CEE 371 Fall 2009

Homework #6.   Coagulation, Flocculation, Sedimentation

1.            The design flow for a water treatment plant (WTP) is 1 MGD (3.8x103 m3/d).  The rapid mixing tank will have a mechanical mixer and the average alum dosage will be 30 mg/L.  The theoretical mean hydraulic detention time of the tank will be 1 minute.  Determine the following:

a)      the quantity of alum needed on a daily basis in kg/d,

b)      the dimensions of the tank in meters for a tank with equal length, width, and depth,

c)      the power input required for a G of 900 sec-1 for a water temperature of 10 oC – express the answer in kW

2.            Flocculation tanks are to be designed for a total flow rate of 49,200 m3/d.  The following conditions apply to the design: water temperature of 10 oC, total mean detention time of 45 min, basin depth of 3.5 m, 3 parallel trains of flocculators (each train receives one third of the total flow), 3 flocculation stages of the same dimensions for each train (so a total of 9 flocculators), the first stage G is 50 sec-1, the second stage G is 35 sec-1, and the third stage G is 20 sec-1.  Determine:

a)       The  dimensions of the tank for each stage as well as the overall dimensions, in meters,

b)      the average G times detention time product (the “Gt” Camp parameter) for the overall process and compare to the Camp criterion, and

c)      the power required for each stage, in kW.

3.            Using Stokes Law for particle settling velocity,

a)      Calculate the settling velocity in m/hr of a 100 mm particle with a density of 1050 kg/m3 for water temperatures of 4, 10, 20, and 30 oC.  Tabulate your results.

b)      Using 20 oC as a reference temperature, compute the relative settling velocity in % (i.e., the settling velocity for the stated temperature divided by the 20 oC settling velocity) for water temperatures of 4, 10, 20, and 30 oC.  Tabulate these results along with those of part a).

4.            Settling of Alum floc

a)      Compute the settling velocity (in m/hr) of alum floc particles of 100 and 200 mm diameter at water temperatures of 4 and 20 oC (four calculations are required). Alum floc has a density of 1010 kg/m3.

b)      Compute the Reynolds numbers for all settling velocities of part a). Does Stokes Law for laminar flow conditions hold for these particles?

5.            For a WTP with a design flow of 1.5 MGD, determine the dimensions (in ft) for a rectangular sedimentation basin with a detention time of 4 hr, an overflow rate of 700 gpd/ft2, and length to width  ratio of 3 to 1.

Assigned: 4 Nov 09

Due: 13 Nov 09