Different from industrial sewage, urban sewage is produced in human life and is one of the main sources of water pollution, mainly excreta and washing sewage. The amount of domestic sewage discharged by urban residents per person per day is 150 ~ 400 L, which is closely related to the living standard. Domestic sewage contains a lot of organic matter, such as cellulose, starch, sugar and fatty protein, and often contains eggs of pathogenic bacteria, viruses and parasites, inorganic salts such as chloride, sulfate, phosphate and bicarbonate, and sodium, potassium, calcium and magnesium. The general characteristic is that it contains nitrogen, sulfur and phosphorus, and it is easy to produce malodorous substances under the action of anaerobic bacteria. Phosphorus removal technology is the main problem that puzzles urban sewage treatment, so it is very important to choose a reasonable treatment process in urban sewage treatment.
Section 1 Scale Division of Urban Sewage Treatment Plants
In the construction of urban sewage treatment, we must first determine the scale of the proposed sewage treatment plant, and then choose the appropriate process according to the scale. According to the actual situation in China, it can be generally divided into large, medium and small sewage treatment plants.
Large-scale sewage treatment plants with a scale of > 10× 104m3/d are generally built in big cities, with an investment of 1 100 million yuan and an annual operating cost of10 million yuan. At present, more than 10 sewage treatment plants have been built in China, the largest of which is Beijing Gaobeidian sewage treatment plant, with the scale of100×104m3/d. ..
The scale of medium-sized sewage treatment plant is (1~10) ×104m3/d, which is generally built in small and medium-sized cities and suburban counties of big cities. The capital construction investment is tens of millions to hundreds of millions of yuan, and the annual operating cost is several million to tens of millions of yuan. At present, dozens of buildings have been built and hundreds are under construction in China, and the number will increase greatly in the future.
Small sewage treatment plants with a scale of < 1× 104m3/d are generally built in small towns, with a capital investment of several million to tens of millions and an annual operating cost of several hundred thousand to several million; Due to the limitation of economic conditions, this kind of sewage treatment plant has just appeared in small towns with developed economy in coastal areas, and there will be more and more in the future. In the end, the number of small sewage treatment plants will exceed that of large and medium-sized sewage treatment plants.
Section II Selection Principles of Urban Sewage Treatment Process
1. Investment province. At present, most sewage treatment projects are invested by state-owned funds. China is a developing country, and there is a huge fund gap for economic development. Controlling investment is of great benefit to the sustainable development of national economy.
2. Low operating cost. Operating cost is an important factor for the normal operation of sewage treatment plants, and it is also one of the main indicators for selecting treatment processes.
3. Less floor space. Land resources are an important factor in urban development planning.
4. The effect of nitrogen and phosphorus removal is good. With the eutrophication of large-scale water environment in China, nitrogen and phosphorus removal from sewage has become an urgent problem. China's national integrated wastewater discharge standard (GB8978- 1996) also clearly stipulates that it is applicable to all sewage units, and very strictly stipulates the discharge standards of phosphate and ammonia nitrogen, that is to say, the problem of nitrogen and phosphorus removal should be considered first in the selection of sewage treatment process.
5. The organic combination of modern advanced technology and environmental protection engineering. The appearance and perfection of modern advanced technology, especially computer technology and automatic control system equipment, has provided strong support for the development of environmental protection engineering. At present, most sewage treatment plants in developed countries abroad adopt advanced computer management and automatic control systems to ensure the normal operation of sewage treatment plants and stable and qualified effluent, but China is still relatively backward in this respect. Computer control and management will also be the development direction of urban sewage treatment plants in China.
Section III Introduction of Urban Sewage Treatment Process
At present, most urban sewage treatment processes adopt primary treatment and secondary treatment. The primary treatment adopts physical methods, mainly through grid interception, precipitation and other means to remove large suspended solids and sand particles in wastewater. This process is very mature, and there is not much difference. The secondary treatment adopts biochemical method, which mainly removes suspended and dissolved organic matter, nitrogen, phosphorus and other nutrients from wastewater by means of microbial life movement. At present, there are many methods of this treatment process, which can be summarized as the above processes, namely SBR, A/O, A2/O, oxidation ditch process and traditional activated sludge process.
