Question

ENV Water Supply

Flocculation tanks need to be sized to match three sedimentation basins 3.5 m deep and 6.0 m wide. The sedimentation basins each handle a flow of 5000 m/d, as will the flocculation tanks. The floc tanks will have two mixing sections with an overall average velocity gradient (G) of 35/s. The average velocity gradient-time (Gt) value has been set at 80,000. The paddles will be the wooden paddle-wheel type and will turn on a horizontal axis perpendicular to flow, with one paddle wheel in each section. A common motor will turn the paddles in both sections of each tank at a constant rotational speed of 5 rpm. The water temperature is 20°C. (a) What are the flocculation tank dimensions? (b) Which of the following would you recommend for the G values for the first (inlet end) and for the second (outlet end) sections of each flocculation tank and why? • G1 = 45/s and G2 = 25/s • G1 = 60/s and G2 = 10/s • G1 = 25/s and G2 = 45/s G1 = 10/s and G2 = 60/s (c) What are the power requirements for the paddles in the first and second sections of each flocculation tank, if you went with the first option?

Answer 1

3.m/ 60m Given flow through sedementation tank - sooo med G: ang velocity gradient = 35/sec. GE = 80,000. rotation speed dy paddes opm T= 20°c. Assuming detention time t=4 hrs. capacity of tank xeqd = 500084 - 833-34 m3 mo 246 Assuming overflow rate do 1000 lt/ or foot of plan area - 1000 1000) = L. @ L = 0.139 m. 833.34 1000x6 Assuming efbeetwe depth in bloculation chamber ar halb depth of tank ie, de = 6:3m. Assuming period of flocculation = 20 min capacity of chamber = floco required in 20 min = 5000820 - 69.50 m² ~70 m² 1440 area required - capacity - 70 - 2333 m? plan 63 depth length of blocculation tank = 70 = 11.7m.

Temporal mean velocity gradient (e where pipower dissipated in watts (ie N.m/sec) u: absolute 61) dynamic viscosity Nshoz. V: volume of raw water to which p is applied. Gafen az ur= b P, G2 = P2G