Encyclopedia of Environmental Science and Engineering, Volume I and II

148 BIOLOGICAL TREATMENT OF WASTEWATER

is related to filter depth, H, volumetric rate of flow per unit

area, Q a , and specific surface area of filter media, A v. Sinkoff,

Porges, and McDermott^37 have proposed the following rela-

tionship based on their experiments:

tcH

A

Q

v

a

n

 1

⎡

⎣

⎢

⎤

⎦

⎥

(23)

c 1 is assumed to be a constant and exponent n ranges between

0.53 and 0.83 depending upon the type of filter medium and

the hydraulic characteristics of the system. Substitution of

this value of t in Eq. 21 gives:

S

S

k

A

Q

e f v Hc

a

n

kHQfan

0

 1 



exp.

⎡

⎣

⎢

⎤

⎦

⎥ e



(24)

Eckenfelder^13 suggests that the amount of active surface

film covering the filter medium decreases with depth H;

therefore, combining Eqs. 22 and 23 and substituting c 1  1/ H^ m^ ,

gives:

(^)
S
S kAHmQ kHmQ
e
fv
n
a
n
fa
n
0
1
11
1
11




( )

( )
.
(25)
For treatment of domestic wastewater on rock filters,
Eckenfelder has obtained the values of n = 0.5, m = 0.33 and
k
f = 2.5 with H in ft and q in MGD/acre. Several empirical
relationships for process efficiency in trickling filters have
been proposed and successfully applied. Most significant
of these are the National Research Council Formula and
Rankin’s Formula which have been described in detail in
ASCE Manual.^36 Eckenfelder and O’Connor^13 have reported
a value of 1.035 for overall temperature coefficient, u, in
Eq. 16. An adjustment in process efficiency due to variation
in temperature should be provided.
Activated Sludge Process
It is a biological treatment process in which biologically
active mass, called activated sludge, is continuously mixed
with the biodegradable matter in an aeration basin in the
presence of oxygen. The combination of wastewater and
activated sludge is called the mixed liquor. The oxygen
is supplied to the mixed liquor either by diffusing com-
pressed air or pure oxygen into the liquid or by mechanical
aeration. The activated sludge is subsequently separated
from the mixed liquor by sedimentation in a clarifier and
a part of this sludge is recirculated to the aeration basin.
The rest of this sludge, indicating net excess production of
biological cell material, is disposed of. Activated sludge
treatment plants vary in performance due to variation in
unit arrangements, methods of introducing air and waste-
water into the aeration basin, aeration time, concentration
of active biomass, aerator volume, degree of mixing, etc.
Some important types of activated sludge processes are
discussed below and their operating parameters are sum-
marized in Table 2.
TABLE 2
Activated sludge process parameters
Parameters Conventional
Step
Aeration
Short
Term Biosorption
Pure
Oxygen
Complete
Mixing
Extended
Aeration
Aerated
Lagoons
Organic Loading Rate—Bv
1b BOD 5 per day per 1000
cubic feet
30–40 50–150 100–400 30–70 150–250 125–180 10–20 5
g BOD 5 per day per
cubic metre
480–640 800–2400 1600–6400 480–1120 2400–3200 2000–2880 160–320 80
Process Loading Factor, U
1b BOD 5 per day per 1b
1b MLVSS
or
kg BOD 5 per day per kg
MLVSS
0.2–0.5 0.2–0.5 2–5 0.2–0.5 0.4–1.0 0.6–1.0 0.05–0.2 0.2
Sludge Age, days, θx 3–4 3–4 0.2–0.5 3–4 0.8–2.3 14– 3–5
Aeration Time, hours, t ̄ 6–7.5 6–7.5 2–4 0.5–1.5
(aeration)
1–3 3–5 20–30 70–120
BOD 5 removal, %, E 90–95 90–95 60–85 85–90 88–95 85–90 85–90 85–90
Normal Return Sludge
Average Resign Flow
(^100) 30 (15–75) 50 (20–75) 20 (10–50) 100 (50–150) 25 (20–50) 100 (50–150) 100 (50–200) 0
Primary Settling Required Yes Yes No Optional Yes Optional No No
Provision in design should be made for these maximum and minimum values.
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