1. The scale division of urban sewage treatment plants
According to the actual situation, it can be roughly divided into large, medium and small sewage treatment plants.
The scale > 10×10^4 m3/d is a large-scale sewage plant, which is generally built in a large city, with an infrastructure investment of 100 million yuan and an annual operating cost of 10 million yuan.
The scale of medium-sized sewage treatment plants is (1~10)×10^4 m3/d, which is generally built in small and medium-sized cities and suburban counties of large cities, with infrastructure investment of tens of millions to hundreds of millions of yuan, and annual operating costs of millions to tens of millions of yuan.
Small sewage treatment plants with a scale of < 1×10^4 m3/d are generally built in small towns, with infrastructure investment of millions to tens of millions, and annual operating costs of hundreds of thousands to millions; Due to economic constraints, such sewage plants usually appear in small towns with developed economies in coastal areas.
2. The main process of urban sewage treatment plant
The main pollutant of municipal wastewater is organic matter, so most of them are currently biological. There are also chemical methods, such as the use of chemically enhanced primary treatment, but the removal rate of this process is not high, the effluent does not meet the specified standards, and is only suitable for some specific places where the effluent quality requirements are not high.
In the biological method, there are two categories of activated sludge method and biological filter, the treatment efficiency of biological filter is not high, the sanitary conditions are poor, there are only a few biological filter urban sewage treatment plants, and the activated sludge method accounts for the vast majority.
There are many types of activated sludge methods, and there are three main types of activated sludge that are the most widely used:
● (1)traditional activated sludge method and its improved A/O and A2/O processes
● (2) oxidation ditches
● (3) SBR processes
The traditional activated sludge method is the earliest application process, it has a high efficiency in removing organic matter, and the sludge produced in the treatment process is stably treated by anaerobic digestion, which is very effective in eliminating the pollution of sewage and sludge, and the energy consumption and operating costs are relatively low, so it is widely used. In the past 20 years, the harm of water eutrophication has become more and more serious, and the removal of nitrogen and phosphorus has been included in the goal of sewage treatment, so the improved A/O method and A2/O method of activated sludge method have appeared. There are two types of A/O methods, one is an anaerobic-aerobic process for phosphorus removal, and the other is an anoxic-aerobic process for denitrification; The A2/O rule is a process that removes both nitrogen and phosphorus.
The oxidation ditch is a variant of the activated sludge method, which is different from the traditional activated sludge method in terms of hydraulic flow regime, and is a kind of end-to-end circulating flow, usually using delayed aeration, and the sludge is stabilized while the sewage is purified. It does not have a primary sedimentation tank or sludge digester, and the treatment facilities are greatly simplified. The oxidation ditch has the advantages of the traditional activated sludge method, which has a high efficiency in removing organic matter and also has the function of denitrification. If an anaerobic pond is added in front of the ditch, phosphorus can also be removed at the same time. The high efficiency and simplicity of the oxidation ditch make it widely used in small and medium-sized urban sewage treatment plants.
SBR is a sequential batch activated sludge method, its basic feature is to complete the biochemical reaction, sedimentation, drainage, and sludge discharge of sewage in a reaction tank, which not only eliminates the primary sedimentation tank and sludge digester, but also eliminates the secondary sedimentation tank and the reflux sludge pump room, the treatment facility is simpler than the oxidation ditch, and the treatment effect is good, and some SBR processes also have a strong denitrification and phosphorus removal function. In recent years, the application of automatic control technology and instruments to sewage treatment has passed, so the SBR process has been vigorously promoted, and has become a process that the industry is very concerned about.
3. The preferred process of large-scale urban sewage treatment plants
The preferred process for large-scale municipal sewage treatment plants is the traditional activated sludge method and its improved A/O method and A2/O method. At present, most of the large-scale sewage plants in most countries in the world (including China) use the traditional activated sludge method, A/O and A2/O method, which is not accidental, because this process has the advantages of being difficult to replace for large-scale sewage plants:
(1)Low energy consumption and low operating expenses
The biggest advantages of the traditional activated sludge method, A/O and A2/O methods over the oxidation ditch and SBR processes are lower energy consumption and lower operating expenses, and the larger the scale, the more obvious this advantage is. For large-scale sewage plants, the annual operating cost is considerable, for example, a sewage plant with a scale of 40×10^4 m3/d, 1 m3 sewage can save 1 cent in treatment fees, saving 1.46 million yuan a year.
The reasons for the low energy consumption and operating cost of this process are: a. setting up a primary sedimentation tank, using the physical method to remove a considerable part of the organic matter and suspended solids in the sewage with the minimum energy consumption and cost, reducing the load of secondary treatment, and significantly saving energy consumption; b. the sludge adopts anaerobic digestion, which significantly saves energy compared with the synchronous aerobic digestion of the oxidation ditch and SBR process, and is a recognized energy-saving process.
