Page 449 - Civil Engineering Formulas

STORMWATER, WASTEWATER, AND ENVIRONMENTAL PROTECTION 375

The weight of oxygen, (lb /day) required to destroy the VSS is approximately
2
(lb VSS/d) (2.3 lb O 2 /lb VSS) (13.58)
W O 2
Then, the volume of air required at standard conditions of 14.7 lb/sq in and
3
68 F with 23.2 percent oxygen by weight and a density of 0.075 lb/ft is:
3
3 ) (0.075 lb/ft ) (0.232) (13.59)
Volume of air (ft ) (W O 2
The volume of the aerobic digester is computed using the following equa-
tion, assuming the digester is loaded with waste activated sludge only:

V Q i X i (13.60)
X(K d P v 1 c )

3
3
where V volume of aerobic digester, ft (m )
3
3
Q influent average flow rate to the digester, ft /d (m /d)
i
X influent suspended solids, mg/L (50,000 mg/L for 5.0% solids)
i
X digester total suspended solids, mg/L
–1
–1
K reaction rate constant, d . May range from 0.05 d at 15°C (59°F)
d
–1
to 0.14 d at 25°C (77°F)
P volatile fraction of digester suspended solids (expressed as a decimal)
v
solids retention time (sludge age), d
The usual design procedure computes needed values for both summer and
winter conditions for the digester.
DESIGN OF A PLASTIC MEDIA TRICKLING FILTER
Due to the predictable properties of plastic media, empirical relationships are
available to predict performance of trickling filters packed with plastic media.
However, the treatability constant must first be adjusted for both the tempera-
ture of the wastewater and the depth of the actual filter.
Adjustment for temperature. The treatability constant is first adjusted from
the given standard at 20°C (68°F) to the actual wastewater temperature using
k 30 20 k 20 20 T 20 (13.61)
where k treatability constant at 30°C (86°F) and 20 ft (6.1 m) filter depth
30/20
k treatability constant at 20°C (68°F) and 20 ft (6.1 m) filter depth
20/20
(as example values)
temperature activity coefficient (assume 1.035)
T wastewater temperature
Note: The actual temperature, °C and depth, ft (m) are used when this for-
mula is applied in practice. The numbers given here are for illustrative purposes
only.