3. 1 SBR process
SBR is a sequencing batch activated sludge process, also known as intermittent aeration process. Its basic feature is that the biochemical reaction, sedimentation, drainage and sludge discharge of sewage are completed in one reaction tank, which not only saves the primary sedimentation tank and sludge digestion tank, but also saves the secondary sedimentation tank and the reflux sludge pump house. The treatment facilities are simpler than the oxidation ditch, and the treatment effect is good. Some SBR processes also have strong nitrogen and phosphorus removal functions.
SBR process requires high automation control. In the past, there were not enough automatic control equipment, so this process could not be popularized. In recent years, the application of automatic control technology and instruments in sewage treatment has passed the test. The third and fourth sewage treatment plants in Kunming, China have successfully operated SBR process for several years, so SBR process has been vigorously promoted and has become a very concerned process in the industry.
3.2 A/O process
A/O process is also called anaerobic aerobic process, and A(Anacrobic) is the anaerobic stage of nitrogen and phosphorus removal. O(Oxic) is an aerobic stage, which is used to remove organic matter from water.
Advantages and disadvantages of A/O process;
1 A/O process
(1) process is simple, no additional carbon source and post aeration tank are needed, and raw sewage is used as carbon source, so the construction and operation cost is low.
(2) Internal circulation is set before denitrification and after nitrification, and organic matter in raw sewage is used as carbon source, which has good effect and full denitrification reaction.
(3) After passing through the aeration tank, the denitrogenation residue can be further removed, and the treated water quality can be improved.
(4) Grade A mixing only suspends sludge to avoid the increase of DO. Strong aeration is used in the front section of O section, and the gas volume is reduced in the back section, so that the DO content of the internal circulating liquid is reduced to ensure the anoxic state of A section.
2. Shortcomings of A/O process
(1) Because there is no independent sludge reflux system, it is impossible to cultivate sludge with unique functions, and the degradation rate of refractory substances is low.
(2) In order to improve the nitrogen removal efficiency, the internal circulation ratio must be increased, thus increasing the operation cost. The internal and external circulating liquid comes from the aeration tank and contains a certain amount of DO, which makes it difficult to maintain the ideal anoxic state in section A and affects the denitrification effect, and the denitrification rate is difficult to reach 90%.
3.3 A2/O process
A2/O process is the abbreviation of anaerobic-anoxic-aerobic biological nitrogen and phosphorus removal process. A2/O process integrates biological nitrogen and phosphorus removal into the same activated sludge system.
Advantages and disadvantages of A2/O process
1 anaerobic anoxic oxic
The treatment efficiency is high, generally reaching 90% ~ 95% of BOD5 and SS, more than 70% of total nitrogen and about 90% of phosphorus. It is generally suitable for large and medium-sized urban sewage plants that need nitrogen and phosphorus removal.
2. The shortcomings of A2/O process
The construction cost and operation cost are higher than those of ordinary activated sludge process, and the requirements for operation management are high. Only when the treated sewage is discharged into a closed water body or a slow-flowing water body to cause eutrophication, thus affecting the water body, this process is adopted.
3.4 Traditional activated sludge process
For a long time, activated sludge process has been widely used in urban domestic sewage, which is the most widely used biological treatment process in the world, with the advantages of high treatment capacity and good effluent quality. The method mainly consists of an aeration tank, a sedimentation tank, a sludge reflux system and a surplus sludge discharge system. The wastewater and the returned activated sludge enter the aeration tank together to form a mixed solution. The aeration tank is a bioreactor. Air is filled into the aeration equipment to dissolve oxygen in the air into the mixed solution to generate aerobic metabolic reaction, and the mixed solution is fully stirred to make it in a suspended state, so that the organic matter, oxygen and microorganisms in the wastewater can fully contact and react. Then the mixed liquid enters the sedimentation tank, and the suspended matter in the mixed liquid precipitates and separates out water, and the purified water flows out of the sedimentation tank. Most of the sludge in the sedimentation tank flows back, which is called reflux sludge. The purpose of reflux sludge is to maintain a certain concentration of suspended solids in the aeration tank, that is, to maintain a certain concentration of microorganisms. The biochemical reaction in the aeration tank causes the proliferation of microorganisms, and the biomass of the proliferated microorganisms is usually removed from the sedimentation tank to maintain the stable operation of the activated sludge system. This part of sludge is called excess sludge. Activated sludge not only has the ability of oxidizing and decomposing organic matter, but also has good coagulation and sedimentation performance, so that the activated sludge can be separated from the mixed liquid and clear effluent can be obtained. The traditional activated sludge process often has high investment and operation cost, large energy consumption, complex management and easy sludge bulking. The equipment cannot meet the requirements of high efficiency and low consumption.
