The discharge of domestic sewage and industrial wastewater has an important impact on the environment of the water body. Among them, the excessive discharge of nutrients such as nitrogen and phosphorus in sewage is one of the main causes of eutrophication of water bodies. Eutrophication of water bodies caused the rapid and massive reproduction of planktonic algae, easy to form a large-scale algae outbreak into a disaster event.
In view of the seriousness of China's water pollution, China's urban sewage treatment plant construction efforts continue to strengthen. The relevant pollutant discharge standards for the discharge of nitrogen and phosphorus requirements are becoming more and more stringent. New sewage treatment plants need to take into account the control of nitrogen and phosphorus emissions, and the sewage treatment plants already built need to be upgraded to enhance or strengthen the function of nitrogen and phosphorus removal.
1 nitrogen and phosphorus for the water environment
A moderate amount of nitrogen and phosphorus to promote the growth of aquatic plants and microorganisms has an important role in maintaining the balance of the water environment also has a certain role, but too much nitrogen and phosphorus and other nutrients into the water body, it will make the water body produce eutrophication, so that the water body of the planktonic algae blooms, even explosive breeding, resulting in a large number of water bodies. Or even explosive reproduction, resulting in the phenomenon of "water bloom". The phenomenon of "bloom" is an obvious manifestation of water pollution and will further aggravate the pollution of water bodies. Massive or explosive reproduction of algae will form a thick or thin cover of floating algae on the surface of the water, resulting in a lack of oxygen in the water body, causing aquatic animals to suffocate and die. Some algae also produce harmful toxins that can damage water ecosystems and cause a reduction in biodiversity.
Three types of indicators of eutrophication of water bodies, nutrient factors, environmental factors and biological factors, of which, nutrient factors are the root cause of eutrophication of water bodies, and in the nutrient factors, nitrogen and phosphorus are the most critical presence. Therefore, controlling the amount of nitrogen and phosphorus entering the water body is crucial to solving the problem of water body eutrophication.
2 Main sources of nitrogen and phosphorus in water bodies
Nitrogen and phosphorus pollution in our country's water bodies mainly comes from domestic pollution, agricultural pollution, and industrial pollution sources.
Living sources of pollution mainly refers to pollutants from the city, such as human excreta, food waste and a variety of synthetic detergents. In this kind of waste, contains a large number of nitrogen and phosphorus substances, if untreated or treatment is not strict into the natural water body, it will become a source of nitrogen and phosphorus pollution in the water body.
Agricultural pollution mainly refers to the large amount of fertilizers or excessive use, the high rate of loss caused by pollution. As we all know, the main ingredient of fertilizer is nitrogen and phosphorus, agriculture without controlling the use of large quantities or excessive use of fertilizers, resulting in the loss of fertilizers is very great, into the water is very easy to become a source of nitrogen and phosphorus pollution in the water body.
Industrial pollution mainly refers to the food processing industry, fertilizer production enterprises to form industrial wastewater, which contains a large amount of nitrogen and phosphorus, if untreated or improperly treated directly into the water body, the nitrogen and phosphorus pollution of the water body has a significant impact.
3 China's wastewater treatment plants to remove nitrogen and phosphorus status
China's urban wastewater treatment plant construction began in Shanghai in the 1920s, after the founding of New China in the 70s and 80s, China began to carry out large-scale construction of urban wastewater treatment plants. In the initial construction of urban sewage treatment plants, the treatment process are used in the activated sludge technology, mainly to deal with organic pollutants and suspended solids in urban sewage, for the treatment of nitrogen and phosphorus in the sewage capacity is relatively weak, the removal rate is low. Later in the early 1980s, some of the new process of sewage treatment began to be applied in the sewage treatment plant, but overall, this stage of China's sewage treatment plant in nitrogen and phosphorus removal process is still at a lower level.
Entering the 1990s, with the increasing environmental pollution of China's water bodies, pollution control began to intensify, has introduced the "groundwater quality standards", "surface water quality standards" and "seawater quality standards" and so on, for the standard value of nitrogen and phosphorus in the water body put forward clear requirements. During this period, China in the construction of sewage treatment plants, for the removal of nitrogen and phosphorus process requirements are increasingly stringent, the new sewage treatment plant must be considered for the control of nitrogen and phosphorus, and has been built and operated sewage treatment plant, it is necessary to carry out the corresponding denitrification and phosphorus removal process transformation.
