Encyclopedia of Environmental Science and Engineering, Volume I and II

BIOLOGICAL TREATMENT OF WASTEWATER 153

this equation becomes similar to Eq. 28. In practice, this

equation has proven to represent a generalized response

function for design of most aerated lagoons.^33

The exact solid level in an aerated lagoon can be

approximated by applying a material balance around

the lagoon, under equilibrium conditions:

Solids In + Net Synthesis In Basin = Solids Out

or

X 0 + ( Y ∆ S – b X e t ) = X e

or

(^)
X
XYS
e bt





0
(^1)
(^) (29)
Because of a very low solid concentration, the deten-
tion time in aeration basins is very high and a large
volume of aeration basins is required. Therefore, the
temperature variation exerts a profound effect on the
rate of BOD removal. Eckenfelder and Ford^10 have
given a relationship for estimating the lagoon tempera-
ture at both extreme conditions. Once this temperature
is established, a corrected k T value should be obtained
from Eq. 16, using u equal to 1.035 and then adopted in
the kinetic Eq. 28.
Several other modifications in the activated sludge
process have been discussed elswhere;^41 but most of these
modifications are similar in concepts to one or more of
the types discussed above. For example, in Hatfield and
Kraus systems the supernatant from digestion tanks or even
digested sludge are added to the reaeration tank to provide
nutrients. Similarly, an Activated Aeration Plant is a com-
bination of a conventional activated sludge process and the
short-term aeration process.
Rotating Biological Contactors
As mentioned earlier, the traditional aerobic biological
wastewater treatment processes have been divided into two
groups: fixed film or stationary contact systems like trickling
filters and suspended contact systems like activated sludge
process. Rotating biological contactors, RBC, are more like
trickling filters in operation, but adopt certain characteristics
of suspended growth systems. In this process, large light-
weight plastic disks of 2–4 m diameter are half submerged
in the wastewater flowing continuously through cylindrical
bottomed tanks. The disks are rotated slowly at a speed of
1–2 rpm. The biomass grows on the plastic disks and the
substrate is absorbed by this biomass while it is submerged
in the wastewater. The oxygen absorption occurs when the
biomass is in direct contact with air, generally at a rate higher
than that obtained in trickling filters.
These units have been operated successfully at extreme
temperature conditions both for municipal and industrial
wastewaters having very high BOD values. Antoine and
Hynek^42 have concluded that RBC are stable, versatile and
competitive with the activated sludge process.
In Canada, an important parameter regulating the pulp
and paper wastewater treatment is toxicity reduction, mea-
sured by rainbow trout standard bioassay tests. The results
of bioassay tests conducted by Antoine^43 showed RBC was
effective in treating the toxic paper mill wastewater, when
RECYCLE
SLUDGE
WASTE
LIQUOR
FEED
OXYGEN
FEED GAS
CONTROL
VALVE
AERATION
TANK COVER
A G I TATO R
GAS RECIRCULATION
COMPRESSORS
EXHAUST
GAS
MIXED
LIQUOR
EFFLUENT
TO
CLARIFIER
STAGE BAFFLE
FIGURE 18 Schematic diagram of “unox” system with rotating sparger.
C002_001_r03.indd 153C002_001_r03.indd 153 11/18/2005 10:15:54 AM11/18/2005 10:15:54 AM