3.5 oxidation ditch
Oxidation ditch is a variant of activated sludge process, and its aeration tank is a closed ditch, so it is different from the traditional activated sludge process in hydraulic flow pattern. It is a circulating flow aeration ditch connected end to end, in which sewage permeates and purifies. The earliest oxidation ditch is an earth ditch with slope protection, with intermittent water inflow and intermittent aeration. From this point of view, oxidation ditch is the earliest sequencing batch sewage treatment technology. The whole process of sewage treatment in oxidation ditch, such as water inflow, aeration, sedimentation, sludge stabilization and water outflow, is concentrated in oxidation ditch, and the earliest oxidation ditch does not need to be equipped with primary sedimentation tank, secondary sedimentation tank and sludge reflux equipment. With the gradual expansion of treatment scale and scope, extended aeration is usually adopted. Continuous inflow and outflow of water can stabilize microbial sludge while purifying sewage by aeration, and there is no need to set up a primary sedimentation tank and a sludge digestion tank, so the treatment facilities are greatly simplified.
Advantages of oxidation ditch process
Oxidation ditch process has become the first choice for small and medium-sized urban sewage plants because of its outstanding advantages of economy and simplicity. It has the advantages of multifunction, stable sludge, good effluent quality and convenient management.
To sum up, choosing a good urban sewage treatment process is of unusual significance to the development of national economy, environmental protection and resource reuse. A good process has the advantages of simple process, reliable equipment, convenient management, less investment, less land occupation, high efficiency, low operating cost, up-to-standard sewage treatment and discharge, and recycling.
Chapter II Process Selection of Urban Sewage Treatment Plants
Section 1 Process Selection of Large Sewage Treatment Plants
The best process of large-scale urban sewage treatment plants is the traditional activated sludge process and its improved A/O process and A2/O process. At present, the traditional activated sludge method, A/O method and A2/O method are mostly used in large-scale sewage treatment plants in most countries (including China). In China, this process has been adopted by Beijing Gaobeidian Sewage Treatment Plant, Tianjin Ji Zhuangzi Sewage Treatment Plant and Dong Jiao Sewage Treatment Plant, Shenyang North Sewage Treatment Plant, Zhengzhou Sewage Treatment Plant, Hangzhou Bao Si Sewage Treatment Plant and Chengdu Sanwayao Sewage Treatment Plant, which is not accidental, because it has irreplaceable advantages of large-scale sewage treatment plants.
Compared with oxidation ditch and SBR, the traditional activated sludge process, A/O process and A2/O process have the greatest advantages of lower energy consumption and operating costs, and the larger the scale, the more obvious this advantage is. For a large sewage plant, the annual operating cost is considerable. For example, for a sewage plant with a scale of 40× 104 m3/d, the sewage saving of 1m3 is 1 cent, and the annual saving is146 million yuan.
The reasons for the low energy consumption and operation cost of this process are: a) setting a primary sedimentation tank to remove a considerable part of organic matter and suspended matter in sewage by physical method with minimum energy consumption and cost, reducing the secondary treatment load and saving energy significantly; B. Sludge is digested by anaerobic digestion, which is more energy-saving than simultaneous aerobic digestion in oxidation ditch and SBR process, and is recognized as an energy-saving process.
Generally speaking, the capital investment of this process is higher than that of oxidation ditch and SBR process, but with the increase of scale, the capital investment of oxidation ditch and SBR also increases exponentially, while the investment growth ratio of conventional activated sludge process is small, and the gap between them is getting smaller and smaller. When the sewage plant reaches a certain scale, the investment of conventional activated sludge method is less than that of oxidation ditch and SBR, so the bigger the sewage plant, the greater the advantages of conventional activated sludge method.
2. The main disadvantages of conventional activated sludge process, A/O process and A2/O process are that there are many treatment units and the operation and management are complicated. In particular, anaerobic digestion of sludge requires a high level of management, and the biogas produced in the digestion process is flammable and explosive, which requires safe operation and increases the management difficulty. However, because large sewage treatment plants are backed by big cities, with strong technical force and high management level, they can meet this requirement, so the shortcomings of conventional activated sludge method will not be the limiting factor.
