After biological treatment, the sewage must enter the secondary Clarifier for sludge water separation, and the clarified treated water can only be discharged. At the same time, a certain concentration of return sludge or a certain amount of treatment water should be provided for the biological treatment facilities. Therefore, the working performance of the secondary Clarifier has a direct relationship with the operation effect of the activated sludge system.

The concept and function of secondary sedimentation tank

1. definition of secondary sedimentation tank

    According to the locations in the sewage treatment process, the sedimentation tank can be divided into primary sedimentation tank and secondary sedimentation tank. The primary sedimentation tank is generally set after the grit chamber and before the aeration tank, while the secondary sedimentation tank is set after the aeration tank and before the advanced treatment or discharge.

After biological treatment, the sewage must enter the secondary sedimentation tank for sludge water separation, and the clarified treated water can only be discharged. At the same time, a certain concentration of return sludge or a certain amount of treatment water should be provided for the biological treatment facilities. Therefore, the working performance of the secondary sedimentation tank has a direct relationship with the operation effect of the activated sludge system. The types of secondary sedimentation tank include horizontal flow sedimentation tank, vertical flow sedimentation tank, radial flow sedimentation tank, inclined plate sedimentation tank, etc.

2. Function of secondary sedimentation tank

The function of the secondary sedimentation tank is to separate the sludge from the water, to clarify the biological treated mixed liquid, and to concentrate the sludge in the mixed liquid. The secondary sedimentation tank is the last link of biological wastewater treatment, which plays a decisive role in ensuring the qualified content of suspended solids in the effluent quality.

If the secondary sedimentation tank is set up unreasonably, even if the biological treatment effect is good, the content of dissolved organic matter in the mixed liquid is small, the effect of sludge water separation of mixed liquid in the secondary sedimentation tank is not ideal, and the effluent quality may still be unqualified. If the sludge thickening effect is not good, it is difficult to guarantee the microbial quantity returned to the aeration tank. The reduction of the concentration of the aeration mixture will lead to the decline of the sewage treatment effect, and then affect the effluent quality.

Requirements for secondary sedimentation tank

1. Setting requirements of secondary sedimentation tank

(1) The hydraulic load is generally 0.5-1.8 m3 / (M2 · h). In the treatment of industrial wastewater, the proportion of organic matter in activated sludge is relatively large, and the SVI of mixed liquor in aeration tank is high. Therefore, the secondary sedimentation tank should adopt a lower surface hydraulic load.

(2) in order to ensure that the sludge can be sufficiently concentrated in the secondary sedimentation tank to supply the required concentration of sludge in the aeration tank, the solid surface load of the secondary sedimentation tank is 150 kg / (M2 · d), and the solid surface load of the inclined tube (plate) secondary sedimentation tank can be extended to 192kg / (M 2 · d).

(3) the water depth at the side of the secondary sedimentation tank should be 2.5 ~ 4 m, and the specific value is related to the size of the tank. The larger the diameter of the secondary sedimentation tank, the water depth at the side of the tank should be increased appropriately, otherwise the hydraulic efficiency of the secondary sedimentation tank will be reduced and the effective volume will be reduced. For the secondary sedimentation tanks with diameters of 10-20m, 20-30m, 30-40m and > 40m, the water depth at the pool side is 3.0m, 3.5m, 4.0m and 4.0m respectively. When the water depth at the edge of the pool can not be reached due to various reasons, a lower surface load value must be adopted in order to keep the sedimentation time unchanged.

(4) the overflow rate (or load) of the outlet weir of the secondary sedimentation tank is 1.5 ~ 2.9L / (m · s).

(5) when mechanical sludge discharge is adopted, the volume of sludge area in secondary sedimentation tank shall be calculated according to the residence time of sludge thickening to the required concentration. The volume of sludge area in the secondary sedimentation tank of activated sludge process is generally 2-4h sludge volume, and continuous sludge discharge measures should be taken. The volume of sludge area in biofilm secondary sedimentation tank is generally 4H sludge volume.

(6) in order to reduce energy consumption, it is better to use screw pump or axial flow pump and other equipment with low head and large flow. If blast aeration is used, air lift pump can also be used
to simplify equipment management and maintenance.

Classification of secondary sedimentation tank

The types of secondary sedimentation tank include horizontal flow sedimentation tank, vertical flow sedimentation tank, radial flow sedimentation tank, inclined plate sedimentation tank, etc.

1. Horizontal flow sedimentation tank

     The surface shape of horizontal flow sedimentation tank is generally rectangular. After entering the sedimentation area through energy dissipation and rectification in the inlet area, the water flows horizontally slowly. The settleable suspended solids in the water gradually sink to the bottom of the tank. The effluent from the sedimentation area overflows the weir and is discharged out of the tank through the outlet tank.

