electrochemical reaction and oxidation reaction rate of organic matter in wastewater, methods such as increasing the overpotential of the electrode and the surface area of ​​the electrode, optimizing the electrode material and improving the electrode structure can be employed. They chose titanium as the electrode substrate, and the surface was covered with a mixture of SnO2 and Sb2O3 to form a composite oxygen evolution electrode; the appropriate activated carbon was selected as the particle group electrode to enhance the mass transfer process; and the anode or cathode of the electrolytic cell was further selected. The chamber is filled with conductive particles as an epitaxial portion of the plate electrode to accelerate the reaction rate. The composite particle group electrolytic reactor can significantly improve the removal rate of COD and chromaticity when used in high-concentration organic wastewater treatment. The role of activated carbon in it is to make the organic matter concentrate on its surface and be concentrated and degraded. The external electric field can make the adsorption process of activated carbon easier (the activated carbon has certain conductivity), and the oxidative degradation rate is greatly accelerated. The amount of saturated adsorption of activated carbon in the reactor is not a necessary performance index.

2.1. Treatment of dye wastewater by activated carbon adsorption:

The development of the textile industry has led to the development of dye production. The survey shows that more than 700,000 tons of dyes are produced every year in the world, 2% of which are directly discharged into the water body in the form of waste water, and 10% are lost in the subsequent textile dyeing process. The dye wastewater has complex composition, large water quality changes, deep color, large concentration and difficult handling. There are many treatment methods for dye wastewater, mainly oxidation, adsorption, membrane separation, flocculation, and biodegradation. Each of these methods has its own advantages and disadvantages. The adsorption method utilizes the adsorption of adsorbents on the pollutants in the wastewater to remove pollutants. The adsorbent is a porous material with a large specific surface area. Activated carbon is one of the most effective adsorbents at present. It can effectively remove the color and COD of wastewater. The activated carbon treatment dye wastewater has been studied at home and abroad, but most of them are coupled with other processes, among which activated carbon adsorption is mostly used for advanced treatment or activated carbon as carrier and catalyst, and activated carbon is used alone. Little research has been done on the treatment of higher concentrations of dye wastewater.

Activated carbon has a good decolorization effect on dye wastewater. The decolorization of acid fuchsin wastewater is the easiest, followed by alkaline fuchsin wastewater, and the active black B 133 wastewater is the most difficult. The decolorization rate of dye wastewater increases with the increase of temperature, pH value It does not have much influence on the decolorization effect of dye wastewater. Under the optimal adsorption conditions, the decolorization rates of acid fuchsin, basic fuchsin and reactive black B 133 dye wastewater are over 97%, the chromaticity dilution ratio of effluent is not more than 50 times, and COD is less than 50mg/L. National level emission standards. Considering that the separated activated carbon still has partial adsorption capacity, and the activated carbon is expensive. Therefore, these activated carbons can be used to treat the dye wastewater to a lower intermediate concentration, and then the new activated carbon can be used to bring the dye wastewater at an intermediate concentration to the discharge standard in order to reduce the cost.

Wastewater containing toxic and difficult-to-degrade pollutants such as aromatic compounds, due to its stable structure and poor biodegradability, is difficult to be effective in conventional treatment methods, and has become a technical problem that needs to be solved in the current water treatment field in China. Advanced oxidation technology and activated carbon (AC) adsorption are two widely studied methods.

In recent years, electrochemical advanced oxidation technology, as a newly developed advanced oxidation technology, has attracted great attention due to its high processing efficiency, simple operation and environmental friendliness. It generates an effective oxidizing hydroxyl radical to effectively degrade pollutants through electrode reaction. Studies have shown that when the concentration of organic pollutants is low, mass transfer will become a controlling factor, causing the degradation process to occur only on the anode surface and rarely in the solution body, and the anode poisoning due to the retention of degradation intermediates. Thereby reducing the processing effect. On the other hand, activated carbon has been widely used in wastewater treatment due to its extremely strong adsorption capacity. However, its cost is high, and it is easy to adsorb and saturate. If it is not recycled, it will not only be economical but also pollute the environment. Commonly used regeneration methods such as thermal regeneration and chemical regeneration. High temperature or high pressure conditions are required, and the cost is high. Recently, the electrochemical regeneration method has attracted the attention of researchers, and its regeneration efficiency can reach 85% under normal temperature and pressure. However, the current reported electrochemical regeneration method lasts for 5 hours, mainly because:

(1) Conventional electrodes such as graphite are used, and active species such as hydroxyl radicals are not easily generated, and the oxidizing property is not strong, resulting in incomplete regeneration.

