144 BIOLOGICAL TREATMENT OF WASTEWATER
Genetically Engineered MicroorganismsOne of the promising approaches in biodegradation of tox-
ic organics is the development of genetically engineered
microorganisms. Knowledge of the physiology and biochem-
istry of microorganisms and development of appropriate
process engineering are required for a successful system to
become a reality. The areas of future research that can benefit
from this system include stabilization of plasmids, enhanced
activities, increased spectrum of activities and development
of environmentally safe microbial systems.^30- TYPES OF REACTORS
Three types of reactors have been idealized for use in bio-
logical wastewater treatment processes:(a) Batch Reactors in which all reactants are added at
one time and composition changes with time;
(b) Plug Flow or Non-Mix Flow Reactors in which
no element of flowing fluid overtakes another ele-
ment; and
(c) Completely Mixed or Back-Mix Reactors in
which the contents are well stirred and are uni-
form in composition throughout.Most of the flow reactors in the biological treatment are
not ideal, but with negligible error, some of these can be con-
sidered ideal plug flow or back-mix flow. Others have con-
siderable deviations due to channeling of fluid through the
vessel, by the recycling of fluid through the vessel or by the
existence of stagnant regions of pockets of fluid.^31 The non-
ideal flow conditions can be studied by tagging and follow-
ing each and every molecule as it passes through the vessel,
but it is almost impossible. Instead, it is possible to measure
the distribution of ages of molecules in the exit stream.
The mean retention time, t- for a reactor of volume V
and having a volumetric feed rate of Q is given by t-VQ. In
non-ideal reactors, every molecule entering the tank has a
different retention time scattered around t-. Since all biologi-
cal reactions are time dependent, knowledge on age distribu-
tion of all the molecules becomes important. The distribution
of ages of molecules in the exit streams of both ideal and
non-ideal reactors in which a tracer is added instantaneously
in the inlet stream is shown in Figure 10. The spread of con-
centration curve around the plug flow conditions depends
upon the vessel or reactor dispersion number, Deul, where D
is longitudinal or axial dispersion coefficient, u is the mean
displacement velocity along the tank length and l is the
length dimension.^32 In the case of plug flow, the dispersion
number is zero, whereas it becomes infinity for completely
mixed tanks.Treatment ModelsLawrence and McCarty^11 have proposed and analyzed
the following three models for existing continuous flowaerobic or anaerobic biological wastewater treatment
configurations:(a) a completely mixed reactor without biological
solids recycle,
(b) a completely mixed reactor with biological solids
recycle, and
(c) a plug flow reactor with biological solids recycle.These configurations are shown schematically in Figure 11.
In all these treatment models, the following equations can
be applied in order to evaluate kinetic constants,^33 where ∆
indicates the mass or quantity of material:- Solid Balance Equation
Cells
ReactorCells
GrowthCells
Decay⎡⎣⎢⎤⎦⎥⎡⎣⎢⎤⎦⎥⎡⎣⎢⎤⎦⎥CCells
Effluent Loss⎡⎣⎢⎤⎦⎥
(19)- Substrate Balance Equation
Substrate
ReactorSubstrate
Influent⎡ Substrat⎣⎢⎤⎦⎥⎡⎣⎢⎤⎦⎥ee
Growth
Substrate
Effluent Loss⎡⎣⎢⎤⎦⎥⎡⎣⎢⎤⎦ ⎥(20)Parameters for Design and OperationVarious parameters have been developed and used in the
design and operation of biological wastewater treatment pro-
cesses and the most significant parameters are:u x – Biological Solids Retention Time, or Sludge
Age, or Mean Cell Retention Time, is definedINFLOWINFLOWOUTFLOWOUTFLOWQQ Q QPLUG FLOW BACK-MIX FLOWPlug Flow Condition
(Dispersion Number = 0)Non-ideal Flow Condition
(Large Dispersion Number)Uniformly Mixed Condition
(Dispersion Number = 0)Time of Flow to Exit / Mean Retention TimeConc. of tracer C/CFIGURE 10 Hydraulic characteristics of basins.C002_001_r03.indd 144C002_001_r03.indd 144 11/18/2005 10:15:51 AM11/18/2005 10:15:51 AM