4 nitrogen and phosphorus removal process in China's wastewater treatment plants
4.1 oxidation ditch process
Oxidation ditch process is characterized by a simple process, stable operation, easy to manage, etc., and the treatment cost is low, compared with other processes, with a Stronger shock load resistance, good effluent water quality, less residual sludge, fewer structures and other advantages. In China, the oxidation ditch process is applied more Carrousel (Carrousel) oxidation ditch, Orbal (Orbal) oxidation ditch, three ditch oxidation ditch and DE type oxidation ditch.
Carrousel (Carousel) oxidation ditch was developed in 1967 by the Dutch DHV company, with investment savings, high processing efficiency, good reliability, easy management and low operation and maintenance costs, etc., in countries around the world have been widely used. China's Kunming First Sewage Treatment Plant, Zhuhai Xiangzhou Sewage Treatment Plant, Zhongshan Sewage Treatment Plant and Chongqing Beibei Sewage Treatment Plant have adopted this process.
Orbal (Orbal) oxidation ditch process is the U.S. USFilterEn-virex company develops and owns the process technology, the process is very suitable for sewage conventional secondary biological treatment, at present, China has achieved the process of its own design and localization of equipment, the western Beidaihe Wastewater Treatment Plant, as well as the central area of the Wenzhou Wastewater Treatment Plant are the application of the process.
Three.
Three ditch oxidation ditch, also known as T-type oxidation ditch, is a typical form of oxidation ditch structure, this process has a simple process, small construction investment, low operating costs, in the structural design does not require the establishment of another primary and secondary sedimentation tanks and sludge reflux device, to a certain extent, to avoid the drawbacks of the oxidation ditch process occupies an area of large. China's Handan Dongjiao Sewage Treatment Plant, Suzhou New District Sewage Treatment Plant, Shenzhen Binhe Sewage Treatment Plant and Luofang Sewage Treatment Plant Phase II have adopted this process design.
DE-type oxidation ditch process is a double ditch system, similar to the three ditch system, the difference is that the DE-type oxidation ditch system has an independent sludge return system. Xi'an Beishiqiao Wastewater Treatment Plant is the use of this process.
Oxidation ditch technology since its inception has received widespread attention, Europe currently has about thousands of oxidation ditch sewage treatment plant in operation, China from the 1980s began to introduce foreign oxidation ditch technology, digestion and absorption of the development of the oxidation ditch process has become one of the main process of municipal wastewater treatment in China.
4.2Application of A/O process
A/0 process has a better effect of nitrogen and phosphorus removal, in the 1980s-90s is the mainstream process of urban wastewater treatment of nitrogen and phosphorus removal. a/0 process includes a/0 phosphorus removal process and a/0 denitrification process, usually phosphorus removal effect can reach more than 90%, denitrification process can reach more than 90%. The effect can reach more than 90%, nitrogen removal effect is more than 80%. The process does not need additional carbon source denitrification, and can fully realize denitrification and easy to control sludge expansion, investment and operating costs are low, in China's early sewage treatment plant has a wide range of applications. For example, Tianjin Dongjiao Sewage Treatment Plant, Beijing Gaobeidian Sewage Treatment Plant, Hangzhou Sibao Sewage Treatment Plant, Shenyang Xijiao Sewage Treatment Plant and so on.
A/0 process has obvious effect on sludge settlement and phosphorus removal, but because of its limited process control, nitrification will reduce the effect of phosphorus removal when it occurs. In addition, when the temperature and influent load of A/0 process are low, the metabolic ability of microorganisms will be weakened, and sludge growth will be slowed down, which has a greater impact on the effect of phosphorus removal.
4.3: Application of A2/O and its improved process
A2/0 process is a commonly used synchronous denitrification and phosphorus removal process in our country, which adds an anoxic tank in the A/0 process that only removes phosphorus, realizing synchronous denitrification and removal of phosphorus, which is simple to operate and inexpensive, and therefore has been widely used in our country's sewage treatment plants. Therefore, it has been widely used in China's sewage treatment plants. Kunming second sewage treatment plant, Guangzhou Datansha sewage treatment plant, Xi'an Dengjiacun sewage treatment plant have applied the process. However, this process can not realize the simultaneous and efficient nitrogen and phosphorus removal, and the process itself is defective, i.e., the nitrifying bacteria, denitrifying bacteria, and phosphorus-polymerizing bacteria in the organic loading, carbon demand, there are contradictions and competition, and it is difficult to realize the simultaneous and efficient removal of nitrogen and phosphorus in the same system.
In order to solve the inherent defects of the A2/0 process, many researchers have carried out various researches on upgrading and improving the process, among which, China has obtained two patented technologies, i.e. the inverted A2/0 process and the A-A2/0 process.
