Analysis of CSOs pollution load reduction based on pipeline sediment control
Fund Project: National Water Pollution Control and Governance Science and Technology Major Project (2008ZX07313-001) Structural Parameters Operating Parameters Project Pipe Diameter / mm Aperture mm Hole Spacing / cm Axial Angle / (.) Tangential Angle / (.) Pressure / MPa Jet Overflow (CSOs) of speed combined drainage system is an important factor causing water pollution. Pollution sources in CSOs include sewage, initial rainwater pollution and pipeline sediments, of which pollution from pipeline sediments accounts for a considerable proportion. When the amount of water on a sunny day is small, sediments are easy to deposit in the pipeline. These sediments carry a large amount of pollutants, which are washed into the nearby water bodies on a rainy day and cause pollution. Studies have shown that 30% to 80% of the polluted load of the receiving water body during storms originates from pipeline sediment discharged by overflow 1. Therefore, controlling the pipeline sediment by cleaning the pipeline is an important measure to reduce the pollution load of CSOs.
1 Project Overview The project is transformed on the basis of the intercepting and confluent drainage system. The system intercepts sewage on sunny days, and when the rainfall exceeds the lifting capacity of the pumping station on rainy days, the gate is opened and the rain and sewage overflow into nearby water bodies. The design scale of the pumping station is 1000m3 / mountain service area is 0.087km2; the service area is mainly living quarters, accounting for 90% of the total area, and the others are shops, clinics, office buildings and catering. A total of three feeder pipelines enter the pumping station. During the implementation of the project, one of the feeders with the larger flow is selected. Its total length is about 420m, the service area is 0.0297km2, the pipe is a concrete prefabricated pipe, and the pipe diameter is DN300 ~ DN500. The longest pipeline between the wells is 40m, and the flow rate at the end of the branch line is 250 ~ 300m3 / d on sunny days. For the purpose of sediment cleaning, the pipeline cleaning needs to be carried out regularly, the flushing cycle is short, and the flushing strength is relatively low. Pipe sediment control technology combined with cyclone separation. The project is divided into two parts: pipeline flushing and hydrocyclone separation. Pipeline flushing generates jets by supplying high-pressure water to a perforated pipe previously laid in the pipe, flushing the pipe sediment and moving it to a downstream inspection well or pumping station. The sediment settled in the inspection well is removed manually or by the suction truck, and the mud-water mixture entering the pumping station is separated by cyclone to remove the sediment and stored in the mud storage tank (see process).
The process pipe is made of low-density high-pressure polyethylene. The structural parameters of the perforated pipe include aperture, hole spacing and opening angle; operating parameters include pressure (or jet velocity) and flushing time; other parameters mainly include installation height. The relevant parameters selected for this project are determined according to the simulation test results and the engineering conditions (pipe diameter is DN400, pipe length is 40m), as shown in Table 1. When the perforated pipe is laid in the pipeline, the orifice is oriented symmetrically along the central axis of the pipeline At every 1m, install the support according to the installation height and fix it in the inspection wells at both ends. After that, the perforated pipe is fixed in the inspection well through 90. The elbow is fixed up along the well wall to 5 ~ 10cm under the manhole cover. Connect to the hose of the pump. During flushing, water is supplied to the perforated pipe through the vehicle-mounted self-priming gasoline pump and the water tank, and both ends of the pipe are simultaneously supplied with water. According to the above parameters and pipeline length, the safety factor is 1.2. After hydraulic calculation and correction, the WP -20X self-priming pump is selected. The flow rate and head are 36m3 / h and 260kPa, respectively. And equipped with a water tank with a water tank capacity of 4m3 and two 25m fire hoses.
