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How to design a settling tank?

Author: Site Editor     Publish Time: 2023-10-08      Origin: Site

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How to design a settling tank?

The settling tank is a device used to settle solid particles suspended in wastewater to the bottom of the tank. It is one of the key components in wastewater treatment. The following are the general steps for settling tank design:

1. Determine treatment needs:

First, determine the specific needs of the wastewater treatment system. This includes determining the wastewater flow rate to be treated, water quality requirements, and the type and concentration of suspended solids that need to be removed.

1.1 Flow and load: This will determine the size and processing capacity of the sedimentation tank. Make sure the settling tank is large enough to handle peak flows and loads.

First, the wastewater flow rate into the sedimentation tank needs to be determined, usually in cubic meters per hour (m³/hr) or cubic meters per day (m³/day).

Then, the concentration of suspended solids or contaminants in the wastewater is known to determine the load (usually in mg/L or ppm).

1.2 Characteristics of wastewater: Understand the characteristics of wastewater, including particle size, concentration, density and pH value of suspended solids. Different types of wastewater may require different types of sedimentation tank designs.

2. Determine the size of the tank:

Calculating the size of the primary settling tank involves determining key parameters such as the length, width, depth and sedimentation time of the tank. The following are general calculation steps:

2.1 Calculate the sedimentation velocity of pollutants:

The rate at which contaminants settle in water depends on their size, shape and density. This can usually be found in the literature, or can be determined experimentally.

Settling velocity in sedimentation tank is usually measured in meters per second (m/s). You can estimate the sedimentation velocity using Stokes' law, which states: sedimentation velocity = (2 * g * (ρp - ρf) * r^2) / (9 * η), where g is the acceleration due to gravity and “ρp”is the particle density, “ρf”is the density of the liquid, “r”is the particle radius, and “η”is the dynamic viscosity of the liquid.

2.2 Determine the required precipitation time:

Settling time refers to the time that water stays in the pre settling tank, usually measured in hours. It depends on the contaminant concentration in the wastewater and the required sedimentation efficiency.

The calculation formula of detention period of sedimentation tank is: sedimentation time = (V / Q), where“V” is the volume of the sedimentation tank and“Q”is the water flow rate entering the sedimentation tank.

2.3 Calculate the volume of the settling tank:

The volume of the water settling tank can be calculated by the following formula: V = (Q * T), where“Q”is the wastewater flow rate and“T”is the required settling time.

Please note that the above calculation is a general method, and the actual settling tank design calculations may need to consider more factors, such as water quality, temperature, pH value, use of sedimentation aids, etc. Additionally, it is best to have a specific sedimentation basin design performed by a professional engineer to ensure that the specific needs and regulatory requirements of the project are met.

3. Select the types of sedimentation tank:

Select the appropriate sedimentation tank types according to the characteristics of the wastewater and treatment needs. Common sedimentation tank types include rectangular sedimentation tanks, circular sedimentation tank, conical bottom tanks, etc.


4.Depth of the pool:

Determine the depth of the sedimentation tank. Deeper pools are generally more effective at removing suspended solids, but also require more space. The depth of the pool is usually related to the residence time. The longer the residence time, the greater the depth can be. The depth of the tank depends on the required sedimentation tank detention time and settling speed, and is usually between 1 and 4 meters.

5. Design the flow pattern:

The water flow pattern of the primary sedimentation tank is very important for removing suspended solids. Water flow patterns are usually divided into horizontal flow and vertical flow. Horizontal flow is suitable for handling low-concentration suspended solids, while vertical flow is suitable for high-concentration suspended solids. Design appropriate water flow patterns to ensure effective settlement.

6. Stirring and clarification systems:

Some clarifier and sedimentation tanks require a stirring system to promote the settling of suspended solids, while other tanks may require a clarification system to remove clean water from the upper part. Ensure these systems are designed to meet processing requirements.

7. Sludge treatment:

Consider the treatment method of sludge. Sedimentation tanks often produce sediment or sludge at the bottom, and a sewage system needs to be designed to handle or dispose of this sludge.

8. Arrangement and Maintenance:

Consider the arrangement of the settlement tank to facilitate maintenance and operation. Settlement tank design that are easily accessible and maintainable can reduce operating costs.

9. Monitoring and control system:

Install an appropriate monitoring and control system to monitor wastewater quality in real time and adjust the operating parameters of the flocculation tank as needed.

10. Safety and environmental considerations:

Design should consider safety and environmental requirements to ensure prevention of leakage and pollution.

11. Comply with regulations and standards:

Ensure that the sedimentation tank design example complies with local, national and international environmental regulations and standards.

Ultimately, as the important waste water treatment equipment, settlement tank design is a complex process that often requires detailed engineering calculations and simulation analysis by engineers and professionals. The design goal is to ensure that the sedimentation tank can effectively remove suspended solids, improve water quality, and meet regulatory requirements.

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