The basic requirements of horizontal flow sedimentation tank are as follows:

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

(2) when mechanical sludge scraping is adopted, sludge hopper is set at the inlet end of sedimentation tank, and the slope of longitudinal sludge hopper at the bottom of tank shall not be less than 0.01, generally 0.01-0.02. The traveling speed of mud scraper shall not be greater than 1.2m/min, generally 0.6-0.9m/min.

(3) when the horizontal sedimentation tank is used as the primary sedimentation tank, the surface load is 1-3m3 / (m · h), and the maximum horizontal velocity is 7mm / S; when the horizontal sedimentation tank is used as the secondary sedimentation tank, the maximum horizontal velocity is 5mm / s.

(4) the population should have rectification measures. There are four common pedestrian flow modes: overflow weir perforated rectifier wall (plate), bottom slab l pedestrian flow baffle combination, submerged orifice pedestrian flow baffle combination and submerged orifice pedestrian perforated rectifier wall (plate) combination. When the perforated rectifier wall (plate) type is used, the total area of the opening on the rectifier wall is 6% ~ 20% of the water flow section, and the flow velocity at the orifice is 0.15 ~ 0.2m/s, and the orifice should be made into a gradually expanding shape.

(5) baffles should be set at the inlet and outlet, 0.1-0.15m above the water surface. The submerged depth of baffle at the inlet shall not be less than 0.25m, generally 0.5-1.0m; the submerged depth of baffle at outlet shall be generally 0.3-0.4m. The baffle at the inlet is 0.5 ~ 1.0m away from the water inlet, and the baffle at the outlet is 0.25 ~ 0.5m away from the weir plate.

(6) when the volume of horizontal flow sedimentation tank is small, perforated pipe can be used for sludge discharge. Most of the perforated pipes are arranged in the mud collecting hopper or on the bottom of the horizontal pool. When multi bucket sludge discharge is adopted in sedimentation tank, the plane of sludge bucket is square or nearly square, and the number of rows is generally no more than two rows. The sludge scraper is usually set up in large horizontal flow sedimentation tank to scrape the sludge from the bottom of the tank from the outlet to the sludge hopper at the inlet, and to scrape the scum to the slag collecting tank at the outlet.

(7) the height of buffer layer should be 0.5m when non mechanical sludge is discharged from horizontal sedimentation tank, and the upper edge of buffer layer should be 0.3m higher than the sludge scraper when mechanical sludge is used.

2. Vertical flow sedimentation tank

    The vertical flow sedimentation tank is round or square. The sewage enters into the tank from the inlet of the central pipe, and distributes around the horizontal section through the baffle plate, and flows upward slowly. The suspended particles whose settling velocity is higher than the rising velocity of water flow sink into the sludge hopper, and the supernatant overflows from the outlet weir around the tank top to the outside of the tank.

The basic requirements of vertical flow sedimentation tank are as follows:

(1) in order to ensure the vertical flow of water in the tank from bottom to top and prevent the flow from being radial flow, the ratio of the diameter of the round pool or the side length of the square pool to the effective depth of the sedimentation area is generally no more than 3, and the diameter of the pool is generally 4.0-7.0m, and the maximum is not more than 10m. When the diameter of the round pool or the side length of the square pool is less than or equal to 7 m, the precipitated water flows out along the periphery; when D ≥ 7 m, the radial branch drainage channel should be added.

(2) the rising velocity of water flow in the vertical flow sedimentation tank is 0.5 ~ 1.0 mm / s, and the settling time is 1 ~ 1.5 h. The flow velocity in the central pipe should be greater than 100 mm / s, and its lower outlet is equipped with a bell mouth and a reflecting plate. The distance between the bottom of the reflector and the center of the reflector is at least 35.3 m, and the distance between the bottom of the reflector and the center of the reflector is 1.3 times of the diameter of the reflector.

(3) the height of the gap between the bottom edge of the bell mouth and the surface of the reflecting plate is 0.25-0.50m. The water flow velocity in the gap should not be greater than 30mm / s when it is used as the primary sedimentation tank, and not more than 20mm / s when it is used as the secondary sedimentation tank.

(4) the inclination angle of the cone-shaped mud hopper is 45-60 degrees, the diameter of the sludge discharge pipe should not be less than 200 mm, the distance between the outlet of the discharge pipe and the bottom of the pool should be less than 0.2 m, and the upper end of the open sludge discharge pipe should not be less than 0.4 m above the water surface. The submerged depth of scum baffle is 0.3 ~ 0.4m, 0.1 ~ 0.25m above the water surface and 0.25 ~ 0.50m away from the water collecting tank.

3. Radial flow sedimentation tank

The flow pattern of the radial flow sedimentation tank is radial manifold, so the sewage enters the sedimentation tank from the center or the periphery.

