The design points for various sedimentation tanks are all here!

1/Horizontal sedimentation tank

The surface shape of a horizontal sedimentation tank is generally rectangular. After energy dissipation and rectification in the inlet area, the water flows slowly and horizontally into the sedimentation area. The suspended solids that can settle in the water gradually settle towards the bottom of the tank. The effluent from the sedimentation area overflows the weir and is discharged outside the tank through the outlet tank.

The basic requirements for a horizontal sedimentation tank are as follows:

(1) The length of the horizontal sedimentation tank is mostly 30-50m, and the width of the tank is mostly 5-10m. The effective water depth of the sedimentation area generally does not exceed 3m, and is mostly 2.5-3.0m. To ensure the uniform distribution of water flow in the pool, the length to width ratio is generally not less than 4:1, and the length to depth ratio is 8-12.

(2) When using mechanical sludge scraping, a sludge hopper is installed at the inlet of the sedimentation tank, and the longitudinal slope of the sludge hopper at the bottom of the tank should not be less than 0.01, generally ranging from 0.01 to 0.02. The travel speed of the mud scraper cannot exceed 1.2m/min, usually 0.6-0.9m/min.

(3) When using a horizontal sedimentation tank as an initial sedimentation tank, the surface load is 1-3m3/(m · h) and the maximum horizontal flow rate is 7mm/s; When used as a secondary sedimentation tank, the maximum horizontal flow rate is 5mm/s.

(4) There should be rectification measures for the population, and the commonly used methods of pedestrian flow include overflow weir perforated rectification wall (plate), bottom trap L pedestrian flow baffle combination, submerged hole pedestrian flow baffle combination, and submerged hole pedestrian flow perforated rectification wall (plate) combination. When using perforated rectifier wall (plate) type, the total area of openings on the rectifier wall is 6% to 20% of the cross section of the water flow, and the flow rate at the orifice is 0.15 to 0.2m/s. The orifice should be made into a gradually expanding shape.

(5) Baffles should be installed at the inlet and outlet, with a height of 0.1-0.15m above the water surface. The submergence depth of the inlet baffle should not be less than 0.25m, generally 0.5-1.0m; The submergence depth of the baffle at the outlet is generally 0.3-0.4m. The inlet baffle is 0.5-1.0m away from the inlet, and the outlet baffle is 0.25-0.5m away from the outlet weir plate.

(6) When the volume of the horizontal sedimentation tank is small, perforated pipes can be used for sludge discharge. Perforated pipes are mostly arranged inside the mud collection hopper, and can also be arranged at the bottom of the horizontal pool. When the sedimentation tank adopts multi bucket sludge discharge, the sludge bucket plane is square or nearly square rectangular, and the number of rows generally cannot exceed two. Large scale horizontal sedimentation tanks are generally equipped with sludge scrapers, which scrape the bottom sludge from the outlet end to the sludge hopper at the inlet end, and at the same time, scrape the floating slag towards the slag collection tank at the outlet end.

(7) The height of the buffer layer in the non mechanical discharge of the horizontal sedimentation tank is 0.5m. When using mechanical discharge, the upper edge of the buffer layer should be 0.3m higher than the scraper.

2/Vertical flow sedimentation tank

The vertical sedimentation tank body is circular or square, and sewage enters the tank from the inlet of the central pipe. It is distributed around the entire horizontal section through the obstruction of the reflector plate and flows slowly upwards. Suspended particles with a settling speed greater than the rising speed of water flow settle into the sludge hopper, and the supernatant overflows from the outlet weir around the top of the tank to the outside.

The basic requirements for vertical sedimentation tanks are as follows:

(1) To ensure the vertical flow of water in the pool from bottom to top and prevent the water from flowing in a radial state, the ratio of the diameter of the circular pool or the side length of the square pool to the effective water depth of the sedimentation zone is generally not greater than 3. The diameter of the pool is generally 4.0-7.0m, with a maximum of 10m. When the diameter of the circular pool or the length of the square pool edge D ≤ 7m, the precipitated water flows out along the periphery; When D ≥ 7m, radial drainage channels should be added.