The inverted A2/0 process is for the A2/0 process anoxic pool and anaerobic pool arrangement position, the process position is inverted, the anoxic pool in an anaerobic pool before. Inverted A2/0 process can be operated without nitrate reflux conditions, the process environment is conducive to the formation of microorganisms stronger phosphorus absorption power, all the sludge will undergo a complete release of phosphorus and phosphorus absorption process so that the ability to remove phosphorus has been enhanced. The application of this process is better than that of Jiangsu Changzhou Qingtan Wastewater Treatment Plant, Changzhou Beicheng Wastewater Treatment Plant, Qingdao Licunhe Wastewater Treatment Plant and so on.
A-A2/0 process is an anaerobic tank before the addition of an anoxic tank, the original A2/0 process by separating the anaerobic tank and the original sewage, can be easily transformed into A-A2/0 process. A-A2/0 process of sufficient reflux sludge retention time ensures that the nitrate in the RAS Thorough denitrification in the RAS is ensured, and sufficient carbon source can be guaranteed. The minimum nitrate content in the anaerobic tank makes the phosphorus removal effect strengthened. Shandong Tai'an wastewater treatment plant, Qingdao Tundao wastewater treatment plant application of the process has achieved good nitrogen and phosphorus removal effect.
4.4: SBR process and its improved application
SBR process is through the automatic control program, the formation of the time series of A2/0 system, with economic and efficient, flexible control features, good results in the removal of nitrogen and phosphorus, for small and medium-sized water amount of sewage treatment plant.
The typical SBR process has certain technical problems, first of all, intermittent water intake, intermittent aeration mode, the blower aerator due to intermittent operation, frequent start and stop, making the whole process of operational stability is greatly affected, the aeration stage of the reaction pool utilization is also relatively low; Secondly, due to the intermittent intake of water, the design of the automatic control system and the sequential intake gate valve installation has become more complicated. When the water intake is large, it is necessary to run several sets of reaction tanks in parallel, and the overall complexity of the system increases; third, for some difficult-to-degrade organic wastewater with high concentration, the reaction time is relatively long. In order to solve the above problems, many researchers have been carried out to improve the typical SBR process variations, the more mature process ICEAS process, DAT-IAT process, CASS process.
The biggest feature of ICEAS process is that a pre-reaction area is added at the inlet end of the reactor, and the operation mode is continuous inlet and intermittent discharge, the pre-reaction area can play a role in regulating the water flow, and the main reaction area is the main body of the aeration and precipitation.
The ICEAS process is also regarded as a continuous inlet and intermittent discharge of the SBR process. Kunming third wastewater treatment plant will be used in this process, the operating effect is good.
DAT-IAT process in the same reaction tank set up DAT pool and IAT pool, separated by a guide wall. DAT pool continuous water intake and continuous aeration, to maintain the hydraulic balance of the system, effectively improve the stability of the system operation, and continuous aeration to enhance the degradation of difficult to degrade organic matter, shorten the treatment time for high concentrations of organic wastewater In addition, continuous aeration strengthens the degradation of difficult-to-degrade organic matter and shortens the treatment time for highly concentrated organic wastewater, and correspondingly shortens the operation time of the blast aerator; in addition, the continuous water intake of the DAT pool can be realized by using an ordinary sewage pump, which greatly reduces the complexity of the system. The process in the Tianjin Economic and Technological Development Area Wastewater Treatment Plant as well as Fushun Sanbotun Wastewater Treatment Plant to achieve better results.
CASS process as an improved SBR process, is in the SBR tank at the inlet end of the increase of a biological selection area, that is, the pre-reaction area, to achieve a continuous water intake, intermittent drainage. The whole process of aeration, sedimentation, drainage and other processes in the same pool cycle cycle operation, eliminating the conventional activated sludge method of the second sedimentation tank and sludge return system. Beijing Aerospace City Wastewater Treatment Plant adopts this process.
5 Conclusion
With the growing prominence of environmental problems in China, China's environmental management of water bodies is also being strengthened, the requirements for the denitrification and removal of phosphorus for wastewater treatment plants are also becoming more and more stringent, but also some of the early construction of the wastewater treatment plant is also faced with the function of denitrification and removal of phosphorus to transform the problem. Comprehensive on the application of the current wastewater treatment plant nitrogen and phosphorus removal process, A2/0 process and its improved, oxidation ditch process, SBR process and its improved is currently widely used and the application of the better choice.
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