Design and operation of all underground treatment facilities and overflow pollution control
The main process of membrane bioreactor (MBR) is used for wastewater treatment, with a design scale of 12,200m3 /d and a storage tank design scale of 5,000m3. The water purification station has the functions of sewage treatment, rainwater storage and urban traffic regulation.
The actual operation shows that the water quality of the water purification station in dry and rainy days is stable and reaches the Grade A standard. Under the condition of full load operation, the CODCr and ammonia can be reduced to 1 533 t/a and 109.5 t/a in dry days. In rainy days, COD can be reduced to 210 t/a and SS to 144 t/a. The land area of the all-underground water purification station is only 6 100m2, and the construction land index is far lower than the limit of 1.50~ 1.20㎡/(m3·d-1). At the same time, the integration with the ground landscape park and the surrounding environment, not only has the characteristics of compact layout, land saving, flexible and efficient operation, but also achieves the purpose of "nimby" to "neighbor benefit". It provides design reference and practical engineering experience for the construction of environment-friendly underground sewage treatment plant.
1. Construction scale and form
1.1 Construction scale
Getang River drainage area of 5.32 square meters, according to the current drainage network, the area is divided into 8 catchment zones, planning to build 3 storage ponds and 2 water purification stations. Among them, the service area of Getang River water purification station is 1.63 km2. According to the estimation of per capita comprehensive water use index method and classified land water use index method, the sewage volume will be 11,600 m3/d by 2025. Therefore, the sewage treatment scale of the water purification station is determined to be 1,200 m3/d. According to the "Nanjing Sponge City Special Planning", when the non-point source pollution reduction rate of a typical watershed reaches the control requirement of 50%, the initial stormwater runoff is about 5 mm, and the runoff coefficient of the general area weighted by area is 0.5~0.8. Therefore, the design scale of the storage tank at the Getang River water purification station is determined to be 5 000m3. The storage tank is used to collect mixed rain sewage during rainfall in the near future, and after the drainage of rain pollution is perfect, it is used to collect initial rain water in the long term.
1.2 Construction form
In order to make full use of public space, a small underground parking lot is also considered for the construction of Getang River water purification station and storage pond. Since the site is located under a square, in order not to affect the above-ground use function of the original square, it is considered to adopt an all-underground water purification station. Generally, the underground sewage treatment plant is divided into double layer and single layer. Due to the small scale of the water purification station, comprehensive investment, surrounding environment and other factors, the single-layer construction form is selected.
2 Design water quality and treatment process
2.1 Design inlet and outlet water quality
There is a municipal sewage treatment plant near Getang River Water purification Station. After statistical analysis of the original design inlet water quality of the municipal sewage treatment plant and the actual inlet water quality in the past three years, the design inlet water quality of the water purification station is finally determined, as shown in Table 1. According to the current national unified requirements for effluent quality of sewage treatment plants and the current discharge standard assessment requirements of Jiangbei New District, Nanjing City, it is determined that the effluent quality of the design of this project adopts the "Pollutant Discharge Standard for Urban sewage Treatment Plants" (GB 18918-2002) Grade A standard.
2.2 Project Difficulties
Through the statistical analysis of water quality and construction conditions, the design and implementation of this project are mainly difficult in the following three projects. (1) The land is tight and the layout is compact: the project is jointly built water purification station, storage tank and parking lot, the site area is only 6 100 square meters (including access channels), and the land saving technology needs to be adopted. (2) Taking into account the needs of above-ground and underground functions: underground facilities should meet the sewage treatment standards and discharge, and consider the access problems of personnel, materials and equipment, and the above-ground public space functions should be restored after construction. (3) Taking into account the operation requirements of dry days and rainy days: the engineering design checks the water quantity and quality of the two working conditions in dry days and rainy days to ensure that the effluent water quality meets the standards.
2.3 Selection of main process scheme
Activated sludge method has more applications in large-scale sewage treatment plants. According to the statistics of the Ministry of Housing and Urban-Rural Development, as of 2010, among the 2 739 urban sewage treatment plants that have been put into operation, there are 190 sewage treatment plants with a scale of more than 100,000 m³/d, and 40% of them use AAO or AO process. Therefore, the AAO process is used as one of the selected processes. The modified sequential batch reactor (MSBR) process has many application cases in municipal sewage treatment due to its small footprint, and can also achieve stable effluent effect, and it is also applied in Jiangsu region. Therefore, it is also one of the alternatives. In recent years, the membrane bioreactor (MBR) process is also widely used in the treatment of sewage plants, and the MBR process as the ultrafiltration facility at the back end of the AAO process can greatly improve the sludge concentration of the AAO system, reduce the volume of the entire biochemical system, and thus greatly reduce the footprint. In this project, the sewage plant adopts the underground layout mode, and the AAO+MBR process can realize the intensive layout and save land, so the AAO+MBR process is also used as the comparison process in this project.