(2) The upward flow rate of water in the vertical sedimentation tank is 0.5-1.0mm/s, and the sedimentation time is l-1.5h. The flow velocity inside the central pipe should generally be greater than 100mm/s, and there is a horn mouth and a reflector plate at the lower outlet. The bottom of the reflector plate should be at least 0.3m away from the mud surface, and the diameter and height of the horn mouth should be 1.35 times the diameter of the central pipe. The diameter of the reflector plate should be 1.3 times the diameter of the horn mouth, and the inclination angle between the surface of the reflector plate and the horizontal plane should be 17.

(3) The height of the gap between the bottom edge of the horn mouth and the surface of the reflector plate is 0.25-0.50m. When used as an initial sedimentation tank, the water flow velocity in the gap should not exceed 30mm/s, and when used as a secondary sedimentation tank, the water flow velocity in the gap should not exceed 20mm/s.

(4) The inclination angle of the conical mud storage hopper is 45 60., The diameter of the mud discharge pipe shall not be less than 200mm, and the distance between the mouth of the mud discharge pipe and the bottom of the pool shall be less than 0.2m. The upper end of the open mud discharge pipe shall not exceed the water surface by less than 0.4m. The submerged depth of the scum baffle is 0.3-0.4m, with a height of 0.1-0.5m above the water surface and a distance of 0.25-0.50m from the collection tank.

3/Radial sedimentation tank

The flow pattern of water in a radial sedimentation tank is radial, therefore, sewage enters the sedimentation tank from the center or periphery.

The inlet pipe of the center inlet radial sedimentation tank is suspended under the bridge or buried in the concrete of the bottom plate of the tank. Sewage first enters the center pipe of the tank, and then when entering the sedimentation tank, it is rectified by the rectifier plate around the center pipe and flows uniformly in all directions. The supernatant overflows through the outlet weir set around the sedimentation tank, and the sludge settles to the bottom of the tank. It is scraped to the mud collection hopper in the center of the sedimentation tank by a mud scraper or suction machine, and then discharged by gravity or pump suction.

The inlet channel of the peripheral inflow radial sedimentation tank is arranged around the sedimentation tank, and the supernatant overflows through the outlet weir set around or in the middle of the sedimentation tank. The discharge method of sludge is the same as that of the central inflow radial sedimentation tank.

What are the basic requirements for radial sedimentation tanks

(1) The ratio of the diameter of a circular or square pool to the effective water depth is generally 6-12, and the diameter of the pool is generally not less than 16m, with a maximum of 100m. The slope at the bottom of the pool is generally between 0.05 and 0.10.

(2) Usually, mechanical scraping is used, and then air lifting or static water head is used to discharge the mud; When the pool diameter is less than 20m, bucket type mud collection (usually four buckets) can also be used. The sludge can be lifted and discharged using compressed air or mechanical pumps (submersible pumps, screw pumps, etc.), or transported to the downstream treatment system using a static water head.

(3) There are three types of layout for inlet and outlet: central inlet and peripheral outlet, peripheral inlet and peripheral outlet.

(4) When the pool diameter is less than 20m, a centrally driven mud scraper is generally used, and its driving device is located on the center aisle board of the pool. When the pool diameter is greater than 20m, a peripheral driven mud scraper is generally used, and its driving device is located at the outer edge of the truss.

(5) The rotation speed of the mud scraper is generally 1-3r/h, and the linear speed of the outer mud scraper cannot exceed 3m/min, usually 1.5m/min.

(6) A scum baffle should be installed before the outlet weir, and the scum should be collected using a scum scraper installed on one side of the scraper truss.

(7) The efficiency of radial sedimentation tanks with peripheral inflow is higher, and compared with radial sedimentation tanks with central inflow and peripheral outflow, the surface load can be increased by about 1 times.

