CIVIL ENGINEERING FORMULAS

STORMWATER, WASTEWATER, AND ENVIRONMENTAL PROTECTION 373

whereRrecirculation ratio Qr/Q
Qrrecirculation flow
Qwastewater flow

The BOD 5 loading for the first stage filter is calculated using

W(influent BOD 5 , mg/L)(wastewater flow, Mgd)

\ (8.34 lb/Mgal/mg/L) (13.49)

The BOD 5 loading for the second stage trickling filter is

(13.50)

where loading to the second stage filter.
The NRC equation for a second stage trickling filter is

(13.51)

The hydraulic loading to each filter is

(13.52)

DESIGN OF A RAPID-MIX BASIN
AND FLOCCULATION BASIN

The hydraulic retention time of typical rapid mix operations in wastewater
treatment range from 5 to 20 seconds. The required volume of the rapid-mix
basin, Fig. 13.6, is

Volume (V)(hydraulic retention time)(wastewater flow)

Hydraulic loading

(1R)(Q)

(area)(1440 min/d)

E 2 

100

1 

0.0561

1 E 1 B

W

VF

WBOD 5

W(1E 1 )W

FIGURE 13.6 Process flow for direct filtration. (Hicks—Handbook of Civil Engineering
Calculations, McGraw-Hill.)

To

disinfection

Rapid mix Flocculation Filtration

(coagulation)

From
secondary
treatment

Polymer

addition