Due to the maintenance and replacement of the membrane, although the operating cost is higher than the other two programs, the AAO+MBR process has better treatment effect, small footprint, and strong adaptability and flexibility. In addition, the effect of energy saving and emission reduction is much higher than that of other processes. In the application of AAO+MBR process in large-scale underground sewage treatment plants, Zhu Yuanyuan et al. found that the AAO+MBR process can stably remove CODCr, TN, ammonia nitrogen, TP and SS, the effluent is better than the standard requirements of GB 18918-2002 Grade A, and the MBR runs stably. Taking into account the development trend of the industry, land use conditions, energy saving and emission reduction and project investment, the project adopts AAO+MBR process.
2.4 Process plan determination
Getang River water purification station has two operating conditions: dry day and rainy day:
In dry days, the domestic sewage directly enters the sewage treatment unit, and the storage tank is vacant.
In rainy days, the mixed rain sewage enters the sewage treatment unit on one side and the storage tank on the other. After the storage tank is full, the rainwater overflows, and the storage water is slowly sent to the sewage treatment unit for treatment after the rain.
When the influent concentration is reduced in rainy days, the influent flow rate of the sewage treatment unit can be appropriately increased for a short time, and the maximum flow rate is considered to increase by 50%. The water intake is controlled by the influent pump room and three variable frequency submersible centrifugal pumps are set.
Water inlet pretreatment is set up in four grates. Among them, the front 50mm coarse grille is set in the total water inlet end, the storage pool is set 20mm coarse grille in front of the water inlet pump room, and the water purification station is set 5mm fine grille and 1mm fine grille after the water inlet pump room to ensure the normal operation of subsequent water treatment equipment and membrane filtration, and the grille is equipped according to the maximum flow rate in rainy days. The aerated sand sedimentation tank is arranged between the fine grid and the fine grid, and the residence time is 7.5 min in dry days and 5.0 min in rainy days.
AAO+MBR process was used for secondary treatment. After comparison and selection, the process is especially suitable for small-scale and compact sewage treatment. Among them, membrane separation saves about 50% of the land than the conventional advection secondary sedimentation tank. At the same time, due to the high concentration of sludge in the membrane tank, the average sludge concentration in the biological reaction tank can be increased by about 60% compared with the conventional activated sludge method through sludge reflux, which has obvious land saving advantages. The effluent water quality is relatively stable with one sludge reflux and two sludge reflux in the secondary treatment.
The tail water disinfection process is ultraviolet disinfection, and the equipment is pipe type and connected with the MBR production pump.
The sludge is dehydrated to 80% moisture content by the stacked screw sludge dewatering machine and disposed of outside.
Deodorizing soil filter adopts biological deodorizing process, and the odor is collected by pipeline and sent to the ground soil filter, combined with the ground greening setting.
The overall process flow is shown in Figure 1.
3Engineering Design
3.1 Graphic Design
According to the water inlet and outlet location and land conditions, the water purification station will be built together with sewage treatment facilities, storage ponds and underground parking lots. The specific plane layout is shown in FIG. 2 and FIG. 3. Among them, the maximum size of the box is 88.5m × 75.5m. In order to facilitate the operation and inspection of the equipment and pump produced by the membrane pool in the underground building, a corridor is set in the length direction of the box, and personnel can enter the corridor through the stairwell to reach the membrane pool and auxiliary room area.
Because the box adopts single-layer underground construction, some equipment, grid slag and sludge need to be repaired or access channels on the ground. The top floor openings are divided into four categories, including equipment inspection openings that are not often opened, steel grille cover plates in the membrane pool area, ground air Wells and personnel access stairwells. Combined with landscape park greening, a soil deodorant biological filter is set on the ground. Combined with the functional needs of the civic square, the ground is completely open. There are 28 parking Spaces in the underground parking lot with separate access ramps.
3.2 Vertical Design
The ground elevation of the public square is 10.7~11.0 m, the box is located below the public square, and the overall buried depth is about 5.5 m; The ground level of underground buildings such as parking lot, dewatering machine room, blower room and power distribution room is 6.35m, and the clearance height is about 4.5m. The top elevation of the pretreatment area is 11.30m, slightly higher than the ground of the square, and the operation space of the area is set in the box; The roof elevation of the biological reaction tank is 10.70 m, the height is 90 cm, and the upper shallow soil cover is 30 cm. The top elevation of the membrane pool was 11.30 m, and the height was 180 cm. The top elevation of the storage tank is 9.90m, and the upper soil cover is 110cm. The profile of the water purification station is shown in Figure 4.
