Four forms of sand settling pond for sewage treatment

There are four types of sedimentation tank: horizontal flow, vertical flow, aeration and swirl flow.

1. Horizontal flow sedimentation tank

The flat-flow sand sedimentation tank is a kind of physical method sand sedimentation tank in the sewage treatment process. Its main purpose is to remove the sand particles with a particle size greater than 0.2mm and a relative density of 2.65 in the sewage to protect pipes, valves and other facilities from wear and blockage. Its working principle is mainly based on gravitational separation. From the structural point of view, the flat-flow sand sedimentation tank is a rectangular plane sand sedimentation tank, the main part is actually a widened and deepened open channel, composed of inlet channel, sand sedimentation area, outlet channel, sand sedimentation bucket and other parts, with ram plates at both ends to control water flow. At the bottom of the pool, there are usually 1 to 2 sand storage buckets, which are connected with sand discharge pipes for storing and discharging deposited sand particles.

Horizontal flow sedimentation tank is widely used in many occasions. In industrial production, such as iron and steel plants, metallurgical plants, pharmaceutical plants, food processing plants, etc., the production process will produce a large number of suspended solids and particulate waste water, the flat-flow sedimentation tank can effectively treat these waste water. In addition, in building construction and agricultural production, the flat-flow sedimentation tank also plays an important role in helping to improve water quality and ensure water safety.

Although the advection sedimentation tank has some advantages in the sewage treatment process, such as simple structure and stable work, there are also some disadvantages, which may affect the treatment effect and use efficiency.

The effect of the advection sedimentation tank on the retention of sand particles covered by a small amount of organic matter is not high. This will lead to the inclusion of about 15% organic matter in the sand, which makes the subsequent treatment of the sand difficult. The presence of organic matter in the sand may not only cause the problem of decay and odor, but also have an adverse effect on the subsequent treatment process.

The treatment effect of advection sedimentation tank is affected by many factors, such as influent flow rate, influent quality, sand depth, tank height, water temperature and so on. Especially when the height of the tank body is large, the water flow at the bottom will become smaller, and the sludge in the sand layer will accumulate a large amount, resulting in the reduction of the permeability of the sand layer, thus affecting the treatment effect of sewage.

The maintenance cost of the advection sedimentation tank is relatively high. It is difficult to discharge the sand layer, and special methods need to be taken to clean and replace the sand layer, which increases the input of manpower, material resources and equipment. At the same time, due to the small water flow at the bottom, it is easy to retain impurities such as sludge, and regular cleaning and maintenance are required after long-term use, which also increases operating costs.

The requirement of effective water depth in advection sedimentation tank is higher. If the water depth is too small, the influence of water temperature, flow and other factors will be more concentrated, thus affecting the water quality treatment effect. For the pool body with too large water depth, it will not only increase the difficulty of cleaning and maintenance, but also increase the treatment cost.

2. Vertical sedimentation basin

Vertical sand sedimentation tank, also known as vertical sand sedimentation tank, is a special precipitation structure, mainly used to remove the density of inorganic particles in the wastewater, especially the particle size of more than 0.2mm. Its working principle is based on the vertical flow of water in the pool, causing the suspended matter in the wastewater to settle under the action of gravity.

The vertical sedimentation tank usually has a round or square plane pattern, and the waste water enters the tank from top to bottom through the water inlet pipe located in the center of the tank. An umbrella baffle is arranged under the inlet pipe to ensure that the waste water is evenly distributed in the pool and slowly rises along the entire cross section. In this process, the suspended material gradually settles to the bottom of the pool, while the clarified water flows out from the pool along the surrounding overflow weir.

Vertical flow sediment basin has certain applications in sewage treatment, but there are also some obvious shortcomings:

Construction difficulties: Due to the large depth of the vertical sand settling pond, the construction difficulty is relatively high. The excavation, construction and installation of deep pools require special technologies and processes, which increase the difficulty and cost of construction.

High cost: Because of its special design and construction requirements, the cost of vertical sand settling tank is often higher than that of traditional sand settling tank. This may be a burden for some sewage treatment plants with limited budgets.

Operation and maintenance difficulties: The internal structure of the vertical sand sedimentation tank is relatively complex, and it needs to be cleaned and maintained regularly. Due to the large depth, the cleaning work is more difficult, and if the operation and maintenance is improper, it may affect the treatment effect and service life.

Poor ability to adapt to impact load and temperature change: The ability of vertical sedimentation tank to adapt to water quality and water quantity change is relatively weak. When water quality changes or water volume shocks are large, its treatment effect may be affected, requiring more frequent adjustment and maintenance.

Pond diameter restriction: The diameter of the vertical sand settling pond should not be too large, otherwise the water distribution may be uneven, affecting the treatment effect. This limits its processing scale and application scope to some extent.

3. Aerated sand settling tank

Aerated sand sedimentation tank, also known as aerated sand sedimentation tank, is a long channel that uses aeration to promote the settlement of suspended solids in wastewater. Its working principle is mainly based on the physical and chemical interactions between the aeration system and the substances in the wastewater.

