Urban sewage is different from industrial sewage, is the sewage produced by human life, is one of the main sources of pollution of water bodies, mainly excreta and washing sewage. The amount of domestic sewage discharged by urban residents per person per day is 150-400L, the amount of which is closely related to the standard of living. Sewage in a large number of organic matter, such as cellulose, starch, sugar and fat proteins, etc., but also often contains pathogenic bacteria, viruses and parasite eggs, inorganic salts of chloride, sulfate, phosphate, bicarbonate and sodium, potassium, calcium, magnesium, etc.. The general characteristics are nitrogen, sulfur, phosphorus, high in anaerobic bacteria, easy to produce malodorous substances. Phosphorus removal technology is plagued by urban sewage treatment is the main problem, so in the city's sewage treatment in a reasonable choice of treatment process becomes very important.
The first section of the urban sewage treatment plant scale
In the construction of urban sewage treatment, first of all, we have to determine the scale of the proposed sewage treatment plant, and then according to the scale of the selection of the appropriate process. According to the actual situation in China, can be roughly divided into large, medium and small sewage treatment plants.
Size > 10 × 104 m3 / d is a large sewage plant, generally built in large cities, capital investment in billions of dollars, the annual operating costs to tens of millions of dollars, the country has built more than a dozen, the largest is the Beijing GaoBeiDian wastewater treatment plant, the scale of 100 × 104 m3 / d.
Medium-sized wastewater treatment plant size (1 to 10) × 104 m3 / d, generally built in medium and small cities and small sewage treatment plants, and then select the appropriate process. Generally built in small and medium-sized cities and large cities in suburban counties, infrastructure investment of tens of millions to hundreds of millions of dollars, the annual operating costs of several million to tens of millions of dollars, the country has built dozens of seats, is being built in the hundreds, the next period of time will also be a large number of increases.
Size <1 × 104 m3 / d is a small sewage treatment plant, generally built in small towns, infrastructure investment of several million to tens of millions of dollars, the annual operating costs of hundreds of thousands of millions of dollars; due to the economic conditions of the limitations of the current type of sewage treatment plant has just appeared in the coastal areas of economically developed small towns, in the future there will be more and more, and ultimately, small sewage treatment plant will be more than the number of medium and large sewage treatment plant.
Section II of the urban sewage treatment process selection principles
1. Investment savings. At present, most of the sewage treatment projects are state-owned capital investment, China is a developing country, the economic development of the required capital gap is huge, the control of investment in the sustainable development of the national economy is very beneficial.
2. Low operating costs. Running costs is an important factor in the normal operation of the sewage treatment plant is to choose one of the main indicators of the treatment process.
3. Occupies less land. Land resources is an important factor in urban development planning.
4. Good nitrogen and phosphorus removal. With the eutrophication of large areas of water environment in China, the de-nitrogenization of sewage phosphorus has become an urgent need to solve the problem. China's implementation of the national "comprehensive sewage discharge standards" (GB8978-1996) also clearly stipulated in all sewage units, very strict provisions of the phosphate discharge standards and ammonia discharge standards, which means that the selection of sewage treatment process should first consider the problem of nitrogen and phosphorus removal.
5. Modern advanced technology and the organic combination of environmental engineering. Modern advanced technology, especially the emergence of computer technology and automatic control system equipment and improve the development of environmental protection projects to provide strong support. At present, most of the sewage treatment plants in developed countries abroad using advanced computer management and automatic control systems to ensure the normal operation of sewage treatment plants and stable and qualified water, while China is still relatively backward in this regard. Computer control and management will be the direction of development of China's urban sewage treatment plant.
Section III of the urban sewage treatment process
Currently, most of the urban sewage treatment process using primary and secondary treatment. Primary treatment using physical methods, mainly through the grid interception, sedimentation and other means of removing large suspended solids and sand in the wastewater and other substances, this process has been very mature, there is little difference. Secondary treatment is used in biochemical methods, mainly through the life of microorganisms and other means to remove suspended, dissolved organic matter in wastewater, as well as nitrogen, phosphorus and other nutrients. At present, this treatment process has a variety of methods, summarized in the main several processes described above, namely, SBR, A/O, A2/O, oxidation ditch process and traditional activated sludge method.
