Sedimentation Tank
What is a Sedimentation Tank?
A sedimentation tank is a treatment structure that removes suspended particles denser than water through gravity settling. It is one of the most widely used units in wastewater treatment, applicable in preliminary treatment, post-biological treatment, and advanced treatment. The principle of sedimentation is applied in:
- Grit chambers to remove inorganic impurities.
- Primary sedimentation tanks to remove suspended solids and other particulate matter.
- Secondary sedimentation tanks to separate activated sludge from biologically treated effluent.
- Thickening tanks to concentrate sludge by separating water.
- Advanced treatment processes where coagulants are added to secondary effluent to remove residual suspended solids.
A sedimentation tank consists of five zones:
- Inlet Zone: Ensures uniform flow distribution, minimizes short-circuiting and turbulence, and reduces dead areas to improve tank efficiency.
- Settling Zone (Clarification Zone): Where settleable particles separate from wastewater.
- Sludge Zone: For storage, concentration, and discharge of settled sludge.
- Buffer Zone: Separates the settling and sludge zones to prevent resuspension of settled particles due to flow disturbances.
- Outlet Zone: Collects clarified water while maintaining uniform flow conditions.
How Does a Sedimentation Tank Work?
Sedimentation tanks operate on the principle that particles with a settling velocity greater than the upward flow velocity of water (or whose settling time is less than the hydraulic retention time) will separate from the flow.
In an ideal sedimentation tank, treatment efficiency depends solely on the surface loading rate (i.e., tank surface area), not depth. Tank depth is only relevant for sludge storage, prevention of scouring, and retention time. However, in practical continuous-flow tanks:
- Particles with settling velocities lower than the upward flow velocity (caused by weir overflow) are carried away.
- Particles with settling velocities equal to the upward flow velocity remain suspended.
- Only particles with settling velocities higher than the upward flow velocity settle.
The time for particles to settle to the bottom is related to the hydraulic retention time, which is influenced by tank depth. Theoretically, shallower tanks allow faster settling, which is the basis for shallow sedimentation technologies like inclined plate or tube settlers. A buffer zone between the settling and sludge zones prevents resuspended particles from rising further by promoting re-settling through particle collisions.
What Are the Common Types of Sedimentation Tanks? Their Advantages, Disadvantages, and Applications.
Based on flow direction, sedimentation tanks are categorized into:
- Horizontal Flow Sedimentation Tanks
- Radial Flow Sedimentation Tanks
- Vertical Flow Sedimentation Tanks
- Inclined Plate/Tube Settlers (developed based on the "shallow depth theory").
The advantages, disadvantages, and applications of each type are summarized in the table below:
| Performance Comparison of Common Sedimentation Tanks | |||||
| Type | Advantages | Disadvantages | Applications | ||
| Horizontal Flow |
1. High sedimentation efficiency 2. Strong adaptability to shock loads and temperature changes |
1. Uneven water distribution, discontinuous sludgedischarge 2. Multi-hopper sludge discharge requires separate pipes for each hopper (high operational effort) 3. Chain scrapers are prone to corrosion of submerged components |
1. Suitable for areas with high groundwater levels and poor geological conditions 2. Applicable to large, medium, and small wastewater treatment plants |
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| Radial Flow |
1.Mechanical sludge discharge with standardizedequipment, easy maintenance 2.Stable operational performance |
1. Complex mechanical sludge discharge equipment 2. High requirements for construction quality |
1. Suitable for areas with high groundwater levels 2.Applicable to large and medium-sized wastewater treatment plants |
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| Vertical Flow |
1. Convenient sludge discharge and simple management 2.Small footprint |
1. Large depth, difficult construction, and high cost 2. Poor adaptability to shock loads and temperature changes 3. Limited tank diameter to avoid uneven water distribution |
Suitable for small wastewater treatment plants | ||
| Inclined Plate/Tube |
1. High hydraulic loading rate and efficiency 2. Short retenton time and small footprint |
1. Complex structure, prone to clogging, requires periodic replacement of plates/tubes 2. Lowr tolerance to high solid loads and shock loads; requires surface washing equipment |
1. Suitable for medium and small wastewater treatment plants 2.Retrofitting existing sedimentation tanks to enhance capacity |
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