The main structure of aerated sand settling tank includes the tank body, aeration system and sand removal system. The pool body is usually made of reinforced concrete and is rectangular or trapezoidal in shape. The waste water flows in from one end of the pool and, after a series of physical and chemical processes, flows out from the other end. The aeration system includes the aeration head, the aeration pipe and the aeration controller and other components, whose main role is to dissolve the oxygen in the air in the sewage, and to promote the sewage to be fully stirred and circulated. In this way, the organic matter in the sewage can be oxidized and decomposed, and the inorganic matter and sand and other particles can be precipitated in the pool.

In the aeration sand settling tank, the aeration device is usually installed 60-90cm away from the bottom of the tank. Due to aeration, the organic particles in the wastewater are often suspended, the sand particles rub against each other and withstand the aeration shear force, and the organic pollutants attached to the sand can be removed, which is conducive to obtaining more pure sand. Aeration also causes suspended matter and organic matter in the water to float to the surface with the bubble, forming scum, which is subsequently crushed by the bubble rupture and the impact of the water flow, making it easier to precipitate.

The sand removal system is responsible for discharging the sand deposited at the bottom of the pool. Through the sludge return system, part of the sludge can also be returned to the aerated sand settling tank for further treatment.

Although the aerated sand sedimentation tank has some obvious advantages in the wastewater treatment process, there are also some shortcomings that cannot be ignored:

High energy consumption: The aeration sedimentation tank requires oxygen supply through air pump or booster pump to maintain the normal operation of the aeration system. This process consumes a large amount of electricity, increasing the energy cost of the entire wastewater treatment process.

High maintenance costs: aeration equipment requires regular inspection and maintenance to ensure its proper operation. In addition, because aeration causes some substances to form scum in the pool, these scum need to be cleaned regularly, which also increases the complexity and cost of maintenance work.

Impact on biological nitrogen and phosphorus removal system: Although aeration helps to remove organic matter in wastewater, it may also adversely affect the operation of the anaerobic or anoxic segment of the biological nitrogen and phosphorus removal system. This may affect the efficiency and stability of the entire biological treatment process.

Limited adaptability to water quality changes: When the influent water quality changes greatly, the treatment effect of the aeration sedimentation tank may be affected. Especially when the water contains a large number of organic substances that are difficult to degrade, the treatment effect of the aeration sedimentation tank may decline.

Large footprint: Because the aeration sedimentation tank requires enough space to set up the aeration equipment and sand removal system, its footprint is relatively large. This can be a problem for some treatment plants where space is limited.

4. Aerated sand settling tank

Also known as eddy sand sedimentation tank, it is generally designed as a circle, the center of the pool is provided with an adjustable rotating slurry plate, the inlet channel is in the tangential position of the circular pool, the outlet channel corresponds to the center of the circular pool, and the sand bucket is provided under the central rotating slurry plate. It can reasonably adjust the rotation speed of the rotating slurry plate, control the flow pattern and velocity of the water, accelerate the sediment of the sand and make the organic matter carried away with the water, and effectively remove the fine sand (sand below 0.1mm) that is difficult to remove in other forms of sand sedimentation tank. It is mainly composed of inflow port, flow outlet, sand settling area, sand bucket, turbine drive device and sand discharge system.

When sewage flows tangentially into the settling area, the drop weir design in the inlet channel allows sand that may have been deposited at the bottom of the channel to slide downward into the settling basin. At the same time, the design of the baffle makes the water flow and sand flow to the bottom of the sand settling tank, which strengthens the wall attachment effect. In the middle of the sand settling pond, a adjustable paddle board is provided to keep the water flow in the pond circulating. The paddle board, baffle and inlet water flow are combined, and the rotating turbine blades cause the sand to flow in a spiral shape, thus facilitating the separation of organic matter and sand.

Due to the different centrifugal force, the relatively dense sand particles are thrown to the pond wall and sink into the sand bucket under the action of gravity. The lighter organic matter is separated from the sand in the middle part of the sedimentation tank and carried out with the water swirl. By adjusting the speed, the best sand settling effect can be achieved. The sand in the sand bucket can be removed by air lifting, sand pumping and other ways, and then through sand water separation to achieve clean sand discharge standards.

The swirling sedimentation tank has the advantages of small area, small effect of water volume change, good sand-water separation effect, low water content, less organic matter content and easy transportation.

The disadvantages of swirl sedimentation tank are mainly reflected in the following aspects:

Relatively high cost: The construction and operation cost of the swirl sedimentation tank is relatively high, mainly due to its complex structure and fine manufacturing process. This increases the initial investment in wastewater treatment and may also require more maintenance costs over the course of long-term operations.

High requirements on inlet water quality: swirl sedimentation tank has more strict requirements on parameters such as sewage flow, concentration and viscosity. If the inlet water quality fluctuates greatly, it may affect the sand settling effect, and may even lead to the blockage or damage of the equipment. Therefore, when using swirl sedimentation tank, it is necessary to strictly control and manage the inlet water quality.

Maintenance and management difficulty: the swirl sedimentation tank needs regular cleaning and maintenance to maintain its good operating condition and sand settling effect. However, due to the complexity of its internal structure, maintenance management can be relatively difficult and requires professional personnel to operate.