3.1 SBR process
SBR is a sequential batch activated sludge method, also known as intermittent aeration method, which is basically characterized by completing the biochemical reaction of sewage, sedimentation, drainage and sludge discharge in a reaction tank, which not only eliminates the primary sedimentation tank and sludge digestion tank, but also eliminates the secondary sedimentation tank and reflux sludge pumping room, and the treatment facilities are simpler than oxidation ditch, and the treatment effect is good. SBR process also has a strong function of nitrogen and phosphorus removal.
SBR process requires high self-control, self-control equipment in the past, this process can not be promoted, in recent years, self-control technology and instrumentation applied to wastewater treatment has been passed, China's Kunming, the third and fourth wastewater plant using the SBR process has been successfully running for several years, and thus the SBR process has been vigorously promoted as a process of great concern to industry insiders.
3.2 A/O process
A/O process is also known as anaerobic aerobic process, A (Anacrobic) is an anaerobic section, used for nitrogen and phosphorus removal; O (Oxic) is an aerobic section, used in addition to organic matter in the water.
Advantages and disadvantages of the A/O process:
1. Advantages of the A/O process
(1) The process is simple, there is no need for an additional carbon source and post-aeration tank, the original sewage as a source of carbon, construction and operation costs are low.
(2) denitrification in the front, nitrification in the back, set up an internal cycle, the original sewage in the organic matter as a carbon source, the effect is good, denitrification reaction is sufficient.
(3) Aeration tank in the back, so that denitrification residues can be further removed to improve the quality of treated water.
(4) A section of the mixing, only to make the sludge suspension, and avoid the increase of DO. O section of the front section of the strong aeration, after the section to reduce the amount of gas, so that the DO content of the internal circulation of the liquid is reduced to ensure that A section of the anoxic condition.
2. Disadvantages of the A/O process
(1) Because there is no independent sludge return system and cannot cultivate sludge with unique functions, the degradation rate of difficult to degrade substances is low.
(2) In order to improve the nitrogen removal efficiency, it is necessary to increase the ratio of internal circulation, thus increasing the operating cost. From the external and internal circulation of liquid from the aeration tank, containing a certain amount of DO, so that the A section is difficult to maintain an ideal anoxic condition, affecting the denitrification effect, nitrogen removal rate is difficult to reach 90%.
3.3 A2/O process
The A2/O process is the abbreviation of anaerobic-anoxic-aerobic biological denitrification and phosphorus removal process.
The A2/O process integrates biological denitrification and phosphorus removal into the same activated sludge system.
The advantages and disadvantages of A2/O process
1. The advantages of A2/O process
High treatment efficiency, generally: BOD5 and SS for 90% to 95%, more than 70% of the total nitrogen, phosphorus for about 90%, generally applicable to the requirements of nitrogen and phosphorus removal of large and medium-sized municipal sewage plants.
2. A2/O process shortcomings
Construction costs and operating costs are higher than the ordinary activated sludge method, operation and management requirements are high, only when the treated sewage is discharged into a closed body of water or slow-flowing body of water caused by eutrophication, thus affecting the water body, the process is used.
3.4 Traditional activated sludge process
For a long time, urban domestic wastewater is mostly used in activated sludge method, which is one of the most widely used biological treatment processes in the world, with the advantages of high treatment capacity and good effluent water quality. The method is mainly composed of aeration tank, sedimentation tank, sludge return and residual sludge discharge system. Wastewater and refluxed activated sludge enter the aeration tank together to form a mixture. Aeration tank is a biological reactor, through the aeration equipment filled with air, the oxygen in the air dissolved into the mixture, resulting in aerobic metabolic reactions, and so that the mixture gets enough stirring and suspended, so that the organic matter in the wastewater, oxygen and microorganisms can be in full contact with the reaction. Subsequently, the mixture enters the sedimentation tank, the suspended solids in the mixture in the sedimentation tank and water separation, out of the sedimentation tank is purified water. Most of the sludge in the sedimentation tank back to the flow, known as 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 amount of proliferating microorganisms is usually excluded from the sedimentation tank to maintain the stable operation of the activated sludge system, which is called residual sludge. In addition to the ability to oxidize and decompose organic matter, activated sludge should have good coagulation and sedimentation performance so that the activated sludge can be separated from the mixture to obtain clarified effluent. The use of traditional activated sludge method, often infrastructure costs, high operating costs, energy consumption, management complexity, prone to sludge expansion phenomenon; equipment can not meet the requirements of high efficiency and low consumption.
