Primary Sedimentation Tank (PST): Function, Design, and Efficiency -

A Primary Sedimentation Tank (PST), also known as a Primary Clarifier, is an essential component in wastewater treatment plants. Its primary function is to remove suspended solids and reduce biochemical oxygen demand (BOD) before secondary treatment. The tank settles out heavier solids as sludge, while lighter materials like grease and oils float to the surface for removal. Typically, a well-designed PST removes 45% to 70% of Total Suspended Solids (TSS) and reduces BOD by 25% to 40%.

2. Detention Time and Flow Characteristics

The detention time (DT) is the theoretical duration that wastewater stays in the tank, calculated using the formula:

The flow-through time is the actual time the water remains in the tank and is determined through tracer studies using substances like NaCl.

2.1 Hydraulic Efficiency (HE)

The hydraulic efficiency (HE) of a PST is calculated as:

An ideal sedimentation tank has HE = 1, but real-world tanks usually have an HE lower than 1 due to inefficiencies such as short-circuiting. A PST with HE ≥ 70% is considered efficient.

3. Characteristics of an Ideal Sedimentation Tank

An ideal sedimentation tank is designed to ensure:

Horizontal flow direction
Uniform velocity in the settling zone
Even distribution of suspended particles across the inflow section
Efficient removal of settled particles at the bottom
Uniform water distribution across the cross-section

4. Zones of a Sedimentation Tank

A sedimentation tank consists of four main zones:

Inlet Zone: Slows down the wastewater and distributes it evenly across the tank.
Settling Zone: Provides space for suspended particles to settle.
Sludge Zone: Stores settled solids and prevents their resuspension.
Outlet Zone: Ensures uniform transition of treated water to the next stage.

5. Impact of Tank Dimensions on Efficiency

Increasing surface area improves efficiency, allowing even smaller particles to settle.
Tank depth has no impact on removal efficiency, as demonstrated in the following example:

PST	Length (m)	Width (m)	Depth (m)	Volume (m³)	Flow (m³/day)	Detention Time (hrs)
PST-1	20	10	3	600	4000	3.6
PST-2	20	10	12	2400	4000	14.4

Despite different detention times, both tanks have the same removal efficiency because surface area is identical.

6. Design Criteria for PST

6.1 Key Design Parameters

Surface Overflow Rate (SOR): 20-40 m³/m²/day
Depth: 3-4 meters
Detention Time: 2-4 hours
Sludge Accumulation Rate: ≈2.5 kg of sludge/m³ of flow/day
Weir Loading Rate: 125-500 m³/m of weir/day (typical value: 250 m³/m of weir/day)

7. Shapes and Flow Patterns of PSTs

7.1 Rectangular Sedimentation Tanks

Flow pattern: Horizontal
Max Length-to-Width Ratio: 4:1
Max Length: 30 meters
Max Width: 6-8 meters
Sludge Removal: Chain and flight collectors or traveling bridge collectors

7.2 Circular Sedimentation Tanks

Flow pattern: Radial
Diameter: 10-30 meters
Sludge Removal: Scrapers moving in a hopper-bottomed structure

Circular tanks are more commonly used today due to their improved efficiency in sludge removal.

8. Inlet and Outlet Design in PSTs

8.1 Well-Designed Inlet System

Reduces entrance velocity to prevent turbulence and short-circuiting
Distributes water uniformly across the tank
In rectangular PSTs, baffles are used to slow water velocity
In circular PSTs, a central inlet pipe with a circular drum slows down the flow

8.2 Well-Designed Outlet System

Outlets are free-falling weirs that allow uniform water flow
V-notches are commonly used in circular PSTs for precise discharge control

9. Example: Designing a Circular PST

For a community of 46,900 people consuming 200 liters per capita per day (Lpcd), using:

SOR = 20 m/day
Detention Time = 3 hours
Weir Loading Rate = 250 m³/m/day
Two PST units

9.1 Flow Calculation

Total daily flow:

$\times 200 = 9,380,000 \text{ L/day} = 9380 \text{ m³/day}$

Flow per unit:

$m³/dayQ_{\text{per unit}} = \frac{9380}{2} = 4690 \text{ m³/day}$

9.2 Volume and Surface Area Calculation

Using DT = Volume / Flow Rate,

$m³\text{Volume} = 4690 \times \left(\frac{3}{24}\right) = 586.25 \text{ m³}$

Using A = Q/SOR,

$\frac{4690}{20} = 234.5 \text{ m²}$

9.3 Tank Dimensions

Diameter:

$\sqrt{\frac{4A}{\pi}} = \sqrt{\frac{4 \times 234.5}{3.1416}} \approx 17.3 \text{ meters}$

Depth:

$\frac{586.25}{234.5} \approx 2.5 \text{ meters}$

Thus, each circular PST should have a diameter of approximately 17.3 meters and a depth of 2.5 meters.

10. Conclusion

Primary Sedimentation Tanks (PSTs) play a crucial role in wastewater treatment by efficiently removing suspended solids and reducing BOD. The surface area (not depth) determines sedimentation efficiency, making horizontal flow design critical. A well-designed PST with optimized detention time, SOR, and hydraulic efficiency ensures effective treatment and smooth operation of downstream treatment processes. Circular PSTs are preferred for their efficient sludge removal mechanisms, while a well-planned inlet and outlet design ensures uniform water distribution and prevents short-circuiting

FAQ

1. Does increasing the depth of a PST improve sedimentation efficiency?

No, increasing the depth of a PST does not improve removal efficiency. Sedimentation efficiency depends on surface area (L × W) rather than depth. Increasing surface area allows more particles to settle, leading to better performance.

2. What is Surface Overflow Rate (SOR) and why is it important?

Surface Overflow Rate (SOR) is the flow rate per unit surface area of the tank and determines how effectively particles settle.

A typical SOR for PSTs ranges from 20-40 m³/m²/day. Smaller particles settle better in tanks with lower SOR, improving efficiency.

3. What are the different shapes of PSTs, and which one is better?

Primary Sedimentation Tank (PSTs) can be rectangular or circular:

Rectangular PSTs have a horizontal flow pattern and are typically longer and narrower (L:W ratio up to 4:1).
Circular PSTs use a radial flow pattern, with a central inlet and sludge removal by scrapers.

Circular tanks are ok today because they offer better hydraulic efficiency and sludge removal.

4. How is sludge removed from a PST?

Sludge is collected at the bottom of the Primary Sedimentation Tank and is removed using scrapers, suction systems, or chain-and-flight collectors. In rectangular tanks, sludge is pushed to collection pits, while in circular tanks, scrapers move the sludge to a central hopper for removal.