Encyclopedia of Environmental Science and Engineering, Volume I and II

BIOLOGICAL TREATMENT OF WASTEWATER 147

to filters is diluted with recirculated flow of treated effluent,

settled effluent, settled sludge, or their mixture, so that it is

passed through the filter more than once. Several recircula-

tion patterns used in high-rate filter systems are shown in

ASCE Manual.^36 Sometimes two filter beds are placed in

series and these are called Two-Stage Filters.

The advantages and disadvantages of recirculation are

listed below:

Advantages of Recirculation

(a) Part of organic matter in influent wastewater is

brought into contact with growth on filter media

more than once.

(b) Recirculated liquid contains active microor-

ganisms not found in sufficient quantity in raw

wastewater, thus providing seed continually. This

continuous seeding with active microorganisms

and enzymes stimulates the hydrolysis and oxi-

dation and increases the rate of biochemical

stabilization.

(c) Diurnal organic load is distributed more uni-

formly. Thus, when plant flow is low, operation is

not shut off. Also, stale wastewater is freshened.

(d) Increased flow improves uniformity of distribu-

tion, increases sloughing and reduces clogging

tendencies.

(e) Higher velocities and continual scouring make con-

ditions less favourable for growth of filter flies.

(f) Provides for more flexibility of operation.

Disadvantages

(a) There is increased operating cost because of

pumping. Larger settling tanks in some designs

may increase capital cost.

(b) Temperature is reduced as a result of number of

passes of liquid. In cold weather, this results in

decreased biochemical activity.

(c) Amount of sludge solids to digesters may be

increased.

The ACE Manual^36 lists the following factors affecting

the design and operation of filters:

(a) composition and characteristics of the wastewater

after pretreatment,

(b) hydraulic loading applied to the filter,

(c) organic loading applied to the filter,

(d) recirculation, system, ratio and arrangement,

(e) filter beds, their volume, depth and air ventilation,

(f) size and characteristics of media, and

(g) temperature of wastewater.

Assuming that the flow through the packed column could be

approximated as plug flow, and if BOD removal rate occurs

by first order reaction, Eq. 13, then the formula to use in

trickling filters will become:

d

d

S

t

qSX=k Sf

or

(^)
S
S
e ktf
0
e.
(21)
Another equation suggested for application in trickling fil-
ters^13 is:
S
SqXtk
e t
0 f
1
1
1
1






(22)
where trickling filter rate coefficient, k f , is a function of
active film mass per unit volume and remains constant for a
given specific area and uniform slime layer. Contact time, t,
TABLE 1
Comparison of low-rate and high-rate filters
Parameters Low-Rate Filters High-Rate Filters
Hydraulic Loading
US gallons per day per square foot 25 to 100 200 to 1000
Million US gallons per day per acre 1.1 to 4.4 8.7 to 44
Cubic metre per day per square metre 1.0 to 4.1 8.1 to 40.7
Organic Loading (BOD)
Pounds of BOD per day per 1000 cubic feet 5 to 25 25 to 300
Pounds of BOD per day per acre-foot 220 to 1100 1100 to 13000
g of BOD per day per cubic metre 80 to 400 400 to 4800
Recirculation Generally absent Always provided R = 0.5 to 3
Effluent Quality High nitrified, lower BOD Not fully nitrified, higher BOD
C002_001_r03.indd 147C002_001_r03.indd 147 11/18/2005 10:15:52 AM11/18/2005 10:15:52 AM