In view of the seriousness of water pollution in China, the construction of urban sewage treatment plants in China has been continuously strengthened. The requirements for nitrogen and phosphorus in pollutant discharge standards are becoming more and more strict. New sewage treatment plants need to consider the control of nitrogen and phosphorus emissions, while existing sewage treatment plants need to be upgraded to enhance or strengthen the function of nitrogen and phosphorus removal.
1 Effects of Nitrogen and Phosphorus on Water Environment
Proper amount of nitrogen and phosphorus plays an important role in promoting the growth of aquatic plants and microorganisms, and also plays a certain role in maintaining the balance of water environment. Excessive nutrients such as nitrogen and phosphorus enter the water body, which will make the water body eutrophic, make the phytoplankton in the water body multiply and even erupt, resulting in the phenomenon of "water bloom". The phenomenon of "water bloom" is an obvious manifestation of water pollution, which will further aggravate the pollution of water bodies. A large number or explosive propagation of algae will form thick or thin floating algae on the water surface, leading to hypoxia in the water body and suffocation and death of aquatic animals. Some algae can also produce harmful toxins, destroy aquatic ecosystems and reduce biodiversity.
There are three indicators of water eutrophication: nutritional factors, environmental factors and biological factors. Among them, nutrition is the fundamental reason of water eutrophication, and nitrogen and phosphorus are the most critical factors. Therefore, it is very important to control the content of nitrogen and phosphorus entering the water body to solve the problem of eutrophication.
2 The main sources of nitrogen and phosphorus in water
The pollution of nitrogen and phosphorus in China mainly comes from domestic pollution, agricultural pollution and industrial pollution.
Domestic pollution sources mainly refer to pollutants from cities, such as human excrement, food waste and various synthetic detergents. In this kind of waste, there are a lot of nitrogen and phosphorus substances, which will become the source of nitrogen and phosphorus pollution in water if they are not treated or not strictly treated.
Agricultural pollution mainly refers to the pollution caused by large or excessive use of chemical fertilizers and high loss rate. As we all know, the main components of chemical fertilizer are nitrogen and phosphorus. Uncontrolled or excessive use of chemical fertilizers in agriculture leads to a large loss rate of chemical fertilizers, which can easily become the source of nitrogen and phosphorus pollution in water bodies.
Industrial pollution mainly refers to industrial wastewater formed by food processing industry and fertilizer production enterprises, which contains a lot of nitrogen and phosphorus. If it is directly discharged into water without treatment or improper treatment, it will have a great impact on nitrogen and phosphorus pollution in water.
3 Status of Nitrogen and Phosphorus Removal in China Sewage Treatment Plant
The construction of urban sewage treatment plants in China began in Shanghai in the 1920s, and large-scale construction of urban sewage treatment plants began in China in the 1970s and 1980s after the founding of New China. At the initial stage of the construction of urban sewage treatment plant, activated sludge technology was used to treat organic pollutants and suspended solids in urban sewage, but the treatment capacity of nitrogen and phosphorus in sewage was weak and the removal rate was low. Later, in the early 1980s, some new sewage treatment processes began to be applied in sewage treatment plants, but overall, the nitrogen and phosphorus removal processes of sewage treatment plants in China are still at a low level.
In 1990s, with the increasingly serious water pollution in China, people began to control pollution, including groundwater quality standards, surface water quality standards and seawater quality standards, all of which set clear requirements for the standard values of nitrogen and phosphorus in water. During this period, the process requirements of nitrogen and phosphorus removal in the construction of sewage treatment plants in China are becoming more and more strict. New sewage treatment plants must consider the control of nitrogen and phosphorus, and the sewage treatment plants that have been built and operated need corresponding nitrogen and phosphorus removal process transformation.
Application of Nitrogen and Phosphorus Removal Process in Sewage Treatment Plants in China
4. 1 oxidation ditch process
Oxidation ditch process has the characteristics of simple process flow, stable operation, convenient management and low treatment cost. Compared with other processes, it has the advantages of strong impact load resistance, good effluent quality, less excess sludge and fewer structures. In China, Carrousel oxidation ditch, Aubert oxidation ditch, triple oxidation ditch and German oxidation ditch are widely used.
