Grit Chamber Calculation [PDF]

Zitiervorschau

CE 2.10 Checking that the Existing Grit Chamber was Sufficient for the Incoming Flow at the Borella Pump Station 2.10.1 Since grit was a major problem at all the pump stations, I carried out preliminary calculations to obtain a rough size for a horizontal flow grit chamber. Peak flow Particle diameter Specific density d

= 10 m3/s = 200 microns = 2.65

Design criteria recommended for a horizontal flow grit chamber is as follows: Vertical velocity Vs mm in diameter Limit design criteria horizontal velocity Vh Detention time (at peak flow) Depth Length Allowance for inlet and outlet turbulence length)

= 0.016-0.021 m/s for particles of 0.2 = 0.2-0.4 m/s usually 0.3 m/s = 30-90s usually 60s = 1-1.5 m = 3-25 m = 25% to 50% (based on theoretical

(From section 5.5.3 of Karia G.L and Christian R.A. (2006) Wastewater Treatment Concepts and Design Approach, Prentice-Hall India, New Delhi and Table 4.6 of Vesilind P. Aarne (2003) Wastewater treatment plant design, Water Environment Federation, VA, USA)

Assuming two horizontal flow grit chambers (which is typical to take into account breakdowns and one chamber shutting down for repair etc.) in parallel and assuming the depth of each of the tanks is 1.5 m: Detention Time

=Depth Settling velocity =1.5 m 0.021 m/s =71 s

Volume of each tank

=(Flow/2) x Detention time =5 m3 x 71s =355 m3

Horizontal area of each tank L x W =Volume Depth LxW

=355 m3/s 1.5 m =237 m2

Length of each tank L

=Horizontal velocity Vh x Detention time =0.3 m/s x 71s =21.3 m =Volume

Therefore width W

Length x Depth =355m3 21.3 m x 1.5 m Width W

=11.1 m

Total depth of tank = net depth + free board + depth for grit collection Total depth of tank = 1.5 + 0.3 + 0.2 Total depth of tank = 2 m Allowing for inlet and outlet turbulence the actual length of each tank is 1.25 x 21.3 m = 26.6 m. Therefore the dimensions of each of the two horizontal flow grit chambers placed parallel to each other is 26.6 m length, 11.1 m width and 2 m depth.