Analysis of key problems in the design of the hott

2022-08-17
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Analysis of key problems in the design of small underground sewage treatment plants

in recent years, with the process of urbanization and the rapid expansion of cities in China, as well as the continuous improvement of the requirements of urban water environment treatment, all regions have accelerated the construction of sewage treatment plants. Under the background of increasingly scarce land resources and rising land prices in big cities, the development momentum of underground sewage treatment plants is particularly rapid

small underground sewage treatment plant is a kind of underground sewage treatment plant, which has the characteristics of small scale, high occupancy of ground facilities and buildings, and difficult to be used as public green space. How to give full play to the advantages of small-scale and high flexibility of small-scale underground sewage treatment plants, improve the degree of intensification, so that they can not only save land, reduce cost, but also improve environmental benefits is a problem worthy of study and analysis

I. characteristics of small underground sewage treatment plant

1 The use of the ground floor

large underground sewage treatment plants generally use the ground floor for the construction of green landscapes, Waterscape, parks, etc., most of which can be opened to citizens and become urban public space. Foreign large-scale underground sewage treatment plants also use the ground floor as a football field

however, due to the needs of the sewage treatment plant's own functions and the necessary settings in ventilation, fire protection, etc. as a large underground building, the ground floor of the underground box structure of the underground sewage treatment plant still needs to set up a series of ground facilities, including: ventilation shaft, smoke exhaust shaft, escape passage, large equipment lifting holes, exhaust funnel of the deodorization system, etc. In consideration of saving energy consumption in operation, many underground sewage treatment plants will also be designed to set up some lighting wells to use natural lighting to reduce the use of lamps in the underground operation floor

began to wait for customers to come to us for power cells. At present, the area occupied by the upper facilities of the underground box structure of sewage treatment plants with a scale of more than 80000 m3/D is generally 1.5%-3% of the top plate area of the box structure; The area occupied by the upper facilities of the underground box structure of sewage treatment plants with a scale of less than 30000 m3/D can reach 3.5%-5% of the top plate area of the box structure In particular, considering that each fire compartment of the underground building needs to be equipped with a smoke exhaust shaft and an escape passage directly to the ground, as well as the air intake shaft and exhaust shaft usually set in each fire compartment, the distribution of ground facilities is relatively scattered, making it difficult for the ground floor of small underground sewage treatment plant to have a large piece of continuous green space for the public to use as a park

2. Layout of auxiliary buildings

considering the safety and convenience of the operation of the sewage treatment plant itself, the underground sewage treatment plant will not place all functional blocks in the underground box structure in the design. Auxiliary buildings usually considered to be placed independently on the ground include: (1) complex building: convenience; (2) Main distribution room: safety; (3) Sludge treatment facilities: the convenience of sludge transport vehicle operation and the safety of some processes; (4) Ozone preparation room: safety of liquid oxygen storage; (5) Dosing room with inflammable and explosive substances: safety; (6) Others

because large underground sewage treatment plants usually have a large plant area, the auxiliary buildings placed on the ground will not significantly affect the greening rate of the whole plant. In terms of the design of small underground sewage treatment plants, it is necessary to build the auxiliary buildings underground as much as possible, which requires giving play to the advantages of small-scale sewage treatment plants, flexible processes, less sludge and less reagent use

3. Project cost

at present, the construction investment cost of underground sewage treatment plants is more than 30% higher than that of surface sewage treatment plants. In actual engineering practice, the increased construction investment of the underground sewage treatment plant compared with the ground form is not only used for the construction of the underground box structure itself at the bell ringing ceremony, but also for foundation treatment, foundation pit enclosure, access road construction, etc

compared with large underground sewage treatment plants, in the construction of small underground sewage treatment plants, foundation pit enclosure and access channels obviously account for a higher proportion of the project engineering cost

II. Design points of small underground sewage treatment plant

1 Design of ground floor

there are three common layouts of underground sewage treatment plants, namely, full underground double-layer capping, semi underground double-layer capping, and full underground single-layer capping. Among them, the layout of all underground double-layer capping has been widely used because of its good sealing performance, good landscape effect and little impact on the surrounding environment. The upper space can be used as a public open area

however, for small underground sewage treatment plants, because of their high opening rate of underground boxes and small overall land occupation, the space on the ground floor is usually not open as a public space and can only be used for plant greening. Under such a premise, the design of the ground floor should consider different strategies from large underground sewage treatment plants, including:

(1) some treatment units with high odor concentration and high noise impact, such as pretreatment, sludge treatment, blower room, etc., should be covered with full underground double layers; Some treatment units with less environmental impact, such as advanced treatment, can be covered with a full underground single layer, which can reduce the depth of structures and foundation pits and reduce the project cost while ensuring the sealing performance

(2) try to arrange the ground auxiliary buildings directly above the underground box structure, and comprehensively consider the design of the underground box and the superstructure. On the one hand, it can reduce the construction cost of the superstructure itself and save the land occupation of the project. At the same time, it can combine and arrange deodorizing exhaust funnel, ventilation shaft, escape passage and other facilities to make the buildings in the plant more compact and beautiful; On the other hand, because such an arrangement leaves more of the upper area of the underground box to buildings rather than greening, the requirements for covering soil on the top of the underground box are reduced. The greening form of thin covering soil on the top of the underground box to plant turf and planting tall trees in the surrounding open space can be adopted to reduce the buried depth of the underground box structure on the premise of ensuring the landscape effect of the plant area, so as to reduce the project cost

(3) because the ground structure and ground building occupy a curve comparison in the plant area of small underground sewage treatment plant; For a larger proportion, the impact of these buildings and structures on the overall landscape effect cannot be ignored. The design of the auxiliary structures of the underground box and the ground buildings should be considered as a whole with the plant landscape, so that these auxiliary structures and the ground buildings themselves become a part of the landscape

2. Design of auxiliary buildings

first of all, in the design of underground sewage treatment plant, considering the humanization and convenience of sewage treatment plant management, the complex building will generally be placed on the ground. At the same time, due to the high risk that the spatial layout of the bunched cable burner in the underground sewage treatment plant will be submerged under extreme weather conditions or accident conditions, it is also recommended to place the main distribution room on the ground in order to enhance the safety of power supply, enhance the security of accident treatment and reduce accident losses. If it is necessary to consider building the main distribution room underground, multiple flood prevention measures should be considered, including but not limited to multiple intake gates, intake quick closing gates, rainwater pump wells and high-power drainage pumps, watertight doors, etc

secondly, unlike large sewage treatment plants, which need to complete sludge thickening, dewatering and treatment and disposal in the plant, small sewage treatment plants produce less sludge and are generally not used as independent sludge treatment and disposal centers. Therefore, it is reasonable to transport the sludge after thickening and dewatering for centralized treatment and disposal or to transport it for further treatment after plate and frame dehydration in the plant. Whether mechanical dehydration or plate and frame dehydration are built underground, there is no safety problem. It is an optional layout form, but the height of the dehydration machine room is high. In the design, attention should be paid to the rational use of the vertical space of the dehydration machine room, such as using the topographic relief of the ground floor to make the top plate elevation of the dehydration machine room higher than the water treatment area. It is convenient for equipment installation and operation, while controlling the bottom plate depth, and then controlling the cost

thirdly, safety should be given priority in the selection of agents, rather than cost. For example, when ozone decolorization process is needed, air preparation with good safety should be preferred rather than liquid oxygen preparation. In this way, the limitation of explosion-proof distance can be compressed or eliminated, and the dosing facilities can be built underground as much as possible. Because the absolute value of the reagent cost of the small sewage treatment plant is small, the land savings brought by adopting a more intensive layout are conducive to the improvement of the overall economy of the project

3. Control of project cost

in addition to the above-mentioned reasonable layout in the design of the ground floor of the plant to control the project cost, there are also some measures that should be paid special attention to in the design:

(1) for underground sewage treatment plants, the buried depth is an important factor in determining the project cost, and the deeper the buried depth means the higher foundation pit enclosure cost, resulting in the higher project cost required for the underground box structure. However, in engineering practice, the design floor elevation of the underground box structure may be close to the elevation of the land bearing layer. In this case, properly adjusting or even deepening the buried depth of the underground box can greatly reduce the cost of foundation treatment, and then save the cost of the whole project, which should be paid attention to in the elevation design

(2) compared with large underground sewage treatment plants, the underground structure of small underground sewage treatment plants has a larger perimeter area ratio, which means that the cost of foundation pit enclosure accounts for a higher proportion in the overall cost of the project. When designing a small underground sewage treatment plant, attention should be paid to the use of regular shaped underground boxes as far as possible to avoid irregular shapes, long strips and other forms with relatively large perimeter areas, so as to save the cost of foundation pit enclosure

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