Page 249 - Water and wastewater engineering

6-26 WATER AND WASTEWATER ENGINEERING

circuiting because of uneven inlet or outlet conditions, and local turbulence or dead spots in the
reactor corners. The mean detention time in real tanks is generally less than the theoretical deten-
tion time calculated from Equation 6-13 .

Selection of G and Gt Values
Both G and the product of the velocity gradient and time ( Gt ), serve as criteria for the design of
mixing systems. The selection of G and Gt values for coagulation is dependent on the mixing
device, the chemicals selected, and the anticipated reactions. As noted previously, coagulation
occurs predominately by two mechanisms: adsorption of the soluble hydrolysis species on the
colloid and destabilization or sweep coagulation where the colloid is trapped in the hydroxide
precipitate. Jar test data may be used to identify whether adsorption/destabilization or sweep
coagulation is predominant using the following procedure:
• Determine the optimum pH and dose from plots of settled turbidity (see, for example,
Figure 6-11 ).

• Plot the optimum pH and dose on Figure 6-9 .
• Determine which is the predominant mechanism from the plotted position.
1
G values in the range of 3,000 to 5,000 s and detention times on the order of 0.5 s are
recommended for adsorption/destabilization reactions. For sweep coagulation, detention times of
1
1 to 10 s and G values in the range of 600 to 1,000 s are recommended (Amirtharajah, 1978).

6-7 MIXING PRACTICE
Although there are some instances of overlap, mixing equipment may be divided into two broad
categories: equipment that is applicable to dispersion of the coagulant into the raw water and that
used to flocculate the coagulated water. Dispersion of the coagulant into water is called flash
mixing or rapid mixing.

Flash Mixing Design Criteria
This equipment is designed to produce a high G. The order of preference in selection of equip-
ment type is based on effectiveness, reliability, maintenance requirements, and cost. Common
alternatives for mixing when the mechanism of coagulation is adsorption/destabilization are:

1. Diffusion mixing by pressured water jets.
2. In-line mechanical mixing.
3. In-line static mixing.

Common alternatives for mixing when the mechanism of coagulation is sweep coagulation are:
1. Mechanical mixing in stirred tanks.
2. Diffusion by pipe grid.
3. Hydraulic mixing.