The capital investment of this process is generally higher than that of the oxidation ditch and SBR process, but with the increase of scale, the capital cost of the oxidation ditch and SBR also increases exponentially, while the investment of the conventional activated sludge process increases in a smaller proportion, and the gap between the two is getting smaller and smaller. When the sewage plant reaches a certain scale, the investment of the conventional activated sludge method is less than that of the oxidation ditch and SBR, so the larger the scale of the sewage plant, the greater the advantages of the conventional activated sludge method.
(2) Improve the management level
The main disadvantages of conventional activated sludge method, A/O and A2/O method are that there are many treatment units and complex operation management, especially sludge anaerobic digestion requires a high level of management, and the biogas produced in the digestion process is a flammable and explosive gas, which requires safe operation, which increases the difficulty of management. However, due to the fact that the large sewage plant is backed by a large city, has strong technical force and high management level, and can meet this requirement, the shortcomings of the conventional activated sludge method will not become a limiting factor.
According to the current reality, due to the imperfection of the drainage network system, the organic composition of most urban sewage is not high, and the management of sludge anaerobic digestion and the utilization of biogas are still not mature, these factors have reduced the economy of the conventional activated sludge method, A/O and A2/O method including sludge anaerobic digestion process. Therefore, for municipal wastewater treatment plants with a scale of (10~20)×10^4 m3/d, it may sometimes be more economical to use SBR and oxidation ditch processes, in which case it is necessary to make a detailed techno-economic comparison of various processes to determine the optimal process.
4. The preferred process of small and medium-sized urban sewage treatment plants
The preferred processes for small and medium-sized municipal wastewater treatment plants are oxidation ditch and SBR, and their common characteristics are:
(1)
The efficiency of removing organic matter is very high, and some can also remove nitrogen and phosphorus, or both nitrogen and phosphorus, and the treatment facilities are very simple, the management is very convenient, and it is currently internationally recognized as an efficient and simplified sewage treatment process, and it is also the preferred process for small and medium-sized urban sewage treatment plants in various countries around the world.
(2)
Below the scale of 10×10^4 m3/d, the infrastructure cost of oxidation ditch and SBR method is significantly lower than that of conventional activated sludge method, A/O and A2/O method. For sewage treatment plants with a scale of (5~10)×10^4 m3/d, the capital construction cost of oxidation ditch and SBR method is usually 10%~15% lower. The smaller the scale, the greater the gap between the two, which is attractive to small and medium-sized cities that lack the funds to build wastewater plants.
Even below the scale of 10×10^4 m3/d, the power consumption and annual operating cost of the oxidation ditch and SBR method are still higher than those of the conventional activated sludge method, but if compared with the capital cost, the total capital cost plus 20 years of operating cost is still lower than that of the conventional activated sludge method. The smaller the scale, the lower the scale, the larger the scale, the smaller the gap, and when the scale is 10×104 m3/d, the total cost of the two types of processes is roughly comparable. Therefore, it is economically advantageous to use the oxidation ditch and SBR method for small and medium-sized wastewater plants.
(3)
The oxidation ditch and SBR process usually do not have a primary sedimentation tank and a sludge digester, and the whole treatment unit is more than 50% less than the conventional activated sludge method, and the operation and management are greatly simplified, which is very suitable for small and medium-sized sewage treatment plants with relatively weak technical force and relatively low management level.
(4)
The equipment of oxidation ditch and SBR process has basically been localized, which can meet the process requirements in terms of quality and is cheap.
(5)
The impact load resistance of the oxidation ditch and SBR process is much better than that of the conventional activated sludge method, which is very beneficial to small and medium-sized sewage plants with drastic changes in water quality and quantity.
It is for all these reasons that oxidation ditches and SBRs develop rapidly. The U.S. Environmental Protection Agency (EPA) has listed a process that saves more than 15 percent of the construction or operating costs of wastewater treatment plants as an innovative alternative technology, with federal financial funding, which has led to the promotion of SBR and oxidation ditch processes, and hundreds of wastewater plants have been built. There are thousands of oxidation ditch wastewater treatment plants in Europe, and nearly 600 SBR process sewage plants have been built in Australia in the past 10 years. In China, oxidation ditch and SBR processes have become the preferred processes for small and medium-sized sewage treatment plants.
5. The main classification and characteristics of the oxidation ditch process
The oxidation ditch process can be broadly divided into four categories:
(1)Multi-groove alternating oxidation ditch
It is characterized by a combined construction type, without a separate secondary sedimentation tank, and adopts rotary brush aeration. It has single-ditch, double-ditch and three-ditch types, the most typical of which is the Handan three-ditch oxidation ditch. This oxidation ditch has the characteristics of SBR process, and can also be regarded as a type of SBR, its denitrification and phosphorus removal effect is unstable, if nitrogen and phosphorus removal is required, some facilities need to be added.
