DESIGN OFA RAPID-MIX BASIN
AND FLOCCULATION BASIN
1.0 Mdg (3785 m
3
/d) of equalized secondary effluent from a municipal waste water treat-
ment facility is to receive tertiary treatment through a direct filtration process which in-
cludes rapid mix with a polymer coagulant, flocculation and filtration. Size the rapid mix
and flocculation basins necessary for direct filtration and determine the horsepower of the
required rapid mixers and flocculators.
Calculation Procedure:- Determine the required volume of the rapid mix basin
A process flow diagram for direct filtration of a secondary effluent is presented in Fig. 6.
This form of tertiary wastewater treatment is used following secondary treatment when an
essentially "virus-free" effluent is desired for wastewater reclamation and reuse.
The rapid mix basin is a continuous mixing process in which the principle objective is
to maintain the contents of the tank in a completely mixed state. Although there are nu-
merous ways to accomplish continuous mixing, mechanical mixing will be used here. In
mechanical mixing, turbulence is induced through the input of energy by means of rotat-
ing impellers such as turbines, paddles, and propellers.
The hydraulic retention time of typical rapid mix operations in wastewater treatment
range from 5 to 20 seconds. A value of 15 seconds will be used here. The required volume
of the rapid mix basin is calculated as follows:
Volume (V) = (hydraulic retention time)(wastewater flow)V= (I* s)(l x 10* gal/day) = 1?4 m 24 ft3 (0>68 m3)
86,400 s/d- Compute the power required for mixing
The power input per volume of liquid is generally used as a rough measure of mixing ef-
fectiveness, based on the reasoning that more input power creates greater turbulence, and
greater turbulence leads to better mixing. The following equation is used to calculate the
required power for mixing:
G= [-^
V /^
FIGURE 6. Process flow for direct filtration.Polymer AdditionFrom
Secondary
Treatment
Rapid Mix
(Coagulation) Flocculation FiltrationTo
Disinfection