The box is located in the whole underground, in order to avoid heavy rainfall when a lot of rain into the box to personnel, electrical equipment and other threats, set a quick closing gate at the inlet end. At the same time, where the cover plate of the equipment manhole is opened relatively frequently, the design elevation of the box top is 30cm higher than the floor of the square, and the cover plate where it is opened infrequently adopts a closed cover plate. The garage ramp is equipped with a flood interception ditch and a strong drainage pump to prevent rainwater from entering the box.
3.3 Design of AAO biological reaction cell and membrane cell
The AAO biological reaction tank is set up in two groups, each group can operate independently, and the effective water depth is 3.6 m. The total hydraulic retention time in dry days was 10.9 h, of which the anaerobic zone was 2.2 h, the anoxic zone was 3.3 h, and the aerobic zone was 5.4 h. The design external reflux ratio is 400%, the aerobic to anoxic internal reflux ratio is 200%, and the anoxic to anaerobic internal reflux ratio is 100%. Due to the multiple backflow places in the system, the mass concentration of sludge in the biological reaction tank showed a gradient change. The anaerobic zone was 3.2 g/L, the anoxic zone was 6.4 g/L, the aerobic zone was 9.6 g/L, and the average sludge mass concentration was 7.4 g/L. The influent concentration in rainy days is reduced by 40% ~ 50% compared with that in dry days, and the biological reaction tank is checked according to the hydraulic load by 50%. The total hydraulic retention time in rainy days is 7.3h, and the standby pump of the external reflux pump is opened to maintain the sludge concentration in the biological reaction tank and ensure the quality of the secondary effluent in dry and rainy days is up to standard.
There are 4 cells in the membrane pool, the water depth is 3.4m, the average filtration rate is 12.1L /((㎡·h), the peak filtration rate in rainy days is 19.4L /(㎡·h), and the mass concentration of sludge in the pool is 12.0g /L. Polyvinylidene fluoride (PVDF) modified hollow fiber membrane with a pore size of 0.03μm was used for membrane filtration, and continuous aerated membrane scrub + intermittent online dosing was used for membrane cleaning. The pump lift was 10 m.
According to the "Urban sewage treatment project construction standards" (construction standard 198-2022), the construction land control index of Class IV (10,000~50,000 m³/d) sewage plant unit water volume should not exceed 1.50~1.20 (㎡/(m³·d-1). The sewage treatment scale of the project is 1,200 m3/d, and the construction land control index of the unit water volume of the project calculated by the interpolation method should not exceed 1.485 (㎡/(m³·d-1), that is, the unit area should not exceed 17 820 (㎡). The AAO biological reaction tank and membrane tank of this project cover a total area of about 2 150 (㎡), which is far less than the construction land area calculated by the construction standard 198-2022, further demonstrating the land saving advantage of this project.
3.4 Storage tank design
The design effective water depth of the storage tank is 3.6m, and the interior is divided into four galleries, with a single lattice width of 5m and a length of 50.5m. Gate-type washing equipment (FIG. 5) is adopted for the flushing of the storage tank. By combining the hydraulic principle and mechanical structure, the storage chamber and washing door set at the beginning of the corridor can be used to realize automatic flushing without power and external water supply after the storage tank is emptied. The working process is a coordinated action between two sets of hydraulically controlled floats (main float and control float) and several sets of washing doors, which are connected to a fully enclosed hydraulic system. The storage tank is connected with the inlet pump room of the water purification station, and the peak is emptied when the rain is sunny.
3.5 Fire Control Design
The fire resistance grade of the underground equipment room is one, and the overall fire risk category is positioned as Class D. The total construction area of the underground equipment room is 1 115.87 square meters, which is divided into 2 fire zones, and the firewall is set up in the middle of the corridor, and each fire zone is set up with 1 safety evacuation port.
The underground parking lot has a total of 28 parking Spaces with a construction area of 1 107.31 square meters. As a fire protection zone, it is equipped with a safety exit and a vehicle access ramp.
4 Operation Management
4.1 Actual Operating status
The Getang River Water purification Station has gradually entered the commissioning period since the second half of 2020, and the water intake and water quality in the past year are shown in Figure 6 and Figure 7. Since the second half of 2021, the water intake has steadily increased, with the daily operating water volume ranging from 8,000 to 10,000 m3/d, and the rainy season is significantly higher than the dry season. From the perspective of influent water quality, the average influent CODCr mass concentration is about 200 mg/L and the average ammonia nitrogen mass concentration is about 20 mg/L in the dry season, while the average influent CODCr mass concentration is about 120 mg/L and the average ammonia nitrogen mass concentration is about 14 mg/L in the rainy season. The data of water quantity and influent quality show that the pipe network in the area needs to be further repaired and improved.
