What is the required plan size of a square sedimentation tank (as the primary sedimentation tank in sweage treatment), given that its effective depth is 3 m, and the flow rate is 40 MLD with admissible surface loading of 100,000 l/ m² / day?

Concept:

Surface Overflow Rate or Surface Loading: Surface overflow rate (SOR) is parameter that describes the settling characteristics of solids in a specific water or wastewater. It is defined as the volume of water flow per unit of time divided by the surface area (or plan area) of the settling basin.

Hence, \({\rm{SOR}} = \frac{{{\rm{Discharge}}}}{{{\rm{Surface\;or\;Plan\;area\;of\;the\;basin}}}}\)

Calculations:

Given, discharge = 40 MLD = 40 × 10⁶ litres/day and surface loading rate = SOR = 100,000 litre/ m² / day.

∴ Surface or Plan area of the basin \(= \frac{{{\rm{Discharge}}}}{{{\rm{SOR}}}} = \;\frac{{40{\rm{\;}} \times {\rm{\;}}{{10}^6}{\rm{\;litres}}/{\rm{day\;}}}}{{100,000{\rm{\;litres}}/{\rm{day\;per\;}}{{\rm{m}}^2}}} = 400\;{m^2}\)

As the sedimentation tank is square in shape so plan dimension is \(\sqrt {400} \;m = 20m.\)

∴ The plan dimension of the square sedimentation tank 20 m × 20 m.