Summary of wastewater treatment technology and its advantages and disadvantages

When the wastewater passes through the filter material layer, the numerous filter material surfaces provide a huge effective area for suspended solids to settle, forming countless small "sedimentation ponds", where suspended solids can easily settle down. This effect belongs to gravitational sedimentation.

Because the filter material has a huge surface area, there is an obvious physical adsorption between it and the suspended matter. In addition, the sand often has a negative surface charge in water, which can adsorb positively charged iron, aluminum and other limbs, thus forming a positively charged film on the surface of the filter material, and then adsorb negatively charged colloids such as colloids and a variety of organic matter, and contact flocculation occurs on the sand.

(2) Precipitation method: Precipitation method is to use the principle of the relative density of suspended matter in sewage and water is different, and the suspended matter is separated from the water by gravity sedimentation. According to the concentration of suspended particles in the water and the flocculation characteristics (that is, the ability to bond to each other) can be divided into four types:

1) Separation settlement (or free settlement) : in the precipitation process, the particles do not polymerize with each other and settle separately. The position of the star is only affected by its own gravity in the water and the resistance of the water flow, and its shape, size and mass do not change, and the descent speed does not change.

2) Coagulation sedimentation (or called flocculation sedimentation) : coagulation sedimentation refers to the condensation of colloids and fine suspended matter in wastewater into separable flocculants under the action of coagulants, and then gravity sedimentation is used to separate and remove. The characteristic of coagulation precipitation is that in the precipitation process, the particles contact and collide and aggregate with each other to form a larger floc, so the size and mass of the particles will increase with the increase of the depth, and the settling speed will also increase with the depth.

The commonly used inorganic coagulants are aluminum sulfate, ferrous sulfate, ferric chloride and polyaluminum; The commonly used organic flocculants are polypropylene cool glue, etc., and coagulants such as water glass and lime can also be used.

3) Regional settlement (also known as crowded settlement, stratified settlement) : when the content of suspended matter in the wastewater is high, the distance between the particles is small, the cohesion can make it set into a whole, and sink together, and the position of the particles between each other does not change, so there is a clear interface between the clarified water and the mixed water, and gradually move downward, such settlement is called regional settlement. Sedimentation in sedimentation tanks with high turbidity water and secondary sedimentation tanks (in the middle and later stages of sedimentation) are mostly of this kind.

4) Compression precipitation: When the concentration of suspended solids in the suspension is very high, the particles contact and squeeze each other, and the water in the gap between the lower particles is squeezed out under the gravity of the upper particles, and the particle population is compressed. Compression precipitation occurs in the sludge hopper or sludge thickening tank at the bottom of the sedimentation tank and proceeds very slowly. According to the different properties of the suspended substances in the water, there are two kinds of equipment: settling tank and settling tank.

Settling basin

The sedimentation tank is used to remove the relatively dense particles such as sand and cinder in the water. The sedimentation tank is generally located in front of the sewage treatment device to prevent other mechanical equipment for treating sewage from being worn out.

The sedimentation tank uses gravity to separate the suspended impurities from the water. It can separate particles with diameters of 20 to 100 µm and above. According to the direction of water flow in the sedimentation tank, it can be divided into three kinds: horizontal flow, radial flow and vertical flow.

① Flat-flow sedimentation tank: The wastewater flows from one end of the pool, flows in the pool in the horizontal direction, and the suspended matter in the water gradually sinks to the bottom of the pool, and the clarified water overflows from the other end.

(2) Radial sedimentation tank: The pool is mostly round, with a large diameter, generally more than 20 ~ 30m, which is suitable for large water treatment plants. After entering the central cylinder through the inlet pipe, the raw water flows radially to the periphery of the sedimentation tank through the hole on the wall of the cylinder and the annular perforated baffle. As the cross section continues to increase, the flow rate gradually becomes smaller, the particles settle down, and the clarified water overflows from its surroundings into the collection tank.

③ Vertical sedimentation tank: the section is mostly round, but also square and polygonal. Water flows into the pool from the lower mouth of the central pipe, distributes around the entire horizontal section through the barrier of the reflection plate, and slowly flows upward. The particles whose settling speed exceeds the rising velocity are sunk into the sludge hopper, and the clarified water spills out of the pond through the surrounding buried orifice.