Backwashing filter oxidation ditch sewage treatment process and its development
Backwashing filter oxidation ditch sewage treatment process and its development
Oxidation ditch, also known as continuous cycle aeration tank, is a variant of the activated sludge process. The oxidation ditch sewage treatment process has been in use since its inception. Due to its technical characteristics of good effluent quality, stable operation and convenient management, it has been widely used in the treatment of domestic sewage and industrial sewage at home and abroad. The types of oxidation ditch that are currently widely used include: Passwell Oxidation Ditch, Carrousel Oxidation Ditch, Alber Oxidation Ditch, T-Type Oxidation Ditch, DE-Type Oxidation Ditch, and Integrated Oxidation Ditch.
This paper mainly introduces the structure, mechanism, existing problems and the latest development of oxidation ditch.
The aerator oxidation ditch is a wastewater biological treatment technology developed by the Dutch Institute of Health Engineering in the 1950s. It is a modification of the activated sludge process and is a special form of delayed aeration. The basic feature is that the aeration tank is closed, the annular runway type, the pool body is long and narrow, the pool depth is shallow, and a surface aeration device is arranged in the groove. The wastewater and activated sludge and various microorganisms are mixed and circulated in the ditch to complete the nitrification and denitrification treatment of the wastewater. The biological oxidation ditch combines the characteristics of a fully mixed, push-flow and oxidation pond. It has the characteristics of high purification degree, impact resistance, stable and reliable operation, simple operation, convenient operation and management, simple maintenance, low investment and low energy consumption. The oxidation ditch forms aerobic zone, anoxic zone and anaerobic zone in space, and has good nitrogen removal function. From the operation mode of the oxidation ditch, it is divided into continuous operation and alternate operation. In the continuous operation mode, the oxidation ditch is used as an aeration tank, and the second settling tank must be set up, so that there is a distinction between division and construction; Err, Carrousel, and Aubel Oxidation Ditch are all continuous operation modes. In the multi-ditch mode, there is often no secondary settling tank, and a part of the oxidation ditch system can be operated alternately as a sedimentation tank. The oxidation ditch process has achieved rapid development in the field of sewage treatment in China. Many universities have done a lot of research on this. Nowadays, the oxidation ditch process has become a mature activated sludge sewage treatment process widely used nationwide.
It is recognized that the current activated sludge process accounts for about 90% of the wastewater treatment process selection, and the mature oxidation ditch process is undoubtedly the first choice for activated sludge treatment. To some extent, it also has anaerobic-hypoxia. ——The function of aerobics is similar to that of AAO biological nitrogen removal process. However, in the construction of large-scale sewage treatment plants, the oxidation ditch process still needs further improvement. The development of oxidation ditch technology is not only reflected in the increase in quantity, but also in the scale of processing and the continuous expansion of processing objects. The treatment capacity of the oxidation ditch is from 5 million population equivalents to 10 million population equivalents. It can be used for both domestic sewage treatment and industrial wastewater treatment. After decades of practice and development, the oxidation ditch technology is considered to be a sewage biological treatment method with good effluent quality, stable operation, low capital investment costs and low operating costs. Oxidation ditch can be divided into different types according to its construction and operation characteristics, and according to different inventors and patents. The representative oxidation ditch commonly used in several projects is described below.
