CIVIL ENGINEERING FORMULAS

STORMWATER, WASTEWATER, AND ENVIRONMENTAL PROTECTION 377

achieve uniform growth and sloughing, higher periodic dosing rates are
required. The required dosing rate in inches per pass of distributor arm may be
approximated using the following:

Dosing rate (organic loading, lb/10^3 ft^3 d)(0.12) (13.67)

Typical dosing rates for trickling filters are listed in Table 13.2. To achieve
the typical dosing rates, the speed of the rotary distributor can be controlled by
(1) reversing the location of some of the existing orifices to the front of the dis-
tributor arm, (2) adding reversed deflectors to the existing orifice discharges,
and (3) by operating the rotary distributor with a variable speed drive.
The rotational speed of the distributor is a function of the instantaneous dos-
ing rate and may be determined using the following:

(13.68)

where nrotational speed of distribution (rpm)
QTtotal applied hydraulic loading rate (gal/minsq ft) (L/sm^2 )
QQR
Qinfluent wastewater hydraulic loading rate (gal/minsq ft) (L/sm^2 )
QRrecycle flow hydraulic loading rate (gal/minsq ft) (L/sm^2 ) Note:
recycle is assumed to be zero in this example.
Anumber of arms in rotary distributor assembly
DRdosing rate (in/pass of distributor arm)

DESIGN OF AN ANAEROBIC DIGESTOR

Anaerobic digestion is one of the oldest processes used for the stabilization of
sludge. It involves the decomposition of organic and inorganic matter in the
absence of molecular oxygen. The major applications of this process are in the
stabilization of concentrated sludges produced from the treatment of wastewater.

n

1.6(QT)

(A)(DR)

TABLE 13.2 Typical Dosing Rates for Trickling Filters

Organic loading lb BOD 5 /10^3 Dosing rate,

ft^3 d (kg/m^2 d) in./pass (cm/pass)

<25 (122.2) 3 (7.6)

50 (244.3) 6 (15.2)

75 (366.5) 9 (22.9)

100 (488.7) 12 (30.5)

150 (733.0) 18 (45.7)

200 (977.4) 24 (60.9)

(Wastewater Engineering: Treatment, Disposal, and Reuse, Metcalf

& Eddy, 3rd Ed, McGraw-Hill.)