The effluent water quality of the water purification station (FIG. 8) has reached the Grade A standard stably, in which the mass concentration of CODCr is basically stable below 40 mg/L, the mass concentration of ammonia nitrogen is no more than 1.0 mg/L, and the mass concentration of TP is stable at about 0.4 mg/L.
3.5 Fire Control Design
The fire resistance grade of the underground equipment room is one, and the overall fire risk category is positioned as Class D. The total construction area of the underground equipment room is 1 115.87 square meters, which is divided into 2 fire zones, and the firewall is set up in the middle of the corridor, and each fire zone is set up with 1 safety evacuation port.
The underground parking lot has a total of 28 parking Spaces with a construction area of 1 107.31 square meters. As a fire protection zone, it is equipped with a safety exit and a vehicle access ramp.
4 Operation Management
4.1 Actual Operating status
The Getang River Water purification Station has gradually entered the commissioning period since the second half of 2020, and the water intake and water quality in the past year are shown in Figure 6 and Figure 7. Since the second half of 2021, the water intake has steadily increased, with the daily operating water volume ranging from 8,000 to 10,000 m3/d, and the rainy season is significantly higher than the dry season. From the perspective of influent water quality, the average influent CODCr mass concentration is about 200 mg/L and the average ammonia nitrogen mass concentration is about 20 mg/L in the dry season, while the average influent CODCr mass concentration is about 120 mg/L and the average ammonia nitrogen mass concentration is about 14 mg/L in the rainy season. The data of water quantity and influent quality show that the pipe network in the area needs to be further repaired and improved. 4.2 Main points of operation management
The operation and management of Getang River water purification station adopts the mode of remote control and regular personnel inspection. Compared with the conventional above-ground sewage treatment plant, the operation and management points include the following three points.
(1) The single use of the storage tank emptying time is not more than 24 hours to ensure the use efficiency of the storage tank. The average discharge is 208 m³/h. Although the current water purification station has not reached the full load, the discharge of storage tank still adopts the form of variable flow, that is, the peak of discharge of storage tank is the low peak of sewage inlet flow. At the same time, the water purification station is coordinated with the municipal sewage treatment plant, so that the water purification station can run relatively smoothly.
(2) Daily slag grid, sludge removal, etc., need to open the ground cover, and the ground is the open space of the civic square, in order to reduce the impact, choose the daily square when the staff is less.
(3) Personnel inspection and remote monitoring pay special attention to the switch status of stairwell entrances and ground cover boards, and take closing and locking measures during non-inspection time to prevent non-related personnel from entering the water purification station or safety accidents.
5 Investment and environmental benefits
The total investment of this project is about 160 million yuan, of which the project cost is about 140 million yuan. When the project is put into operation at full capacity, the CODCr and ammonia nitrogen can be reduced to 1 533 t/a and 109.5 t/a respectively. Based on 120 typical annual rainfall events in Nanjing, the CODCr and SS of pollutants entering the river on rainy days can be reduced to 210 t/a and 144 t/a.
6. Project summary and reflection
(1) Getang River Water purification Station adopts the integrated construction of sewage treatment facilities, storage tank and underground parking lot, which has the functions of sewage treatment, rainwater storage and urban traffic regulation, and has the characteristics of compact layout, land saving, flexible operation and efficient technology. The area of the project is far less than the relevant requirements of the construction standard 198-2022, which further demonstrates the advantages of land saving of the project.
(2) The single-layer all-underground construction method is adopted to make full use of the underground space of public land, and does not affect the above-ground use function of public land after completion, which has certain reference significance for the site selection and construction of such decentralized water purification stations. At the same time, it also provides design reference and practical engineering experience for the construction of environment-friendly underground sewage treatment plant.
(3) A soil deodorant biological filter is set up on the ground in combination with greening by using the deodorant technology combined with biological method and physical chemical adsorption. It can not only effectively reduce the odor concentration of the underground sewage treatment plant and facilitate the operation and management, but also integrate with the ground landscape park and the surrounding environment to achieve the purpose of "nimby" and "neighbor benefit ".
(4) The project will rain rain and sewage mixed water treatment standards in accordance with the national standard A standard design, consistent with the dry day sewage treatment standards. At this stage, most of such water purification stations in other cities in China are implemented according to this standard, and the feasibility of increasing the inflow flow on rainy days and appropriately reducing the treatment standards remains to be further explored by the industry.