Judging from the current reality in China, urban sewage treatment is in its infancy, the laws and regulations are not perfect, and the requirements for sludge stabilization are not clearly defined. At the same time, due to the imperfect drainage pipe network system, the organic composition of most urban sewage is not high, and the management of anaerobic digestion of sludge and the utilization of biogas are still lacking in mature experience. These factors reduce the economy of traditional activated sludge process, including A/O and A2/O processes of sludge anaerobic digestion. Therefore, it is sometimes more economical to adopt SBR and oxidation ditch process for urban sewage treatment plants with the scale of (10 ~ 20) ×104m3/d. In this case, it is necessary to make a detailed technical and economic comparison of various processes to determine the best process.
Section 2 Optimization Process of Sewage Treatment Plants in Small and Medium-sized Cities
Oxidation ditch and SBR are the best processes for sewage treatment plants in small and medium-sized cities, and their similarities are as follows:
1, the removal efficiency of organic matter is very high, and some can also remove nitrogen and phosphorus, and the treatment facilities are very simple and the management is very convenient. It is internationally recognized as an efficient and simplified sewage treatment process, and it is also the first choice for sewage treatment plants in small and medium-sized cities around the world.
2. Under the scale of 10× 104 m3/d, the capital cost of oxidation ditch and SBR process is obviously lower than that of conventional activated sludge process, A/O process and A2/O process; For a sewage plant with a scale of (5 ~ 10) × 104m3/d, the capital cost of oxidation ditch and SBR process is usually low 10% ~ 15%. The smaller the scale, the greater the gap between them, which is very attractive for small and medium-sized cities that lack funds to build sewage treatment plants.
Even at the scale of 10× 104 m3/d, the power consumption and annual operating cost of oxidation ditch and SBR process are still higher than those of conventional activated sludge process, but if compared with the capital cost, the total capital cost plus 20-year operating cost is still lower than that of conventional activated sludge process. The smaller the scale, the lower it is, and the bigger the scale, the smaller the gap. When the scale is 10× 104 m3/d, the total cost of the two processes is almost the same. Therefore, for small and medium-sized sewage treatment plants, it is economically beneficial to adopt oxidation ditch and SBR process.
3. Oxidation ditch and SBR process usually have no primary sedimentation tank and sludge digestion tank, and the whole treatment unit is less than 50% compared with the conventional activated sludge process, which greatly simplifies the operation and management, and 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 technological requirements in quality, and the price is several times cheaper than that of foreign equipment, which also saves all kinds of troubles in applying for foreign exchange imported equipment.
5. Oxidation ditch and SBR process have much better impact load resistance than conventional activated sludge process, which is beneficial to small and medium-sized sewage treatment plants with drastic changes in water quality and quantity.
It is precisely because of the above reasons that oxidation ditch and SBR have developed rapidly at home and abroad. The United States Environmental Protection Agency (EPA) listed the process that saved the construction cost or operation cost of sewage treatment plants by more than 65,438+05% compared with the conventional activated sludge process as an innovative alternative technology, and the federal government gave financial support, so it vigorously promoted SBR and oxidation ditch process, and hundreds of sewage treatment plants have been built. There are thousands of oxidation ditch sewage treatment plants in Europe, and nearly 600 SBR process sewage treatment plants have been built in Australia in recent 10 years. In China, oxidation ditch and SBR process have become the first choice for small and medium-sized sewage treatment plants.
6, the main classification and characteristics of oxidation ditch process
Oxidation ditch process can be generally divided into four categories:
① Multi-ditch alternating oxidation ditch, characterized by combined construction, no separate secondary sedimentation tank, and brush aeration. There are single ditch, double ditch and three ditch, and the most typical one is Handan three ditch oxidation ditch. This oxidation ditch has the characteristics of SBR process and can also be regarded as a kind of SBR. Its nitrogen and phosphorus removal effect is unstable. If nitrogen and phosphorus need to be removed, some facilities need to be added.
(2) Carrousel oxidation ditch, which is built separately, has a separate secondary sedimentation tank and is aerated by a surface aerator. The ditch depth is greater than that of multi-ditch alternating oxidation ditch. The second water purification plant in Changsha is such a process, and the effect of nitrogen and phosphorus removal is not ideal. If nitrogen and phosphorus need to be removed, some facilities need to be added.