The inlet pipe of the central inlet radial flow sedimentation tank is suspended under the bridge or embedded in the bottom concrete of the tank body. The sewage first enters into the central pipe of the tank body. Then, when entering the sedimentation tank, it is rectified by the rectifying plate around the central pipe and radiates to the surrounding evenly. The supernatant overflows through the outlet weir around the sedimentation tank, and the sludge settles to the bottom of the tank by the mud scraper Or scraping suction machine scrapes to the sludge collecting bucket in the center of sedimentation tank, and then it is pumped and discharged by gravity or pump.

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

What are the basic requirements of radial flow sedimentation tank?

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

(2) mechanical scraping is usually used, followed by air lifting or static head discharge; when the pool diameter is less than 20m, bucket type sludge collection (generally four buckets) can be used. The sludge can be lifted by compressed air or discharged by mechanical pump (submersible sewage pump, screw pump, etc.), and the sludge can also be transported to the bottom by the static head_ Level processing system.

(3) the layout of inlet and outlet water includes three forms: central inlet and peripheral outlet, peripheral inlet central effluent and peripheral influent peripheral effluent.

(4) when the pool diameter is less than 20 m, the mud scraper with central drive is generally adopted, and its driving device is set on the central aisle board of the pool. When the pool diameter is more than 20 m, the mud scraper with peripheral drive is generally used, and its driving device is set 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 peripheral mud scraper can not exceed 3m / min, usually 1.5m/min. (6) scum baffle shall be set in front of outlet weir, and scum shall be collected by scum scraper installed on one side of scraper truss.

(7) the efficiency of radial flow sedimentation tank with peripheral water inflow is higher than that of radial flow sedimentation tank with central inflow and peripheral effluent, and the surface load can be increased by about one time.

4. Inclined plate (tube) sedimentation tank

Inclined plate (tube) sedimentation tank is a kind of sedimentation tank with inclined plate or honeycomb inclined tube added in the sedimentation tank according to the principle of "shallow sedimentation", so as to improve the sedimentation efficiency. According to the relative movement direction of water flow and sludge, there are three types of inclined plate (pipe) sedimentation tank, i.e. different direction flow, same direction flow and side flow. In sewage treatment, the upflow counter flow inclined plate (pipe) sedimentation tank is mainly used.

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 primary sedimentation tank of urban sewage and the oil separation process of small flow industrial wastewater. Its treatment effect is stable and the maintenance workload is small. 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 large, the impact load resistance capacity of inclined plate (tube) sedimentation tank is poor, and the effect is unstable; moreover, due to the large dissolved oxygen content in the mixed liquid, algae can easily grow on the inclined plate (pipe) to form biofilm, which may block the overflow area of the inclined plate (pipe) after running for a period of time, It's very difficult to clean up.

The surface load of inclined plate (tube) sedimentation tank is about twice as high as that of ordinary sedimentation tank, so inclined plate (tube) sedimentation tank can be used when the potential of original sedimentation tank needs to be tapped or the land occupation of sedimentation tank needs to be compressed.

What are the basic requirements of inclined plate (tube) sedimentation tank?

The influent of inclined plate (tube) sedimentation tank usually enters from the lower part of the inclined plate (pipe) layer, and flows through the inclined plate (pipe) from bottom to top. The suspended particles settle on the bottom surface of the inclined plate (pipe), and then slide down to the mud collecting bucket after accumulating to a certain extent, and the supernatant is collected by perforated pipe or overflowed by triangular weir on the surface of sedimentation tank. The basic requirements of inclined plate (tube) sedimentation tank are as follows:

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

(2) the upper end of the inclined plate should be inclined to the inlet end of the sedimentation tank. In order to prevent short-circuit of water flow, a baffle plate should be installed at the gap between the tank wall and the inclined plate.

(3) generally, water distribution and rectification devices are set in the water inlet mode, such as perforated water distribution plate and gap water distribution plate. The flow rate of rectification water distribution hole is generally lower than 0.15m/s. In general, the way of water outlet is to set up multiple collecting flumes on the pool surface, and the water collecting mode of the water collecting tank is eyelet type or triangular weir type.

(4) the inclined plate (tube) sedimentation tank generally uses sludge collecting hopper to collect sludge and then discharge sludge by gravity, discharging sludge 1-2 times a day, or increasing the frequency of sludge discharge according to the specific situation, or even discharging sludge continuously.

(5) the hydraulic retention time of the primary sedimentation tank is generally less than 30 min, and that of the secondary sedimentation tank is not more than 60 min.

(6) the inclined plate (pipe) sedimentation tank must be equipped with facilities for flushing the inclined plate (pipe), which can be emptied during maintenance or temporary shutdown. The sludge accumulated in the inclined plate (pipe) can be thoroughly washed and cleaned with high-pressure water to prevent the sludge from blocking the inclined plate (pipe) and affecting the sedimentation effect.

(7) the surface load of the upflow inclined plate (tube) sedimentation tank is generally 3-6m3 / (M2 · h), which is about twice as high as that of the ordinary sedimentation tank. The hydraulic retention time in the tank is generally 30-60min.