(2) The regeneration device rarely considers mass transfer, resulting in long regeneration time.

Based on the above research background, a new "phase transfer" wastewater treatment method that combines activated carbon adsorption and electrochemical advanced oxidation is proposed. First, the organic pollutants are quickly adsorbed through the activated carbon fluidized bed. Then, the activated carbon on-site regeneration is realized by a special electrochemical device in the bed, so that the organic pollutants transferred to the activated carbon are degraded, and the activated carbon can ensure the repeated operation of the system after regeneration. At present, the regeneration of activated carbon has certain limitations, which limits the application of activated carbon. If the regeneration problem is solved, the application of activated carbon in the treatment of wastewater will be more extensive.

2.2. Treatment of chromium-containing electroplating wastewater by fly ash activated carbon:

According to statistics, the annual discharge of electroplating wastewater in China is about 40×108m3, which contains not only highly toxic components such as cyanide, but also metal ions such as chromium, zinc, copper and nickel. Chromium is a kind of metal raw material with a large amount in electroplating. In the wastewater, hexavalent chromium exists in the form of CrO2-4, HCrO-4, Cr2O2-7, etc. depending on the pH value. The study found that hexavalent chromium is carcinogenic and its toxicity is 100 times stronger than trivalent chromium. There are many treatment methods for chromium-containing electroplating wastewater, mainly including chemical precipitation method, activated carbon method, electrolysis method and membrane treatment method. In the activated carbon method, fly ash activated carbon is used as adsorbent and reducing agent for electroplating wastewater containing Cr(VI). deal with.

The pH value has a great influence on the adsorption amount and removal rate. When the pH value is about 3, the adsorption amount reaches the maximum, and the Cr(VI) removal effect is the best. When the pH value is too high or too low, the fly ash activated carbon is applied to Cr (VI). The adsorption capacity is low.

The adsorption time has a certain influence on the adsorption amount and removal rate. With the extension of time, the adsorption amount and removal rate increase. When the time is 1.5h, the adsorption is basically complete, the time is further extended, and the adsorption amount and removal rate increase. Not obvious. Cr(VI) adsorbed by activated carbon is chemically reduced to form Cr3+. Under acidic conditions, Cr3+ is desorbed from activated carbon, so that activated carbon can be regenerated. The method of regeneration is to soak the activated carbon with 5% H2SO4 solution to completely desorb Cr3+. Then rinse with water and dry. The effect of activated carbon on the removal of Cr (VI) was slightly reduced. The Cr3+ solution is neutralized with a base to form Cr(OH)3, and Cr(OH)3 can be recycled to prevent secondary pollution.

3. Review and outlook:

High-concentration organic wastewater is characterized by complex pollution and frequent changes in water quality and quantity. It is difficult to treat industrial wastewater, and it is difficult to find a treatment method with universal applicability. Various activated carbon adsorbents have been widely used in the treatment of such wastewaters due to their special application properties.

In the primary physicochemical treatment process of wastewater, activated carbon is mainly used for the adsorption of organic pollutants that are difficult to biodegrade or poisonous to microorganisms or assists in the physicochemical separation process such as flocculation.

In the secondary biochemical degradation treatment process, activated carbon is used as a biofilm carrier filler for various new high-load biochemical reactors to enrich organic matter and increase the rate of biodegradation reaction and the final conversion rate. At its root, the physicochemical separation characteristics of activated carbon are still utilized.

In the three-stage deep treatment process of wastewater, activated carbon is mostly used to adsorb residual toxic and harmful substances in water, so that the effluent water quality meets the requirements of production reuse index.

Various electrolytic reactors based on the principle of electrochemical reaction are biochemical pretreatment devices dedicated to the treatment of some special high-concentration organic wastewater. The activated carbon has a high fixed carbon content, a certain conductivity and a large specific surface area and adsorption capacity. There is great potential for application in reactors, and research and development needs to be strengthened.

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