(3) Aubert oxidation ditch is also built separately, with a separate secondary sedimentation tank, which is aerated by rotary table, and the ditch is deep. Now it is used in Sichuan, Beijing, Shandong, Zhejiang and other places, and the nitrogen removal effect is good, but the phosphorus removal efficiency is not high enough, so some measures should be taken when phosphorus removal is needed.
④ The integrated oxidation ditch is combined, and the sedimentation tank is built in the oxidation ditch, which has been applied in Chengdu Xindu Sewage Treatment Plant in Sichuan and gaomi city Sewage Treatment Plant in Shandong. It is not only continuous water inlet and outlet, but also combined construction, which is the most economical and reasonable in theory, but it is not mature in some specific technical problems, which affects its popularization and use.
7, the main classification and characteristics of SBR process
SBR process mainly has the following types:
① In the traditional SBR process, all operations are intermittent and periodic, and Sichuan Bazhong Sewage Treatment Plant is such a process. Its nitrogen and phosphorus removal effect is not stable enough. If nitrogen and phosphorus need to be removed, some improvements need to be made.
(2) ICE AS process, that is, intermittent circulation extended aeration activated sludge process, divides the reaction tank into two parts with partition walls, with the pre-reaction area before and the main reaction area after. Continuous water inflow, intermittent aeration, sedimentation, drainage and sludge discharge have been used in the third and fourth sewage treatment plants in Kunming. Nitrogen and phosphorus can be removed, but the effect is not ideal.
③ DAT-IAT process, that is, the process of combining continuous aeration with intermittent aeration. The middle of the reaction tank is divided into two parts by a partition wall. DAT in front is continuous aeration, and IAT in the back is intermittent aeration, sedimentation, drainage and sludge discharge, which have been used in Tianjin Development Zone Sewage Treatment Plant. Its nitrogen and phosphorus removal function is general, so it is necessary to increase facilities to improve nitrogen and phosphorus removal efficiency.
④CAST process, that is, the circulating activated sludge process, in which the reaction tank is divided into a selection area and a main reaction area by a partition wall, and the water inlet, aeration, sedimentation, drainage and sludge discharge all operate intermittently and periodically. Its nitrogen and phosphorus removal effect is good, and its performance of preventing sludge bulking is good. At present, some sewage treatment plants in Shenzhen, Tianjin and Yunnan have adopted this process.
⑤UNITANK process, in which three rectangular tanks are connected in parallel and work in a periodic operation mode similar to the three-ditch oxidation ditch. However, changing the rotary brush aeration into blast aeration can increase the depth of the pond, and changing the adjustable weir into a fixed weir to simplify the drainage. Shanghai Shidongkou Sewage Treatment Plant adopts this process, and its function is similar to that of Sangou oxidation ditch.
8. Comparison between oxidation ditch and SBR process
Oxidation ditch and SBR process have many common characteristics, but they also have their own characteristics and applicability, so they need to be carefully analyzed when choosing the scheme.
First, from the perspective of infrastructure investment, SBR process is a combined building type. Generally, the cost of land acquisition and civil works is lower than that of oxidation ditch, while the cost of equipment is higher than that of oxidation ditch. The total cost depends on the specific situation.
B, high land price, unfavorable to the oxidation ditch. High influent BOD concentration and high ratio of reaction volume to precipitation volume are beneficial to oxidation ditch; Lower BOD concentration and lower ratio of reaction volume to precipitation volume are beneficial to SBR.
C, from the operating cost, SBR process usually adopts blast aeration, and oxidation ditch process usually adopts mechanical aeration. Generally speaking, under the same oxygen supply, blast aeration saves electricity than mechanical aeration; On the other hand, SBR process is combined, which does not need sludge reflux (some of them have small reflux), and oxidation ditch process is separated, which needs a lot of reflux and consumes a lot of electricity; Thirdly, SBR process runs at variable water level, which increases the pump head. Generally speaking, the power consumption of oxidation ditch process is higher than that of SBR process, and the operating cost is also higher.
D, oxidation ditch process is continuous operation, and automatic control is not needed, which is only needed when energy is saved; SBR process runs periodically and intermittently, and each process changes frequently, so it needs automatic control.
E, SBR process is static precipitation and oxidation ditch process is dynamic precipitation, so the sedimentation efficiency of SBR is higher and the effluent quality is better.