4/Slanted plate (pipe) sedimentation tank

A inclined plate (tube) sedimentation tank is a type of sedimentation tank that adds inclined plates or honeycomb inclined tubes to the sedimentation tank based on the principle of "shallow sedimentation" to improve sedimentation efficiency. According to the relative movement direction of water flow and sludge, there are three forms of inclined plate (pipe) sedimentation tanks: counter flow, co flow, and lateral flow. In sewage treatment, upflow counter flow inclined plate (pipe) sedimentation tanks are mainly used.

The inclined plate (tube) sedimentation tank has the advantages of high sedimentation efficiency, short residence time, and less land occupation. It is often used in the initial sedimentation tank of urban sewage and the pre-treatment process of oil separation for small flow industrial wastewater. Its treatment effect is stable and the maintenance workload is not large. It is rarely used in the secondary sedimentation tank process of sewage treatment because the solid content in the mixed liquid after biological treatment is high, and the impact load resistance is poor when using inclined plate (tube) sedimentation tanks for treatment, resulting in unstable effects; Moreover, due to the high dissolved oxygen content in the mixed solution, algae are prone to grow and form biofilms on the inclined plate (tube), which may block the drainage area of the inclined plate (tube) after running for a period of time, making it very difficult to clean.

The surface load of the inclined plate (tube) sedimentation tank is about twice that of a regular sedimentation tank. Therefore, when it is necessary to tap the potential of the original sedimentation tank or compress the land occupied by the sedimentation tank, the inclined plate (tube) sedimentation tank can be used.

What are the basic requirements for inclined plate (tube) sedimentation tanks

The inlet water of the commonly used inclined plate (pipe) sedimentation tank enters from the lower part of the inclined plate (pipe) layer, flows from bottom to top, passes through the inclined plate (pipe), and suspended particles settle on the bottom surface of the inclined plate (pipe). After accumulating to a certain extent, they slide down to the mud collection hopper and are discharged from the tank by the perforated pipe. The supernatant is collected by the perforated pipe on the surface of the sedimentation tank or overflowed by the triangular weir. The basic requirements for inclined plate (tube) sedimentation tanks are as follows:

(1) The vertical clearance of the inclined plate is generally 80-120ram, and the diameter of the inclined tube is generally 50-80mm. The length of the inclined plate (pipe) is generally 1.0-1.2m, and the inclination angle is generally 60 °. The upper water depth of the inclined plate (pipe) and the height of the bottom buffer layer are generally 0.5-1.0m.

(2) The upper end of the inclined plate should be installed inclined towards the inlet end of the sedimentation tank. To prevent short circuits in water flow, a flow blocking baffle should be installed at the gap between the pool wall and the inclined plate.

(3) The water inlet method is generally equipped with a water distribution rectifier water distribution device, commonly used are perforated water distribution plates and gap water distribution plates, etc. The flow rate of the rectifier water distribution hole is generally less than 0.15m/s. The water discharge method generally adopts the method of not placing multiple collection tanks on the pool surface, and the collection tank's collection method is the orifice type or triangular weir type.

(4) The inclined plate (pipe) sedimentation tank generally uses a sludge collection hopper to collect sludge and then relies on gravity to discharge sludge. The sludge is discharged 1-2 times a day, or the frequency of discharge may be increased depending on the specific situation, or even continuously discharged.

(5) The hydraulic retention time of the initial sedimentation tank generally does not exceed 30 minutes, and that of the secondary sedimentation tank generally does not exceed 60 minutes.

(6) The inclined plate (pipe) sedimentation tank must be equipped with facilities for flushing the inclined plate (pipe). Flushing can be carried out by emptying the sedimentation tank during maintenance or temporary shutdown, and using high-pressure water to thoroughly flush and clean the accumulated sludge inside the inclined plate (pipe) to prevent sludge from blocking the inclined plate (pipe) and affecting the sedimentation effect.

(7) The surface load of upflow inclined plate (tube) sedimentation tanks is generally 3-6m3/(m2 · h), which is about twice the design surface load of ordinary sedimentation tanks. The hydraulic retention time in the tank is generally 30-60 minutes.