What are the common methods of urban sewage treatment? What are their advantages and disadvantages?

Several common urban sewage treatment methods

Activated sludge treatment method

At present, the so-called activated sludge process is the most widely used method for municipal domestic sewage, which has the advantages of strong treatment capacity and good water quality after treatment. Its general composition includes aeration tank, sedimentation tank, sludge discharge and reflux system. Sewage to be treated and activated sludge reflux into the aeration tank to mix, and then contact with the air in it to increase the oxygen content and produce metabolic reaction. The fully stirred mixed liquid becomes suspended, so the organic pollutants and oxygen in it can react with microorganisms. Next, it enters the sedimentation tank, and the original suspended matter will settle down and be isolated, so the effluent from the sedimentation tank is pure water. Generally, the sludge in the sedimentation tank will flow back, thus ensuring a certain concentration of suspended solids and microorganisms in the aeration tank. The reaction in the aeration tank will make microorganisms multiply, so excessive microorganisms should be discharged from the sedimentation tank to maintain the stability of the whole system. In addition to oxidizing and decomposing organic matter, activated sludge must also have a certain coagulation and sedimentation ability, so that it can be separated from the mixed liquid and then pure water can be obtained at the outlet. The disadvantage of activated sludge process is that the capital cost is too high and it is not easy to implement.

Biofilm treatment method

The so-called biofilm process is a method to treat organic pollutants in sewage through microorganisms attached to the surface of some solid objects. It is basically consistent with the development time of activated sludge process. The so-called "biofilm" is the image of microorganisms attached to the solid surface, which is generally an ecosystem composed of very dense aerobic bacteria, anaerobic bacteria, protozoa and algae. The solid medium attached to the biofilm is called carrier or filter material, so the outward biofilm can be divided into anaerobic layer, aerobic layer, adhesion layer and moving water layer. The basic operation process of the whole method is that the biofilm first absorbs the organic matter in the water layer, then decomposes by aerobic bacteria, and then anaerobically decomposes by anaerobic bacteria, and the moving water layer constantly updates the biofilm through flow, thus repeatedly realizing the purification of sewage.

Biofilm method is generally applied to the treatment of sewage in small and medium-sized cities. The treatment structure used is biological filter or biological turntable, and biological filter is generally used in southern China. Due to the continuous innovation of materials and technology, biofilm technology has made great progress in recent years. Because microorganisms in biofilm process are generally fixed on fillers, the ecosystem is relatively stable, the energy consumed by microorganisms is much smaller than that of activated sludge process, and there is less excess sludge. Biofilm method has the advantages of high efficiency, strong impact resistance, low sludge output and convenient transportation, which makes it very competitive in various treatment methods. The disadvantages of biofilm process are high cost and low unit treatment efficiency. Therefore, further reducing cost and improving efficiency are the main directions of biofilm research in the future.

Oxidation treatment method

Oxidation treatment is a widely used pretreatment method of urban sewage, which has great potential. According to the type of oxidant and reactor, it can be divided into chemical oxidation, catalytic oxidation and photocatalytic oxidation. Among them, the chemical oxidation method is relatively simple to operate, but the effect is not obvious enough, the running cost is high, and it is not widely used in practical work. In order to improve the treatment effect and reduce the cost, people have found some other oxidation technologies.

One of these new methods is photocatalysis. It is characterized by simple equipment, mild conditions, strong oxidation ability and thorough treatment effect. It is widely welcomed in sewage treatment.

Photocatalytic reaction is a chemical reaction that occurs through the action of light. In the process of reaction, molecules are transformed into excited States by absorbing light waves with specific wavelengths, and then undergo chemical reactions to form new substances, or become intermediate chemical products to promote thermal reactions. The activation energy required by photochemical reaction comes from light, and the utilization of light energy in solar energy through photoelectric conversion and photochemical conversion is a very hot research field.

Photocatalytic oxidation technology combines O2, H2O2 and other oxidants with light radiation through photo-excited oxidation. The light used is mainly ultraviolet light, including uv-H2O2, uv-O2 and other processes, which can be used to treat refractory substances such as CHCl3, CCl4 and PCBs in sewage. In addition, in the Feton system with ultraviolet light, there is a synergistic effect between ultraviolet light and iron ions, which greatly accelerates the decomposition of H2O2 into hydroxyl radicals and promotes the oxidative removal of organic matter.

The so-called photochemical reaction is a chemical reaction that can only be carried out under the action of light. In this reaction, the light energy absorbed by the molecule is excited to a high energy state, and then the electronically excited molecule carries out a chemical reaction. The activation energy of photochemical reaction comes from the energy of photons. In the utilization of solar energy, photoelectric conversion and photochemical conversion have always been very active fields in photochemical research. The application of photochemistry in environmental protection began to be studied in the early 1980s, in which the photochemical degradation of pollution was paid special attention, including photocatalytic degradation without catalyst and catalytic degradation. The former mostly uses ozone and hydrogen peroxide as oxidants to oxidize and decompose pollutants under ultraviolet irradiation; The latter is also called photocatalytic degradation, which can be generally divided into homogeneous and heterogeneous. Homogeneous photocatalytic degradation mainly takes Fe2+ or Fe3+ and H2O2 as the medium, and degrades pollutants through photo-Fenton reaction, which can directly utilize visible light. Multiphase photocatalytic degradation is to add a certain amount of photosensitive semiconductor materials into the polluted system, and at the same time, combine with light radiation with a certain energy, so that the photosensitive semiconductor is excited to generate electron-hole pairs under the irradiation of light, and dissolved oxygen and water molecules adsorbed on the semiconductor react with the electron-hole to generate radical with strong oxidation, such as OH, and then all or almost all pollutants are mineralized through addition, substitution and electron transfer with hydroxyl groups of pollutants, and finally other ions such as CO2, H2O and NO3- are generated. Compared with photochemical degradation without catalyst, photocatalytic degradation is more active in environmental pollution control.

At present, oxidation treatment has attracted wide attention because of its advantages of low cost and high efficiency. In addition, it has a good prospect in the advanced treatment of sewage and the treatment of refractory organic wastewater, and has become an active research topic at home and abroad. Many people think that oxidation will become an important method of wastewater treatment in 2 1 century.