1 oxidation ditch
1.1 Characteristics of the oxidation ditch
The oxidation ditch is a single-channel annular oxidation ditch with a water depth of more than 5 m. The water and sludge are mixed and flowed continuously in the ditch. A vertical surface aerator is installed at the same end of each group of ditches, and the aerator has both oxygen supply and push flow agitation. The unique pool type of the oxidation ditch and the corresponding aeration equipment layout form an oxygen-rich zone near the downstream of the aerator and an anoxic zone upstream of the aerator and the outer ring, which not only facilitates biocoagulation but also activates the fouling. The mud is easy to precipitate. Since the energy intensity of the local area around the aerator is much higher than that of the conventional activated sludge aeration tank, the oxygen transfer efficiency is greatly improved, and the average oxygen transmission efficiency reaches at least 2.1 kg/(kW·h). Therefore, the oxidation ditch has a strong mixing and impact resistance. When the organic load is low, the operation of some aerators can be stopped, and the energy consumption can be saved under the premise of ensuring the flow of the water flow mixing and mixing cycle. Compared with other oxidation ditch processes, the oxidation ditch has a small footprint, low civil construction cost, significant energy saving effect, and simple management and maintenance. The BOD removal rate is as high as 95% to 99%, the denitrification rate is over 90%, and the phosphorus removal rate is about 50%. If combined with a coagulant, the phosphorus removal effect can reach 95%. Therefore, it is the most widely used in all oxidation ditch treatment processes.
1.2 Development of oxidation ditch
The development process can be divided into three generations: The first generation of ordinary oxidation ditch is mainly used to remove BOD. The system has a fuzzy A/O system. Nitrification and denitrification occur in the same pool, which has certain nitrogen and phosphorus removal effects. . The second generation oxidation ditch system strengthens the function of nitrogen and phosphorus removal in the common oxidation ditch system. This system adds an anaerobic tank and an anoxic tank in front of the common oxidation ditch to facilitate nitrogen and phosphorus removal. The third generation of oxidation ditch is to add a biological selection zone before the oxidation ditch system. The biological selection zone is to screen the strains with high organic load, inhibit the growth of filamentous bacteria, and increase the removal rate of each pollutant. The subsequent process principle is the same as that of the oxidation ditch system.
2 Oxidation ditch
Oxidation ditch is one of the new processes for denitrification and dephosphorization. Due to its unique technical and economic advantages, it has attracted the attention of the domestic and international wastewater treatment industry. Oxidation ditch is a multi-channel oxidation ditch system. It is generally composed of 3 concentric circles or elliptical channels, and the channels are connected. The influent water is first introduced into the outermost channel, and while circulating continuously, it enters the next channel in turn, which is equivalent to a series of completely mixed reaction cells connected in series, and finally discharged from the central channel. A pilot valve is arranged in the channel to make the inlet water downstream of the water outlet to avoid short-flowing of sewage. Aeration vehicles are mostly used in aeration equipment. The number of rotary discs depends on the amount of dissolved oxygen required in the tank. The water depth is generally 3.5 m to 4.5 m, and the flow velocity at the bottom of the ditch is 0.3 m/s to 0.9 m/s. In the three channel systems, from the outside to the inside, the volume of the first channel is about 60% to 70% of the total volume, the second channel is about 20% to 30% of the total volume, and the third channel is only the total volume. 10%. The dissolved oxygen of the first, second and third channels should be kept at 0 mg/L, 1 mg/L, 2 mg/L, which is the 0- 1- 2 gradient of the so-called three-channel DO. The oxygen absorption rate in the first channel is very high, usually higher than the oxygen supply rate, and most of the dissolved oxygen supplied is immediately consumed; in the second and third channels, the oxygen absorption rate is low, despite the oxygen supply in the reaction cell. The amount is relatively low, and the amount of dissolved oxygen can be kept at a high level. This oxygen supply method has the following advantages: First, the oxygen supply of the first channel can meet the needs of degrading BOD, and can maintain zero dissolved oxygen in the channel, which can save energy and meet denitrification conditions. Second, under the condition of lack of oxygen in the first channel, microorganisms can release phosphorus, so that they can absorb phosphorus in wastewater under aerobic environment and achieve phosphorus removal effect. Third, large dissolution is formed in three ditches. The oxygen gradient is beneficial to increase the oxygenation efficiency.
With the demonstration of the superiority of the oxidation ditch process, the number of sewage treatment plants using this process is increasing. We should carry out further practical application investigation and performance research on the process, and provide reference and technical guidance for future engineering design and operation management, so that it is suitable for domestic needs and can maximize its efficacy.