Table 1 Technical parameters of perforated pipe flushing Hydrocyclone separation is to provide a high-pressure water head to the cyclone separator through a water pump to generate a high-strength centrifugal force field in the separator to separate the solid particles in the mud-water mixture. After separation, it is relatively clean The water is discharged through the top overflow port, and the concentrated liquid is discharged through the bottom flow port, and the purpose of purifying the water quality and reducing the pollution load is achieved by removing the particulate matter therein. The project uses XQ-25 hydraulic cyclone separator with a processing capacity of 25m3 / h and a separation capacity of 20 ~ 50pm. The processing capacity of the submersible pump is the same as that of the cyclone separator, also 25m3 / h, head H = h ++ hf, where is the height from the bottom of the collecting tank to the inlet of the cyclone, 6.3m; h2 is the maximum inlet pressure of the cyclone separator, which is 120kPa (12m water column); hf is the resistance loss and local resistance along the way The sum of the losses is calculated as 24kPa (2.4m water column). After calculation, the required pump head is 207kPa. Choose the 22-4 submersible sewage pump, the flow rate is 25m3 / h, the head is 220kPa, and the power is 4kW. 1.3 The flow rate of the bottom flow port of the cyclone separator of the mud storage tank accounts for 4% of the inlet flow rate. %, Flush the upstream pipeline once every two weeks, the consumption of flushing water is 8 ~ 12m3 per flush, the water volume of the collection well pipeline is 15 and the cyclone separator needs to run about 1h after each pipeline flushing, and the volume of mud produced each time It is designed for 1m3. The size of the mud storage tank is 1.5mX1.5mx1.5m. At the same time, a hole is opened at 1.0m and a valve is provided to allow the supernatant to return to the pump station after sedimentation, effectively use the volume of the mud storage tank and reduce the frequency of silt removal .
2 Operation mode and effect Flushing is carried out section by section from upstream pipeline to downstream. After the water tank is filled with water, the flushing vehicle is placed in the middle of the pipe section that needs to be flushed. Connect the hose and the water pump, and then connect the hose and the flushing pipe with a movable joint. Turn on the water pump. After flushing for 2 ~ 3 minutes, turn off the water pump to stop flushing. Repeat the above operation for one pipe section. Run the cyclone separator while flushing the pipes. After the flushing is completed, the hydrocyclone is stopped when the water level of the collection tank drops to the minimum, the sediment of the inspection well is cleaned, the valve of the mud storage tank is opened, and the supernatant is discharged into the collection well of the pump station.
The flushing cycle is determined according to the pipeline sedimentation intensity, the flushing capacity of the flushing tube and the rainfall cycle, mainly considering that the pipeline sediment is discharged out of the system before the overflow occurs.
In this project, the washing cycle is 2 weeks.
2.1 Flushing curve ~ 4) Flushing was carried out, and the data of the pipeline section is shown in Table 2. Table 2 Basic situation of the flushing pipeline section After flushing, the ss concentration in the downstream outlet water sample changes with time as shown.
It can be seen that the SS concentration of the effluent rises rapidly within 30 seconds after the start of washing, reaches the highest value after 60 to 90 seconds of washing, and then drops to a lower level during 150 to 180 seconds, basically completing the washing process.
See the flushing effect of the simulation test.
Perforated pipe flushing effect 2.2 The separation effect of the cyclone is 1 mg / L, and the inlet and outlet water D5 are 55.2%, 6.3%, and 32.5%, respectively, indicating that the cyclone has a significant effect on the removal of pollutants. The removal effect of the cyclone separator is mainly related to the particle size distribution of the particles in the influent. The larger the median particle size D50, the higher the removal efficiency. After separation, the median particle size of suspended particles becomes smaller.
2.3 Change of pollutant concentration before and after project implementation See the change of CSOs pollutant concentration before and after project implementation.
After the implementation of the project, the concentration of overflow pollutants decreased significantly, and the average concentrations of SS and COD decreased by 23.9% and 23.2% respectively. The monitoring results show that cleaning the pipeline has a significant effect on reducing the pollution load of CSOs.
Comparison of overflow water quality before and after the implementation of the project Table 3 Engineering operation cost items Fuel oil water and electricity charges manual total value 5.124.56-36.843.890123.46 -135.74 Note: Water price, electricity price and fuel oil price refer to the price level of the project site.
