Industrial wastewater is an effective method for advanced treatment and purification of polluted water. In the 1960s, particulate Activated Carbon was used for the treatment of carbon disulfide wastewater in China. Since the early 1970s, Granular Activated Carbon has been used to treat industrial wastewater. Both in terms of technology, as well as in terms of application range and processing scale, they have developed rapidly. For example, large scale applications have been made in the treatment of refinery wastewater, explosive wastewater, printing and dyeing wastewater, chemical wastewater, and electroplating wastewater, and satisfactory results have been achieved.
With the wide application of granular activated carbon, the recovery of granular activated carbon has begun to gain attention. If used granular activated carbon cannot be recovered, except that the treatment cost per ton of wastewater will increase by 0.83 to 0.90 yuan, secondary pollution will also be caused to the environment. Therefore, the regeneration of granular activated carbon has an extremely important significance.
1 Traditional granular activated carbon regeneration method
1.1 Thermal regeneration
The thermal regeneration method is currently the most widely used and most industrially mature regeneration method for granular activated carbon [2, 3]. In the process of regeneration, granular activated carbon after organic wastewater treatment is generally divided into three stages of drying, high temperature carbonization and activation according to the change of organic matter when heated to different temperatures. In the drying stage, the volatile components on the granular activated carbon are mainly removed. The high-temperature carbonization stage boils off and vaporizes and desorbs a part of the organic matter adsorbed on the granular activated carbon. A part of the organic matter decomposes to generate small molecular hydrocarbons and desorption occurs. The residual components remain in the pores of the granular activated carbon to become a “fixed carbon”. At this stage, the temperature will reach 800-900°C. In order to avoid the oxidation of granular activated carbon, it is generally carried out under vacuum or inert atmosphere. In the subsequent activation stage, gases such as CO2, CO, H2, or water vapor are introduced into the reactor to clean the pores of the granular activated carbon, so that the adsorption performance is restored. The activation stage is the key to the entire regeneration process. Although the heat regeneration method has the characteristics of high regeneration efficiency and wide application range, in the regeneration process, energy must be added for heating, and the investment and operation costs are high.
1.2 Biological regeneration method
The biological regeneration method utilizes domesticated bacteria to analyze the organic substances adsorbed on activated carbon and further digest and decompose into H2O and CO2 [1, 2]. The biological regeneration method is similar to the biological method used in sewage treatment, and there are also aerobic and anaerobic methods. Since the pore size of granular activated carbon itself is very small, and some are only a few nanometers, microorganisms cannot enter such pores. It is generally considered that cell autolysis occurs during regeneration, that is, cell enzymes flow to the outside, and granular activated carbon adsorbs enzymes. As a result, an enzymatic center is formed on the surface of the charcoal to promote decomposition of the contaminants and achieve regeneration.
Biological methods are simple, low investment and operating costs, but they take a long time and are greatly affected by water quality and temperature. Microorganisms are highly targeted for the treatment of pollutants and require specialized domestication of specific substances. In the process of degradation, all organic matters cannot be completely decomposed into CO2 and H2O. The intermediate product still remains on the granular activated carbon and accumulates in the micropores. After many cycles, the regeneration efficiency will be significantly reduced. This limits the industrial application of the biological regeneration method.
1.3 Wet oxidation regeneration method
Under the condition of high temperature and high pressure, using oxygen or air as oxidant, a method of oxidizing and decomposing organic matter adsorbed on granular activated carbon in liquid phase into small molecules is called wet oxidation regeneration method . The regeneration conditions are generally 200 to 250°C, 3 to 7M Pa, and the regeneration time is usually within 60m in. The wet oxidation regeneration method has a wide range of objects. The reaction time is short, the regeneration efficiency is stable, and no additional heating is required after regeneration is started. However, for certain refractory organics, more potentially toxic intermediates may be produced.
The Institute of Environmental Studies of Tongji University used the change of phenol adsorption isotherms as the evaluation criteria, systematically studied the main influencing factors in the wet regeneration of granular activated carbon, and theoretically discussed its regularity; discussed the synergy between the main factors. Effect; The possibility of repeated recycling of saturated charcoal was investigated; and the changes in the structure of granular activated carbon itself during wet oxidation were studied. The optimum regeneration conditions for the granular activated carbon obtained in the experiment were: regeneration temperature 230 °C, regeneration time 1 h, oxygenation pO20. 6 MPa, carbon addition amount 15 g, and water volume 300 mL. The regeneration efficiency reached (45±5)%. After 5 cycles of regeneration, the regeneration efficiency only decreased by 3%. Partial oxidation of the micropores on the surface of granular activated carbon is the main reason for the decrease of regeneration efficiency.
In addition to the disadvantages of traditional granular activated carbon regeneration technology, there are usually three common defects: (1) Granular activated carbon loss during regeneration is often large; (2) Adsorption capacity of granular activated carbon after regeneration will be significantly reduced; (3) The exhaust gas generated during regeneration will cause secondary air pollution. As a result, people have either improved traditional recycling technologies or explored new recycling technologies.