Design of rectangular sedimentation tank| waste water purification engineering

Design of Sedimentation tank

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Sedimentation tank purifies water by the process of sedimentation, which affects suspended particles to settle to bottom of tank. The density currents and turbulences caused by wind, typhoon or high tide carry these particles in water. In sedimentation tank, by controlling turbulence suspended particles in water settle to bottom of tank as sludge. The plain sedimentation in normal can remove as much as 70% of the suspended impurities present in water.

Factors effecting sedimentation are

• Velocity of flow of waters : If the maximum permissible velocity of the flow reduces below 0.3m/sec gravity takes effect on suspended particles to settle.
• Detention period : The detention time for plain sedimentation ranges from 4 to 8hours. For coagulant sedimentation ranges from 2 to 4hours.
• Size and shape of particles : Particles in the size range of 0.5-5 μm are likely to deposit by sedimentation (Darquenne and Prisk 2004, O'Callaghan and Barry 1997, Pilcer and Amighi 2010, Stahlhofen et al. 1980).
• Viscosity of water: High viscous fluid reduces the sedimentation. Sedimentation of oil and greasy substances reduces sedimentation efficiency.
• Temperature: When temperature rises, sedimentation increases. The cold water reduces sedimentation process.

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Maximum daily Demand of water purification Million liters/day.

Detention period in hours hours.

Velocity of flow centimeter/minute cm/minute.

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To design sedimentation tank dimensions like length, width and height. We need three inputs namely maximum daily demand, detention period, velocity of flow, assume depth and free board. These mentioned variables are placed in input box. The page automatically calculates dimensions by clicking on calculate button.

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Where,

• Q_w = Maximum daily demand ( liters/day ).
• Q_dp = Quantity of water to be treated during the detention period.

\[\mathop Q\nolimits_{\mathop d\nolimits_p } = \frac{{\mathop Q\nolimits_w }}{{24}} \times \mathop d\nolimits_p \]

• d_p = detention period ( hours ).

\[\mathop V\nolimits_{f - m} = \mathop V\nolimits_{f - cm} \times 10\]

• V_f-m = Velocity of flow ( cm/minute or m/minute ).

\[\mathop L\nolimits_T = \mathop V\nolimits_{f - m} \times \mathop d\nolimits_p \]

• L_T = Length of tank ( m ).

\[\mathop C\nolimits_s = \frac{{\mathop Q\nolimits_{\mathop d\nolimits_p } }}{{\mathop L\nolimits_T }}\]

• C_s = Cross sectional are ( m² ).

\[\mathop W\nolimits_T = \frac{{\mathop C\nolimits_s }}{{\mathop D\nolimits_T }}\]

• W_T = Width of tank ( m ).
• D_T = Depth of tank ( m ).

Note: above mentioned formulas are used to design rectangular sedimention tank.

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Quantity of water to be treated in 24hrs liters.

Quantity of water to be treated during detention period liters.

capacity of tank m³.

Length of tank m.

Area of tank m².

Assume Depth of tank m.{ Type the value here. }

Width of tank m.

Assume Free board m. { Type the value here. }

Overall depth of tank m.

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Overall dimension of rectangular tank is

1. Length =
   m.
2. Width =
   m.
3. Height =
   m.

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