Rotary oxidation ditch was developed by Dutch DHV company in 1967. It has the advantages of low investment, high treatment efficiency, good reliability, convenient management and low operation and maintenance cost, and has been widely used in all countries of the world. Kunming No.1 Sewage Treatment Plant, Zhuhai Xiangzhou Sewage Treatment Plant, Zhongshan Sewage Treatment Plant and Chongqing Beibei Sewage Treatment Plant all adopt this process.
Aubert oxidation ditch process is a process developed and owned by USFilterEn-virex company, which is very suitable for conventional secondary biological treatment of sewage. At present, China has realized the independent design of the process and the localization of equipment, and Beidaihe West Sewage Treatment Plant and Wenzhou Central Sewage Treatment Plant should adopt this process.
Tri-ditch oxidation ditch, also known as T-ditch, is a typical structural form of oxidation ditch. The process has the characteristics of simple process, less construction investment and low operation cost. In structural design, there is no need to set up additional primary and secondary sedimentation tanks and sludge reflux devices, which avoids the disadvantage of large area occupied by oxidation ditch process to some extent. The process design has been used in sewage treatment plants such as the eastern suburb of Handan, Suzhou New District, Shenzhen Binhe and Luofang.
DE oxidation ditch process is a double ditch system, similar to the triple ditch system, but the difference is that DE oxidation ditch system has an independent sludge reflux system. Xi Anbei Shiqiao Sewage Treatment Plant adopts this process.
Oxidation ditch technology has been widely concerned since it came out. At present, there are thousands of oxidation ditch sewage treatment plants in operation in Europe. Since 1980s, oxidation ditch technology has been introduced from abroad, and after digestion and absorption, oxidation ditch technology has become one of the main technologies for urban sewage treatment in China.
Application of 4.2A/O process
A/0 process has a good effect of nitrogen and phosphorus removal, and it is the mainstream process of nitrogen and phosphorus removal in urban sewage treatment in 1980s and 1990s. A/0 process includes A/0 phosphorus removal process and A/0 nitrogen removal process. Usually, the phosphorus removal effect can reach more than 90%, and the nitrogen removal effect is more than 80%. This process does not need additional carbon source for nitrogen removal, and can fully realize denitrification, easily control sludge bulking, and has low investment and operating costs, so it has been widely used in early sewage treatment plants in China. Such as Tianjin Dong Jiao Sewage Treatment Plant, Beijing Gaobeidian Sewage Treatment Plant, Hangzhou Bao Si Sewage Treatment Plant and Shenyang Xijiao Sewage Treatment Plant.
A/0 process has obvious effect on sludge sedimentation and phosphorus removal, but its process control is limited, which will reduce the phosphorus removal effect when nitrification occurs. In addition, when the temperature and influent load of A/0 process are low, the metabolic capacity of microorganisms will be weakened, and the growth of sludge will be slowed down, which will have a great impact on the phosphorus removal effect.
4.3: Application of A2/O and Its Improved Process
A2/0 process is a commonly used simultaneous nitrogen and phosphorus removal process in China. An anoxic tank was added to A/0 process with only phosphorus removal function, which realized simultaneous nitrogen and phosphorus removal. It has been widely used in sewage treatment plants in China because of its simple operation and low cost. This process has been applied in Kunming No.2 Sewage Treatment Plant, Guangzhou Datansha Sewage Treatment Plant and Xi 'an Dengjiacun Sewage Treatment Plant. However, this process can't achieve simultaneous and efficient nitrogen and phosphorus removal, and its own defects, that is, the contradiction and competition between nitrifying bacteria, denitrifying bacteria and phosphorus accumulating bacteria in organic load and carbon source demand, make it difficult to achieve simultaneous and efficient nitrogen and phosphorus removal in the same system.
In order to solve the inherent defects of A2/0 process, many researchers have carried out various studies to upgrade and improve the process, among which China has obtained two patented technologies, namely, the inverted A2/0 process and the A-A2/0 process.