(2) Carrousel oxidation ditch
It is a separate type, there is a separate secondary sedimentation tank, the use of surface aerator aeration, the depth of the ditch is greater than the multi-ditch alternating oxidation ditch, Changsha water purification plant II is this process, its denitrification and phosphorus removal effect is not ideal, if the requirements of denitrification and phosphorus removal, also need to add some facilities.
(3) Obel oxidation ditch
It is also divided into two sedimentation tanks, using rotary disc aeration, the depth of the ditch is also larger, now Sichuan, Beijing, Shandong, Zhejiang and other places are adopting, its denitrification effect is very good, but the phosphorus removal efficiency is not high enough, and some measures need to be taken when requiring phosphorus removal.
(4) Integrated oxidation ditch
It is a joint type, and the sedimentation tank is built in the oxidation ditch. It is not only a continuous inlet and outlet of the water, but also a joint construction type, and does not need to switch the function, theoretically the most economical and reasonable, but in some specific technical issues are not very mature, so it affects its popularization and use.
6. The main classification and characteristics of the SBR process
There are several main types of SBR processes:
(1) Traditional SBR process
All of its operations are intermittent and cyclical. Its denitrification and phosphorus removal effect is not stable enough, such as the requirement of denitrification and phosphorus removal, some improvements need to be made.
(2) ICEAS process
That is, the intermittent circulating delayed aeration activated sludge method, which divides the reaction tank into two parts with a partition wall, the front is the pre-reaction area, and the back is the main reaction area, which adopts continuous water intake, intermittent aeration, sedimentation, drainage, and sludge discharge, which has been used in the third and fourth sewage plants in Kunming. It can remove nitrogen and phosphorus, but the effect is not ideal.
(3) DAT-IAT process
That is, the process of combining continuous aeration and intermittent aeration, the middle part of the reaction tank is divided into two parts with a partition wall, the DAT in the front is continuously aerated and the IAT in the rear is intermittently aerated, sedimentated, drained and discharged, which has been used in the sewage treatment plant of Tianjin Development Zone. Its denitrification and phosphorus removal function is average, and additional facilities are needed to improve the efficiency of nitrogen and phosphorus removal.
(4) CAST process
That is, the circulating activated sludge method, its reaction tank is divided into a selection area and a main reaction area with a partition wall, and the water intake, aeration, sedimentation, drainage, and sludge discharge are all intermittent periodic operation. It has good nitrogen and phosphorus removal effect and good performance to prevent sludge expansion.
(5) UNITANK process
It is three rectangular pools in parallel, which work in accordance with the periodic operation mode similar to the three-ditch oxidation ditch, but the rotary brush aeration is changed to blast aeration, which can increase the depth of the pool, and the water outlet adjustable weir is changed to a fixed weir, which simplifies the drainage, and its function is similar to that of the three-ditch oxidation ditch.
7. Comparison of oxidation ditch and SBR process
The oxidation ditch and SBR processes have many characteristics in common, as well as their own characteristics and applicability, which need to be carefully analyzed when selecting a solution.
(1)From the perspective of infrastructure investment
In general, the land acquisition fee and civil construction fee are lower than those of the oxidation ditch, and the equipment cost is higher than that of the oxidation ditch, and the total cost depends on the specific situation.
a. The high land price is not good for the oxidation ditch. b. The high BOD concentration of the influent water and the high ratio of reaction volume to precipitation volume are beneficial to the oxidation ditch; The low concentration of BOD and the low ratio of reaction volume to precipitation volume are beneficial to SBR.
(2) From the perspective of operating expenses
The SBR process is usually aerated with blast air, and the oxidation ditch process is usually aerated mechanically. Generally speaking, under the same oxygen supply, blast aeration is more energy-efficient than mechanical aeration; In the second aspect, the SBR process is a joint construction type, without sludge reflux (some small amount of reflux), and the oxidation ditch process is a separate construction type with a large amount of reflux, and the power consumption is large; Thirdly, the SBR process is a variable water level operation, which increases the head of the inlet water lifting pumping station. All things considered, the power consumption of the oxidation ditch process is usually larger than that of the SBR process, and the operating cost is higher.
(3) From the perspective of operational differences
The oxidation ditch process is continuous operation and does not require automatic control, but only uses automatic control when energy saving is required; The SBR process is intermittent during the cycle, and the various processes are frequently changed and need to be controlled automatically.
(4) From the perspective of sedimentation differences
The SBR process is static precipitation, and the oxidation ditch process is dynamic precipitation, so the sedimentation efficiency of SBR is higher and the effluent quality is better.