The principle of sedimentation tanks and the advantages and disadvantages of various sedimentation tanks

What is a sedimentation tank?

Sedimentation tank is a treatment structure that uses gravity sedimentation to remove suspended particles with a higher density than water. It is one of the most widely used treatment units in wastewater treatment, and can be used for wastewater treatment, post-treatment of biological treatment and advanced treatment. In the grit chamber, the sedimentation principle can be used to remove inorganic impurities in the water. The sedimentation principle can be used in the primary sedimentation tank to remove suspended solids and other solids in the water. The sedimentation principle in the secondary sedimentation tank can be used to remove the activated sludge from the biological treatment effluent. In the concentration tank, the sedimentation principle is used to separate the water in the sludge, and the sludge is concentrated. In the deep treatment field, the sedimentation principle of the second settling tank effluent plus flocculant can be used to remove the suspended solids in the water.

Sedimentation tank
The sedimentation tank includes five parts: the influent zone, the sedimentation zone, the buffer zone, the sludge zone and the water discharge zone. The function of the inlet and outlet zones is to make the water flow evenly through the sedimentation tank, avoiding the short-flow and reducing the adverse effects of turbulence on the sedimentation of the cattle, reducing the dead water area and increasing the volume utilization of the sedimentation tank; The clarification zone, ie the working area of ​​the sedimentation tank, is the area where the sedimentary particles are separated from the wastewater; the sludge zone is the area where the sludge is stored, concentrated and discharged; the buffer zone is the zone of the water layer separating the sedimentation zone and the sludge zone. It is ensured that the precipitated particles are not floated again due to the agitation of the water flow.

What is the principle of the sedimentation tank?

The sedimentation tank realizes the purification of water by utilizing the principle that the sedimentation velocity of the suspended impurity particles in the water flow is greater than the flow velocity of the water flow, or the time when the downward sedimentation time is less than the time when the water flow flows out of the sedimentation tank.

The treatment efficiency of the ideal sedimentation tank is only related to the surface load, that is, to the surface area of ​​the sedimentation tank, and is independent of the depth of the sedimentation tank. The depth of the pond is only related to the time and quantity of sludge storage and the prevention of sludge scouring. In the actual continuous operation of the sedimentation tank, since the overflow of the water flow from the effluent dome will bring about the upward flow velocity of the water flow, the particles whose sedimentation velocity is lower than the upward flow velocity will flow with the water, and the sedimentation velocity is equal to the bu-liter flow rate. In the pool, only particles with a sedimentation rate greater than the rising flow rate will precipitate in the pool. The time during which the precipitated particles settle to the bottom of the sedimentation tank is related to the hydraulic retention time of the water flow in the sedimentation tank, that is, the depth of the pool body.

In theory, the shallower the pool, the easier it is for the particles to reach the bottom of the pool. This is the theoretical basis for shallow sedimentation tanks such as inclined tubes or inclined plate sedimentation tanks. In order to precipitate the particles in the sedimentation tank slightly larger than the rising flow rate and prevent the settled sludge from being re-floated by the disturbance of the influent water flow, a buffer zone is left between the sedimentation zone and the sludge storage zone to make these precipitates After the particles in the pool which are slightly larger than the rising flow rate or the re-floated particles come into contact with each other, they are precipitated again.

First, the advantages and disadvantages of inclined tube (plate) sedimentation tank

advantage

1. The precipitation area increases;

2. High sedimentation efficiency and large water production;

3. Good hydraulic conditions, Re is small, Fr is large, which is conducive to sedimentation;

Disadvantage

1. Due to the short residence time, its buffering capacity is poor;

2. High requirements for coagulation;

3. Maintenance and management is difficult, and the sloping plate (tube) needs to be replaced after a period of use.

Applicable conditions

1. Secondary sedimentation tank suitable for small and medium-sized sewage plants

2, can be used to tap the potential of the existing advection sedimentation tank

Second, the advantages and disadvantages of the advection sedimentation tank

advantage

1. The amount of treated water is not limited, and the sedimentation effect is good.

2. Strong adaptability to changes in water volume and temperature.

3. Compact layout, convenient construction and low cost.

Disadvantage

1. The water in and out of the water is not easy to be even.

2. When multiple buckets are drained, each bucket needs to be equipped with a drain pipe (valve), which is manually operated and complicated to work. It is easy to rust when using mechanical scraping.

Advection Sedimentation Tank – Scope of Application

1. Applicable to areas with high groundwater level and poor geological conditions.

2. Large, medium and small sewage treatment projects can be used.

Third, the advantages and disadvantages of vertical flow sedimentation tank

advantage

The vertical flow sedimentation tank has a good effect, and the floor space is small, and the mud discharge is easy.

Disadvantage

The depth of the pool is large, construction is difficult, and the cost is high. It is commonly used in small sewage treatment plants with a water volume of less than 20,000 m3/d.

Applicable conditions

Suitable for small sewage treatment plants

Fourth, the advantages and disadvantages of the radial flow sedimentation tank

advantage

The advantage of the radial flow sedimentation tank is that it uses mechanical mud discharge, which has better operation and simple management, and the mud discharge equipment has been shaped.

Disadvantage

The mechanical mud discharge equipment is complex and requires high construction quality;

Applicable conditions

1. Applicable to areas with high groundwater level

2, suitable for large and medium-sized sewage treatment plants

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