3.5 Oxidation ditch
Oxidation ditch is a variant of activated sludge method, its aeration tank is closed ditch type, so it is different from the traditional activated sludge method in hydraulic flow. It is a kind of circulating flow aeration ditch connected to the head and tail, sewage infiltration into which to get purified, the earliest oxidation ditch is to be bermed earth ditch, is intermittent into the water intermittent aeration. From this point of view, the oxidation ditch is the earliest in a sequential batch of sewage treatment technology. Oxidation ditch sewage treatment process such as water intake, aeration, sedimentation, sludge stabilization and effluent are all concentrated in the oxidation ditch to complete the earliest oxidation ditch do not need to set up a separate primary sedimentation tank, the second sedimentation tank and sludge reflux equipment. With the gradual expansion of the scale and scope of treatment, usually using delayed aeration. Continuous water in and out, the microbial sludge produced in the sewage aeration and purification at the same time to stabilize, without the need to set up the primary sedimentation tank and sludge digestion tank, treatment facilities greatly simplified.
Advantages of oxidation ditch process
Oxidation ditch process has become the first choice of small and medium-sized municipal wastewater treatment plant with its outstanding advantages of economic simplicity. It has the advantages of versatility, sludge stabilization, good effluent quality and easy management.
Summary, choose a good urban wastewater treatment process, both for the development of the national economy and environmental protection, resource reuse has an unusual significance. A good process has a simple process, reliable equipment, convenient management, investment, occupies an area of less, high efficiency, low operating costs, sewage treatment can meet the discharge standards and can be reused and other advantages.
Chapter 2: Process Selection of Municipal Wastewater Treatment Plant
Section 1: Process Selection of Large-scale Wastewater Treatment Plant
The preferred process of large-scale municipal wastewater treatment plant is the traditional activated sludge method and its improved A/O method and A2/O method. At present, most of the large sewage plants in the vast majority of countries in the world (including China) mostly use the traditional activated sludge method, A/O and A2/O method, and this process is used in Beijing Gaobeidian Sewage Treatment Plant, Tianjin Jizhuangzi Sewage Treatment Plant and Dongjiao Sewage Treatment Plant, Shenyang Northern Sewage Treatment Plant, Zhengzhou Municipal Sewage Treatment Plant, Hangzhou City Four Castles Sewage Treatment Plant, Chengdu City Sanyuayao Sewage Treatment Plant and so on in our country, and it is not accidental, as this process for the Large sewage plants have irreplaceable advantages:
1, traditional activated sludge, A/O and A2/O method with oxidation ditch and SBR process compared to the biggest advantage is lower energy consumption, lower operating costs, the larger the scale of this advantage is more obvious. For large sewage plants, the annual operating costs are substantial, such as the scale of 40 × 104 m3 / d sewage plant, 1 m3 of sewage to save 1 cent of the treatment fee, a year to save 1.46 million yuan.
This process of low energy consumption and operating costs are due to: a. Setting up a primary sedimentation tank, the use of physical methods to minimize energy consumption and costs to remove a significant portion of the sewage organic matter and suspended solids, reducing the load of the secondary treatment, a significant savings in energy consumption; b. Sludge using anaerobic digestion, which is a recognized energy-saving process than the oxidation ditch and the simultaneous aerobic digestion of the SBR process to significantly save energy consumption.
The capital investment of this process is generally higher than that of oxidation ditch and SBR process, but as the scale increases, the capital cost of oxidation ditch and SBR increases exponentially, while the investment of conventional activated sludge method increases by a smaller percentage, and the gap between the two is getting smaller and smaller. When the sewage plant reaches a certain size, the investment of conventional activated sludge method than oxidation ditch and SBR also save, so the larger the scale of the sewage plant, the greater the advantage of conventional activated sludge method.
2, the main disadvantage of conventional activated sludge, A/O and A2 / O method is that the treatment unit is more complex operation and management, especially anaerobic digestion of sludge requires a high level of management, digestion process produces methane gas is flammable and explosive gases, more requirements for safe operation, which increases the difficulty of management. However, due to the large sewage plant backed by big cities, strong technical force, high level of management, can meet this requirement, and thus the shortcomings of conventional activated sludge method will not become a limiting factor in the use of.