3 alternating oxidation ditch
The alternating oxidation ditch has two oxidation zones (D-type) and three-pool alternate (T-type) operation of the oxidation ditch. They can be operated continuously without a secondary settling tank, and the groove depth can be adjusted from 2 m to 3.5 m. The D-type oxidation ditch is composed of two pools of the same volume of A and B, which are moved in series and alternately used as an aeration tank and a sedimentation tank. Generally, the movement period is 8 h.
The D-type alternating oxidation ditch is mainly used to remove BOD. To simultaneously remove nitrogen and phosphorus, an anaerobic pool and a sedimentation tank are added before and after the oxidation ditch. The three-channel oxidation ditch for nitrogen and phosphorus removal can be completed in the same reactor. In summary, this alternate operation process improves the processing efficiency, while the effect of nitrogen and phosphorus removal is significant. However, its volume utilization rate is relatively low, making its footprint much larger than other oxidation ditch processes. Therefore, when selecting a process, it should be carefully considered and weighed the pros and cons.
4 integrated oxidation ditch
The integrated oxidation ditch is also known as the combined oxidation ditch. Broadly speaking, an integrated oxidation ditch is an oxidation ditch that does not have a separate settling tank and a sludge return device. The oxidation ditch in this sense includes the oxidation ditch and the early intermittent operation. The narrowly defined oxidation ditch refers to making full use of the larger volume and water surface of the oxidation ditch. Without affecting the normal operation of the oxidation ditch, the mud-water separation process is improved by improving the structure of the partial area of ​​the oxidation ditch or by providing a certain device in the ditch. Oxidation ditch completed in the trench.
Compared with other oxidation ditch, the effect of integrated oxidation ditch is not very good. The main reason is: First, due to the integrated design of solid-liquid separator and oxidation ditch, the effluent is easily affected by water quality and water fluctuation, affecting the effluent. The second effect is that the design and installation of the solid-liquid separator are relatively high, but the engineering often fails to meet the requirements. However, the integrated oxidation ditch also has significant advantages: Because the secondary settling tank is omitted, the floor space is small, the sludge is automatically recirculated, the management is more convenient, the energy consumption is less, and the investment cost and operating cost are lower. Therefore, the process is particularly suitable for the sewage treatment in small cities and small towns, the domestic sewage treatment of tourist attractions and the like.
5 Development direction of various types of oxidation ditch
(1) Develop biological phosphorus and nitrogen removal technologies. So far, the oxidation ditch system is mostly activated sludge method. It can learn from the biofilm theory and increase the total amount of microorganisms in the unit reactor, thus increasing the organic load, but it is necessary to focus on the hydraulic design problem.
(2) Increase the activity of microorganisms in the oxidation ditch. It is the research direction of the oxidation ditch technology to add a specific strain in the oxidation ditch, to input iron salt, to acclimate the microorganism into bio-iron, to put in activated carbon to enhance the formation of the micelle and to improve the toxicity and impact resistance.
(3) Improve the performance and monitoring technology of the oxidation ditch equipment to further save energy in the oxidation ditch system. Improve the performance of surface aerators and underwater propellers and reduce maintenance workload. The use of multi-target monitoring technology such as DO, ORP and frequency conversion technology is the only way for the scientific operation of oxidation ditch in the future.
(4) Improve the cold resistance and poison resistance of the oxidation ditch, reduce the floor space and project cost.
6 Conclusion
As a mature biological treatment method, the oxidation ditch has the advantages of good treatment effect, low energy consumption, low capital investment, simple operation management, less sludge production and impact load resistance. In the actual design, the application status of the technology in different projects should be fully analyzed, the successful experience of domestic engineering design should be absorbed and digested, and the type of oxidation ditch and specific design parameters should be comprehensively determined based on the water quantity, water quality characteristics and economic development level.