What is the difference between the secondary sedimentation tank and the primary sedimentation tank in a sewage plant?

What is the difference between the secondary sedimentation tank and the primary sedimentation tank in a sewage plant?

Primary sedimentation tank: The first sedimentation treatment of the incoming water can play the role of a regulating tank, have a certain degree of homogenization effect on the water quality, and slow down the impact of water quality changes on the subsequent biochemical system. The structures of the primary sedimentation tank are: horizontal flow, radial flow, vertical flow, inclined plate (tube) type.

Secondary sedimentation tank: The secondary sedimentation tank is located after the aeration tank (aerobic biochemical tank). It is a place for mud and water separation to ensure tailwater discharge and sludge return. The structures of the secondary sedimentation tank are: horizontal flow, radial flow, vertical flow, inclined (tube) plate.

Principle

The sedimentation tank uses the principle that the suspended impurity particles in the water flow can be separated from the water flow when the downward sedimentation speed is greater than the downward flow speed of the water flow, or the downward sedimentation time is less than the time for the water flow to flow out of the sedimentation tank to achieve water purification.

The treatment efficiency of the ideal sedimentation tank is only related to the surface load, that is, it is related to the surface area of ​​the sedimentation tank, but not to the depth of the sedimentation tank. The depth of the tank is only related to the storage time and quantity of sludge and the prevention of sludge from being washed away. In the actual continuous operation of the sedimentation tank, since the overflow of water from the top of the outlet weir will bring the rising flow rate of the water flow, the particles with a sedimentation rate less than the rising flow rate will flow away with the water, and the particles with a sedimentation rate equal to the rising flow rate will be suspended in the tank. Only the particles with a sedimentation rate greater than the rising flow rate will settle in the tank. The time it takes for the sedimentation particles to 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, it is related to the depth of the tank body.

Theoretically, the shallower the tank body, the easier it is for the particles to reach the bottom of the tank. This is the theoretical basis of shallow sedimentation tanks such as inclined tube or inclined plate sedimentation tanks. In order to make the particles in the sedimentation tank slightly greater than the rising flow rate settle down and prevent the settled sludge from being disturbed by the influent water flow and re-floating, a buffer zone is left between the sedimentation area and the sludge storage area, so that the particles in these sedimentation tanks that are slightly greater than the rising flow rate or the re-floating particles can contact each other and settle again.

Object

a. Primary sedimentation tank: mainly suspended matter, some organic matter

b. Secondary sedimentation tank: activated sludge mixed liquor, which has the characteristics of high concentration, flocculation, light weight, slow sedimentation rate, etc.

Load

Primary sedimentation tank: sedimentation time 1.0~2.5h, surface load 1.2~2.0 (m3/m2.h), sludge moisture content 95~97%, weir load less than or equal to 2.9L/(s.m);

Secondary sedimentation tank (after activated sludge method): sedimentation time 2.0~5.0h, surface load 0.6~1.0 (m3/m2.h), sludge moisture content 99.2~99.6%, weir load less than or equal to 1.7L/(s.m);

Secondary sedimentation tank (after biofilm method): sedimentation time 1.5~4.0h, surface load 1.0~1.5 (m3/m2.h), sludge moisture content 96~98%, weir load less than or equal to 1.7L/(s.m).