Vertical settling tanks in wastewater treatment play a very important role by separating and removing sediment and suspended solids from water. These tanks are vertically designed with a cylindrical body and a conical bottom to enhance the settling process. These tanks can handle other types of sediments, such as biological sediments, organic sediments, and flocs from the flocculation process. In addition, these tanks also help to separate microbial sludge as well as reduce moisture in the sludge, thereby optimizing the sludge treatment process later.
The vertical settling tank in wastewater treatment is one of the important steps for improving efficiency in the overall wastewater treatment system. Therefore, it is necessary to clearly understand the characteristics of the settling tank as well as how to design it to optimize the wastewater treatment process. Let’s explore the content below with NTS Engineering Co.,Ltd.
1/ What is a vertical settling tank in wastewater treatment?
The vertical settling tank in wastewater treatment is a type of tank designed to separate settleable solids from wastewater, with the aim of completely removing pollutants before proceeding to the next treatment stage.
The vertical settling tank is one of the most important stages in a wastewater treatment system. It is commonly used in most industrial and domestic wastewater treatment systems.
Structure of vertical settling tank
Material and shape
The vertical settling tanks are constructed from steel coated with acid-resistant paint or built from concrete. The choice of tank material depends on engineering requirements, the needs of the investor, as well as the investment cost of each specific project.
These tanks are usually designed in a cylindrical shape with a conical bottom, which enhances sedimentation efficiency by creating a vertical flow pattern and reducing water velocity.
Main components of the tank
Depending on the scale and characteristics of the wastewater treatment system, vertical settling tanks can be configured in different ways. Inside the tank, a clear water collection system is typically installed at the top, a sludge collection hopper at the bottom to collect settled sludge. In addition, some modern settling tanks also have automatic sludge separators to help separate sludge more efficiently and easily.
The vertical settling tank has a fairly simple central reaction compartment with a diameter not exceeding 3 times the depth, along with components such as:
- Sludge discharge pipes and floating sludge outlets
- Effluent troughs, water collectors, and drainage channels
- Central inlet pipe
Classification of vertical settling tanks
Vertical settling tanks are divided into two types:
| No. | Type of vertical settling tanks | Use |
|
1 |
Primary settling tank |
Remove water-insoluble organic matter before applying biological treatment methods |
|
2 |
Secondary settling tanks |
Sedimentation of microorganisms and sludge before discharge into the environment |
2/ How a vertical settling tank works
Main operating principle
The central pipe brings wastewater into the tank, water flows from bottom to top and goes into the water collection grooves arranged around the tank wall.
The settling process takes place in a working area with a height of 4 to 5 meters, with an upward flow velocity maintained at about 0.5 – 0.6 m/s.
To ensure settling efficiency, the outlet water velocity is designed to be smaller than the inlet velocity, allowing suspended particles and dirt to be easily swept up and settled to the bottom.
Challenges in the operation of vertical settling tanks
One of the most common challenges in vertical settling tanks is the formation of turbulence or vortex zones, caused by the sudden transition of flow from the central pipe to the clarification zone.
To limit this issue, the central pipe of the tank is equipped with a baffle plate to help regulate the water velocity as it exits the distribution funnel below the central pipe, ensuring it does not exceed 0.02 m/s.
To concentrate sludge efficiently into the sump pit at the bottom center of the tank, the conical bottom of the vertical settling tank is designed with a minimum inclination angle of 50 degrees. This inclined angle helps the sludge slide quickly into the sump. However, such a design requires an increase in the overall depth of the tank, resulting in higher structural demands and construction costs.
In some cases, a sludge scraper system is installed inside the tank. Its operating principle is similar to that of a centrifugal settling tank. However, a sludge scraper can reduce sedimentation efficiency because the scraping action disturbs the flow in the settling zone, leading to decreased treatment performance. In addition, investment, operation, and maintenance costs may increase.
3/ Applications of vertical settling tanks
Sludge settling application – removing sludge in wastewater
Vertical settling tanks are applied in wastewater treatment systems for the following purposes:
- Removing biological and organic solids from the wastewater
- Sand settling
- Gravity sludge compression helps reduce the moisture during sludge treatment
- Removing chemical flocs formed during the coagulation–flocculation process
- Separating microbial sludge from water
Sludge settling efficiency of vertical settling tanks
The settling efficiency of vertical settling tanks is relatively low, only about 45–48%. However, this technology is still chosen in some cases because the tank has a small construction area, making it suitable for projects with a limited scale wastewater treatment system. In addition, vertical settling tanks also allow users to discharge sludge more easily.
4/ Key Factors to consider when designing a vertical settling tank
Wastewater flow rate
The wastewater flow into the tank is the basis for calculating the tank volume and the settling surface area. This is an important parameter that the designer needs to consider.
Determining the wastewater flow not only helps to design a suitable system, but it also ensures that the system meets the needs and treatment flow of the project.
Suspended solids in wastewater and allowable concentration after settling
This parameter affects the tank size, settling rate, and sludge discharge frequency. Normally, the higher the two parameters are, the larger the tank size must be and vice versa.
The conditions for sedimentation
There are 3 main types of sedimentation
- Discrete settling: Applies to small, independent particles
- Flocculent (zone) settling: Applies to suspended particles that influence each other
- Compression settling: Applies particles at the bottom of the tank
Each type of sedimentation mode requires a different tank height, so it is necessary to calculate correctly to ensure sedimentation efficiency.
Sludge discharge time
The design of the tank must also take sludge discharge time into account. The designer needs to ensure the system has enough time to discharge sludge without interrupting operation or the settling process.
To better understand the construction of vertical settling tanks and their role in wastewater treatment, businesses can contact NTS Engineering Co.,Ltd. Our experts are available to answer questions and provide advice based on actual needs.
NTS Engineering Co.,Ltd is a leading company in wastewater treatment, trusted and chosen by the majority of businesses. We are always proud to offer your business the most economical and effective solutions in wastewater treatment
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