According to the current reality in China, urban sewage treatment is in its infancy, the regulations and systems are not sound enough, and there is no clear regulation on the requirements of sludge stabilization, and at the same time, due to the lack of perfect drainage network system, most of the urban sewage is not high in organic content, coupled with the management of anaerobic digestion of sludge and the use of biogas is still a lack of mature experience, all these factors have reduced the number of conventional activated sludge which contains anaerobic digestion process of sludge, the number of sludge is not high. These factors reduce the economics of conventional activated sludge, A/O and A2/O methods that include sludge anaerobic digestion processes. Therefore, for municipal wastewater treatment plants with a size of (10-20) × 104 m3/d, it may sometimes be more economical to use the SBR and oxidation ditch processes, in which case it is necessary to carry out a detailed techno-economic comparison of the various processes to determine the optimum process.
Section II, small and medium-sized municipal wastewater treatment plant of the preferred process
Small and medium-sized municipal wastewater treatment plant of the preferred process is oxidation ditch and SBR, their **** with the same characteristics are:
1, the removal of organic matter is very high efficiency, and some of the nitrogen, phosphorus, or both nitrogen and phosphorus, and the treatment facilities are very simple, very easy to manage, it is the internationally recognized efficient, simplified wastewater treatment plant. Recognized as a highly efficient, simplified sewage treatment process, but also the world's small and medium-sized municipal sewage treatment plants preferred process.
2, in 10 × 104 m3 / d scale, oxidation ditch and SBR method of infrastructure costs significantly lower than conventional activated sludge, A/O and A2 / O method; for the scale of (5 ~ 10) × 104 m3 / d sewage plant, oxidation ditch and SBR method of infrastructure costs are usually 10% to 15% lower. The smaller the scale, the greater the gap between the two, which is very attractive to small and medium-sized cities that lack the funds to build sewage plants.
Even in 10 × 104 m3 / d scale, oxidation ditch and SBR method of power consumption and annual operating costs are still higher than conventional activated sludge method, but if the cost of capital together with a comparison of the capital costs, capital costs plus 20 years of operating costs total or lower than conventional activated sludge method. The smaller the scale, the lower the more, the larger the scale, the smaller the gap, when the scale of 10 × 104 m3 / d, the total cost of the two types of processes is roughly equivalent. Therefore, for small and medium-sized sewage plants using oxidation ditch and SBR method is economically advantageous.
3, oxidation ditch and SBR process usually do not set up primary sedimentation tank and sludge digestion tank, the entire treatment unit than the conventional activated sludge method is less than 50%, the operation and management is greatly simplified, which is relatively weak for the technical force, the management of the relatively low level of small and medium-sized sewage treatment plant is very suitable.
4, oxidation ditch and SBR process equipment basically localized, in quality to meet the process requirements, the price is several times cheaper than foreign equipment, and also eliminates the need to apply for foreign exchange imported equipment of all kinds of trouble.
5, oxidation ditch and SBR process shock load resistance than the conventional activated sludge method is much better, which is for the water quality, water quality changes in small and medium-sized sewage plant is very favorable.
It is for all these reasons, oxidation ditch and SBR at home and abroad are developing rapidly. U.S. Environmental Protection Agency (EPA) sewage treatment plant construction costs or operating costs than the conventional activated sludge method to save more than 15% of the process as an innovative alternative technology, the federal government to give financial support, SBR and oxidation ditch process has been vigorously promoted, has been built hundreds of sewage plants. Oxidation ditch sewage plant in Europe has thousands, Australia in the past 10 years, nearly 600 SBR process sewage plant. In the country, oxidation ditch and SBR process has become the first choice of small and medium-sized sewage treatment plants.
6, the main classification and characteristics of the oxidation ditch process
Oxidation ditch process can be broadly divided into four categories:
① Multi-ditch alternating oxidation ditch, which is characterized by the combined construction type, there is no separate secondary sedimentation tank, the use of rotary brush aeration. It has a single ditch, double ditch and three ditch type, the most typical is Handan three ditch oxidation ditch. This oxidation ditch has the characteristics of the SBR process, can also be considered a type of SBR, its nitrogen and phosphorus removal effect is not stable, if the requirements of nitrogen and phosphorus removal, need to add some facilities.
② Carrousel oxidation ditch, it is a sub-built, with a separate secondary sedimentation tank, the use of table aeration machine aeration, the ditch depth is greater than the ditch alternating oxidation ditch, Changsha water purification plant is this process, it is not enough to remove nitrogen and phosphorus effect is not ideal, if you ask for removal of nitrogen and phosphorus, but also need to add some facilities.
