How can I calculate the size of a primary settlement tank in a sewage treatment plant?

Overview:

To size a primary settlement tank for domestic-type sewage (e.g., from a house, school, hotel, or campsite), the goal is to achieve an upward flow velocity of less than 0.9 m/hr. This requires calculating the necessary surface area based on the inflow rate.

Step-by-Step Calculation:

1. Determine the Flow Rate:

   • Flow is calculated based on Population Equivalent (PE):

     • 1 PE = 200 litres/day of flow and 60 g/day of BOD.

   • Example for 400 PE:
     400 PE×200 L/PE=80,000 L/day400 \, \text{PE} \times 200 \, \text{L/PE} = 80,000 \, \text{L/day}400PE×200L/PE=80,000L/day
     Convert to hourly flow:
     80,000 L/day÷24=3,333 L/hr=3.33 m³/hr80,000 \, \text{L/day} \div 24 = 3,333 \, \text{L/hr} = 3.33 \, \text{m³/hr}80,000L/day÷24=3,333L/hr=3.33m³/hr

2. Calculate Minimum Surface Area:

   • Using the standard upward flow velocity of 0.9 m/hr:
     3.33 m³/hr÷0.9 m/hr=3.7 m²3.33 \, \text{m³/hr} \div 0.9 \, \text{m/hr} = 3.7 \, \text{m²}3.33m³/hr÷0.9m/hr=3.7m²

   • The tank must have at least 3.7 m² of surface area to achieve proper settlement.

3. Account for Settlement at 3 x DWF:

   • Standard practice is to size the tank for 3 x Dry Weather Flow (DWF):
     3×3.7 m²=11.1 m²3 \times 3.7 \, \text{m²} = 11.1 \, \text{m²}3×3.7m²=11.1m²

   • The tank must have at least 11.1 m² of surface area to handle peak flows effectively.

Example Using BMS Settlement Tanks:

• Surface Area Per Linear Metre:

  • Standard BMS horizontal flow settlement tanks provide 1.64 m² per linear metre.

• Required Tank Length:
  11.1 m²÷1.64 m²/m=6.8 m11.1 \, \text{m²} \div 1.64 \, \text{m²/m} = 6.8 \, \text{m}11.1m²÷1.64m²/m=6.8m

  • A BMS settlement tank must be 6.8 m or longer to provide adequate settlement.

Innovation: Lamellae Plate Settlers

• Compact Design:

  • Lamellae plates significantly expand the available surface area, achieving the same upward flow velocity in 1/3 of the space.

• Advantages:

  • More compact settlement tanks.

  • Critical for space-saving designs, such as in the BMS Blivet, the world’s most compact all-in-one package sewage treatment system.

Applications:

• Standard Horizontal Flow Tanks:

  • Used where space is available for larger tanks.

• Lamellae Plate Settlers:

  • Ideal for sites with space constraints or requiring compact solutions.

For more information on BMS settlement tanks or the innovative BMS Blivet, contact BMS or visit our Education Blog for detailed examples and technical guidance.

Examples of standard BMS Horizontal Flow Settlement Tank & BMS Lamellae Plate Primary Settlement Area built in a BMS Blivet.

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About BMS

BMS is a quality manufacturer of package products for wastewater and stormwater treatment:

• Established: 1986

• Global Reach: Exported to over 50 countries.

• Family Business: A third-generation company with deep expertise.

BMS Product Range

BMS provides a wide array of surface water and wastewater treatment products, including:

• Attenuation systems

• Interceptors

• Holding tanks (chemical-resistant options available)

• Hydrobrakes

• Rainwater harvesting tanks

• Vortex silt/oil/debris separators

• Fully fitted pumping stations

• Recyclone: A plug & play vehicle wash water recycling system

View the full product catalogue here

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