Inverted A2/0 process is to invert the technological positions of anoxic tank and anaerobic tank in A2/0 process, and put anoxic tank in front of anaerobic tank. Inverted A2/0 process can be operated with or without nitrate reflux, and the process environment is conducive to the formation of stronger phosphorus absorption capacity of microorganisms. All sludge will undergo a complete process of phosphorus release and absorption to improve phosphorus removal capacity. The application effect of this process in Jiangsu Changzhou Qingtan Sewage Treatment Plant, Changzhou Beicheng Sewage Treatment Plant and Qingdao Licunhe Sewage Treatment Plant is good.
A-A2/0 process is to add an anoxic tank in front of the anaerobic tank. The original A2/0 process can be easily transformed into A-A2/0 process by separating the anaerobic tank from the raw sewage. A-A2/0 process can ensure the complete denitrification of nitrate in RAS and sufficient carbon source, and the lowest nitrate content in anaerobic tank can enhance the phosphorus removal effect. Shandong Taian Sewage Treatment Plant and Qingdao Tuandao Sewage Treatment Plant have achieved good results in nitrogen and phosphorus removal.
4.4:SBR process and its improved application
SBR process forms A2/0 system in time series through automatic control program, which has the characteristics of economy, high efficiency and flexible control, and has good nitrogen and phosphorus removal effect, and is suitable for small and medium-sized sewage treatment plants.
There are some technical problems in typical SBR process. Firstly, intermittent water intake and intermittent aeration, intermittent operation and frequent start-stop of blast aerator greatly affect the operation stability of the whole process, and the utilization rate of reaction tank in aeration stage is also low; Secondly, due to intermittent water inflow, the design of automatic control system and the installation of sequential water inflow gate valve become more complicated. When the water inflow is large, multiple sets of reaction tanks need to run in parallel, and the overall complexity of the system increases. Thirdly, for some high-concentration refractory organic wastewater, the reaction time is longer. In order to solve the above problems, many researchers have improved the typical SBR process, and the more mature processes include ICEAS process, DAT—IAT process, CASS process and so on.
The biggest feature of ICEAS process is that a pre-reaction zone is added at the inlet end of the reactor, and the operation mode is continuous water inlet and intermittent drainage. The pre-reaction zone can adjust the water flow, and the main reaction zone is the main body of aeration and precipitation. ICEAS process can also be regarded as SBR process with continuous water inflow and intermittent drainage. The third sewage treatment plant in Kunming adopted this process, and the operation effect is good.
DAT—IAT process sets DAT tank and IAT tank in the same reaction tank, separated by diversion wall. DAT tank is continuously filled with water and aerated to keep the hydraulic balance of the system and effectively improve the stability of the system operation. In addition, continuous aeration strengthens the degradation of refractory organic matter, shortens the treatment time of high-concentration organic wastewater, and correspondingly shortens the operation time of the aerator. In addition, the continuous water intake of DAT pool can be realized by ordinary sewage pump, which greatly reduces the complexity of the system. This process has achieved good application results in Tianjin Economic and Technological Development Zone Sewage Treatment Plant and Fushun Sanbaotun Sewage Treatment Plant.
As an improvement of SBR process, CASS process adds a biological selection zone, that is, pre-reaction zone, at the inlet end of SBR tank, realizing continuous water inflow and intermittent drainage. The whole process of aeration, sedimentation and drainage runs periodically in the same tank, which saves the secondary sedimentation tank and sludge reflux system of the conventional activated sludge process. Beijing Aerospace City Sewage Treatment Plant adopted this process.
5 concluding remarks
With the increasingly prominent environmental problems in China, the governance of water environment in China is also constantly strengthened, and the requirements for nitrogen and phosphorus removal in sewage treatment plants are becoming more and more strict. Some early sewage treatment plants are also faced with the problem of transformation of nitrogen and phosphorus removal functions. Based on the current application of nitrogen and phosphorus removal processes in sewage treatment plants, A2/0 process and its improvement, oxidation ditch process and SBR process and their improvement are the choices with wide application scope and good application effect.
More articles on sewage treatment technology can be found in Yijing Water Purification Network.
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