③ Aubert oxidation ditch, it is also a sub-built, with a separate secondary sedimentation tank, the use of rotating disk aeration, the ditch depth is also larger, and now Sichuan, Beijing, Shandong, Zhejiang and other places are used, it is a very good effect of nitrogen removal, but phosphorus removal is not efficient enough, the requirement for removal of phosphorus needs to be taken when some measures.
④ integrated oxidation ditch, is a combined type, sedimentation tank built in the oxidation ditch, has been applied in Chengdu, Sichuan Xindu sewage plant and Shandong Gaomi City sewage plant. It is both continuous in and out of the water, but also the combined type, and do not have to invert the function, from the theory of the most economical and reasonable, but in some specific technical issues are not very mature, so it affects the popularization of its use.
7, the main classification and characteristics of the SBR process
SBR process mainly has the following types:
① traditional SBR process, all its operations are intermittent, cyclical, Sichuan Bazhong sewage plant is this process. It is not stable enough to remove nitrogen and phosphorus, such as the requirement of nitrogen and phosphorus removal, need to make some improvements.
② ICEAS process, that is, intermittent cyclic delayed aeration activated sludge method, which uses a partition wall to divide the reaction tank into two parts, in front of the pre-reaction area, behind the main reaction area, using a continuous water intake, intermittent aeration, sedimentation, drainage, sludge discharge, has been used in the third and fourth sewage plant in Kunming. It can remove nitrogen and phosphorus, but the effect is not ideal.
③ DAT-IAT process, that is, the combination of continuous aeration and intermittent aeration process, the middle of the reaction pool is divided into two parts with a partition wall, the front of the DAT continuous aeration, the back of the IAT intermittent aeration, sedimentation, drainage, mud, has been used in Tianjin Development Area Sewage Treatment Plant. It has a general function of nitrogen and phosphorus removal, and needs additional facilities to improve the efficiency of nitrogen and phosphorus removal.
④ CAST process, i.e., cyclic activated sludge method, its reaction tank is divided into the selection area and the main reaction area with a partition wall, and the water intake, aeration, precipitation, drainage and mud discharge are intermittent cyclic operation. It has good effect of nitrogen removal and phosphorus removal, and good performance of preventing sludge expansion. At present, this process is adopted in some sewage treatment plants in Shenzhen, Tianjin and Yunnan.
⑤ UNITANK process, is three rectangular pools in parallel, according to the similar three ditch oxidation ditch cycle operation mode work, but the rotary brush aeration changed to blast aeration, can increase the depth of the pool, the outlet adjustable weir changed to a fixed weir, to simplify the drainage, the Shanghai Shidongkou wastewater treatment plant is the use of such a process, which is similar to the three ditch oxidation ditch function.
8, oxidation ditch and SBR process comparison
Oxidation ditch and SBR process has a lot of *** with the same characteristics, but also their own characteristics and applicability, in the selected program needs to be carefully analyzed.
a, from the point of view of infrastructure investment, the SBR process is a joint construction, in general, land acquisition and civil construction costs lower than the oxidation ditch, while the equipment costs higher than the oxidation ditch, the total cost of the total cost of the decision to be made on a case-by-case basis.
b, high land prices, the oxidation ditch is not favorable. Inlet BOD concentration is high, the reaction volume and sedimentation volume ratio is high, favorable to the oxidation ditch; BOD concentration is low, the reaction volume and sedimentation volume ratio is low, favorable to the SBR.
c, from the point of view of operating costs, the SBR process is usually aeration with a blast, oxidation ditch process is usually mechanical aeration. Generally speaking, in the case of the same amount of oxygen supply, air blower aeration than mechanical aeration to save electricity; the second aspect, the SBR process is built, without sludge reflux (some small amount of reflux), oxidation ditch process is built to be a large number of reflux, power consumption is greater; the third aspect, the SBR process is a variable water level operation, increasing the intake lifting pumping station head. Comprehensive consideration, usually oxidation ditch process power consumption is larger than the SBR process, the operating costs to be higher.
d, oxidation ditch process is a continuous operation, does not require automatic control, only in the requirements of energy saving with automatic control; SBR process is a cycle of intermittent operation, the conversion of each process is frequent, the need for automatic control.
e, SBR process is static precipitation, oxidation ditch process is dynamic precipitation, thus SBR precipitation efficiency is higher, better water quality.