Question

Please explain selection of answer in #1

Sedimentation A town is designing a rectangular, 4m deep settling tank for treating surface water intake. The tank will have a flow velocity of 5 cm/s. The untreated water has particles with the following properties: d=0.025 mm and density of 2,650 kg m 3 The settler is placed following a coagulation-flocculation step which is able to do one of the following: a) Triple the diameter of the particles but have no impact on the particle density. b) Triple the density of the particles but have no impact on their diameter. 1. Which of the options would you select (a or b)? (Please explain your selection in the next question) 2. According to your selection, calculate the length of the settler (in m) required to remove the particles assuming they reach terminal velocity.

Answer 1

Details : liven • Flow velocity , Ve = 5 cms = 0.05 m/s •depth of tank, H = 4 m Density of solids, Is=2650 kg/m3 diameter of particle, d = 0.025mm = 0.025810 m • Kinematic viscosity, @ 20°C , P = -106m% Bly option @ is correct: Settles triples the diameter of the particles but no to impact on the particle density i because mere bringing the particles together does not increase The density of soleds. -- - 132. Now, New diameter of solids, d'= 3xd d'= 0.075 x 103 m To calculate Terminal velocity using stokes law Vs = (Is - Sw) g d² lore Vs = (2650-1000)* 9.81*(0.075X10-3)? 18 x 10-6 Vy = 5.058 m/s Settling time, t = = = 0.791 seconds Settler, L = Vp xt L= 0.05% 0.791 a Scanned with L = 0.04 m Sand CamScanner Length of