Factors Affecting Respiratory Quotient
— Application to Pure Oxygen Systems
JAMES A.  MUELLER, Assistant Professor
Environmental Engineering and Science
Manhattan College
Bronx, New York 10471
AJIT SINGH PANNU, Staff Engineer
Hydroscience, Inc.
Westwood, New Jersey 07675
INTRODUCTION — PURE OXYGEN SYSTEM APPLICATION
In the covered tank pure oxygen aeration system, three gas transfer mechanisms occur:
1) oxygen is transferred from the gas to the liquid phase, 2) nitrogen originally present in the
liquid phase is transferred to the gas phase, and 3) carbon dioxide (C02) produced by the
biological reaction is transferred to the gas phase. The rate of oxygen transferred across the
gas liquid interface is proportional to the driving force for oxygen transfer (C - C. ) as
follows:
dCL      =     KLa(Cs-CL) (1)
dt
where C      =   O2 concentration in liquid, mg/1
C       =   02 saturation concentration, mg/1
Kx a   =   Oxygen transfer coefficient, hr   , a function
of system turbulence determined by the type, size, and quantity of aeration devices.
The oxygen saturation concentration is related to the oxygen partial pressure
as follows:
Cs=   Hp (2)
where: H =   Henry's constant, mg/1 - atm, and
p   =   O2 partial pressure in gas phase, atm.
For air aeration systems, where oxygen partial pressure is substantially constant (20.9%),
the driving force is known for a given C^ value. For a specific oxygen uptake rate, the
required K. a can be determined from Equation 1 allowing sizing of the aeration
equipment.
For pure oxygen aeration, approximately 100% oxygen gas is fed to the first stage of a
covered tank and is immediately diluted in this and subsequent stages by the stripped
nitrogen and carbon dioxide, until a 50% oxygen partial pressure typically exists in the vent
gas. Design of the aeration equipment for this system requires that the partial pressure in
each stage be known. To accomplish this a model relating the gas transfer kinetics to the
liquid phase biological oxidation kinetics has recently been developed by Mueller, et al (1).
In their model the respiratory quotient (RQ), the moles of C02 produced/mole O,
utilized, was used to relate C02 production per unit substrate removed to the
experimentally measured oxygen utilization per unit substrate removed, a'. In verifying the
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