2.4 Estimated rainfall data for pollution load reduction. There are 62 rainfall events with a rainfall of> 5mm, with an average rainfall of 11.8mm and an average rainfall duration of 9h. ~ 300m3 / d, with an average of 12m3 / h, according to the surface runoff coefficient of 2 cities 0.8. If the infiltration and seepage of the pipeline are not considered, the rainfall is calculated as 17383m3 through the average rainfall and service area, and the interception multiple is 2, then the total annual overflow is 3 991m3. Some studies have reported that the overflow The average values ​​of COD and SS are 614 and 84mg / L3, respectively, and the annual overflow pollution load is calculated based on the annual total overflow: COD is 2 730kg. After the project is implemented, the average concentration of COD and SS has been reduced by 23.2% and 23.9% respectively, and thus reduced The COD and SS loads are 568.4 and 52.5kg respectively. 3 Economic analysis does not include the investment of the washing vehicle, the total cost of equipment materials is 13,800 yuan, the installation cost is 12,300 yuan, the construction cost is 18,200 yuan, the total investment of the project It is RMB 44,300, and the investment cost is 320 yuan / m pipeline. With the increase of the laying length of the flushing pipe, the investment cost per unit pipe length will decrease. Assuming that the cleaning vehicle travels 5km each time, the water consumption is 8 ~ 12m3, the power consumption is 7.2kWh, and two workers each work a total of 2h, the operating cost is shown in Table 3.4. 4 Discussion 4.1 Technical characteristics and CSOs end control In contrast, sediment-based CSOs control is to take measures before CSOs occur, so it is more proactive and less difficult to implement. Traditional and mainstream pipeline desilting mainly relies on machinery, or high-pressure hydraulic cleaning combined with suction trucks. These technologies are mainly aimed at the serious siltation of pipelines. However, sediment-based overflow pollution control requires frequent pipe cleaning. The flushing technology of the embedded perforated pipe just meets such technical requirements, which can effectively clean the pipeline, at the same time, the cleaning time is short, and the operation is convenient. The hydrocyclone has a small footprint and can be combined with multiple cyclones to adapt to changes in water volume. It is easy to assemble, has high separation efficiency, and does not require excavation.
4.2 Applicable occasions Due to the limited flushing strength and flushing range of the perforated pipe, the flushing effect of the sediment layer of the large-diameter pipe and the compacted pipe is poor, so it is suitable for small and medium-diameter drainage pipes. Before laying the perforated pipe, it is best to dredge the pipe thoroughly. In order to reduce the installation cost, it is recommended to bury perforated pipes during pipeline laying and reconstruction. Perforated pipe flushing can meet the cleaning of most pipelines with a length of 40m between inspection wells. For pipelines with particularly long lengths, due to the large loss of resistance, the local flushing strength is weak, and a good flushing effect cannot be achieved. The larger the required diameter of the perforated pipe is, the smaller the diameter of the drainage pipe may affect its conveying function.
4.3 Relevant considerations for the control of pipeline cleaning CSOs Pipe cleaning can effectively reduce the pollution load of CSOs. It is of great significance to develop appropriate cleaning technologies and prepare corresponding cleaning regulations according to the characteristics of pipelines in specific occasions. The requirements for pipeline cleaning technology are economic, effective, fast and convenient. The pipeline cleaning procedures should include the pipeline cleaning cycle and cleaning process. The cleaning cycle should comprehensively consider factors such as pipeline deposition strength, the flushing capacity of the pipeline flushing technology, rainfall cycle and rainfall intensity to meet economic requirements.
5 Conclusion Install a perforated pipe in the upstream pipeline of the interception pump station of the combined drainage system, provide high-pressure water to the perforated pipe to generate a jet of 7.5m / s or more to flush the pipeline, and set up a cyclone separation system in the interception pump station to remove the flushing water Sediments. The operation shows that for the shallow water layer, the diameter of the pipe is DN400, the length is 40m, and the thickness of the sediment layer is 3cm, the flushing can increase the SS in the pipeline flow from 2500 to 3000mg / L from before the flushing. Rinse well within 3 minutes. The removal rate of SS in the flushing water by the cyclone separator is 55.6%. The routine cleaning and maintenance of the pipeline once every two weeks can reduce the concentration of pollutants in CSOs by about 25%.
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