374 CHAPTER THIRTEEN
The power input per volume of liquid is generally used as a rough measure
of mixing effectiveness, 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:
(13.53)
whereGmean velocity gradient (s–1)
Ppower requirement (ftlb/s) (kW)
dynamic viscosity (lbsq ft) (Pas)
Vvolume of mixing tank (ft^3 ) (m^3 )
Gis a measure of the mean velocity gradient in the fluid. Gvalues for rapid
mixing operations in wastewater treatment range from 250 to 1500 s–1.
The required power for mixing is
PG^2 V (13.54)The required volume of the flocculation basin is
(13.55)
Gvalues for flocculation in a direct filtration process range from 20 to 100 s–1.
The power required for flocculation is
(13.56)
If the flows to the rapid mix and flocculation basin vary significantly, or
turn down capability is desired, a variable speed drive should be provided for
each mixer and flocculator.
It should be noted that the above analysis provides only approximate values
for mixer and flocculator sizes. Mixing is in general a “black art,” and a mixing
manufacturer is usually consulted regarding the best type and size of mixer or
flocculator for a perticular application.
DESIGN OF AN AEROBIC DIGESTER
The volume of digested sludge is
(13.57)
where Vsludge volume (ft^3 ) (m^3 )
Wsweight of sludge (lb) (kg)
density of water (62.4 lb/ft^3 ) (994.6 kg/m^3 )
s.g.specific gravity of digested sludge (assume s.g. 1.03)
% solids percent solids expressed as a decimal
V
Ws
(ρ)(s.g.)(% solids)PG^2 V
V
(retention time, min) (flow rate of secondary effluent, Mgd)
(min per day)