BOD 5 loading for a first stage filter in a two stage system typically ranges from 60 to
120 lb/10
3
ft
3
-d (0.96 to 1.93 kg/m
3
-d). The second stage filter loading typically ranges
from 5 to 20 lb/10
3
ft
3
-d (0.08 to 0.32 kg/m
3
-d).
The hydraulic loading to each filter is calculated as follows:(1 + R)(0
Hydraulic loading = — —T-TT
(Area)(l ,440mm/d)First stage filter:
(1+2)(1 x 106 gal/day)
Hydraulic loadingy 6 = —: ——
(2,644 ft^2 )( 1,440 min/d)= 0.79 gal/min-ft
2
(0.54 L/s-m
2
)Second stage filter:(1+2)(1 x 10
6
gal/day)
Hydraulic loadingJ = —: :
(9,190 ft^2 )( 1 ,440 mm/d)= 0.23 gal/min-ft^2 (0.156 L/s-m^2 )Hydraulic loading for two stage trickling filter systems typically ranges from 0.16 to 0.64
gal/min-ft^2 (0.11 to 0.43 L/s-m^2 ).
Related Calculations. In practice, the diameter of the two filters should be
rounded to the nearest 5 ft (1.52 m) to accommodate standard rotary distributor mecha-
nisms. To reduce construction costs, the two trickling filters are often made the same size.
When both filters in a two stage trickling filter system are the same size, the efficiencies
will be unequal and the analysis will be an iterative one.
DESIGN OFA PLASTIC MEDIA
TRICKLING FILTER
A municipal wastewater with a flow of 1.0 Mgd (694 gal/min) (3,785 m^3 /d) and a BOD 5
of 240 mg/L is to be treated in a single stage plastic media trickling filter without recycle.
The effluent wastewater is to have a BOD 5 of 20 mg/L. Determine the diameter of the fil-
ter, the hydraulic loading, the organic loading, the dosing rate, and the required rotational
speed of the distributor arm. Assume a filter depth of 25 feet (7.6 m). Also assume that a
treatability constant (k 20 /2o) °f 0.075 (gal/min)^0 5 /ft^2 was obtained in a 20 foot (6.1 m)
high test filter at 2O
0
C (68
0
F). The wastewater temperature is 3O
0
C (86
0
F).
Calculation Procedure:- Adjust the treatability constant for wastewater
temperature and depth
Due to the predictable properties of plastic media, empirical relationships are available to
predict performance of trickling filters packed with plastic media. However, the treatabil-