UFC 3-280-04
17 DEC 2003
k
= coefficient of permeability (assumed 5 under most conditions of water
filtration)
Y
=
sphericity
dynamic viscosity (Ns/m2)
=
kinematic viscosity (m2/s)
v
=
Nr
=
Reynolds number
In a clean filter stratified by backwashing, the equations presented calculate the head
loss as the sum of the losses in successive layers of the media. The head loss calcula-
tions are performed on the basis of a sieve analysis of the material and consider that
the particles between adjacent sieve sizes are uniform. The modified equations for
stratified media are as follows.
5-1.2.7.4 Modified Fair-Hatch
kL (1- α )2 6
pi
( )∑ 2
h=
vu
α 3 Yd
g
di
where pi = percentage of weight retained by sieve and di = geometric mean size be-
tween adjacent sieves.
5-1.2.7.5 Modified Carmen-Kozeny
fpi
h = LK ∑
di
where f = friction factor for each layer, and
1 1- α u 2
K=
φα g
5-1.2.7.6 Modified Rose
1 u2
CdPi
1.067
∑ d
h=
L 4
φ
α g
i
where Cd = drag coefficient for each layer.
5-1.2.8 Configuration. Single media, dual media, and multi-media filters have been
used in water filtration. A bed should be configured on the basis of water stream, efflu-
ent quality, availability of materials, and backwash design. If necessary and practicable,
pilot testing may be used for selecting the media type and configuration. Pilot testing will
provide information on head loss and resultant effluent quality for each medium consid-
ered. Pilot testing is addressed in Paragraph 3-4. If pilot testing is not done, experience
with similar water streams provides guidance in selecting media type and configuration.
Backwash requirements should also be considered in making the final media selection.
5-8
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