Dynamics of Nitrification in the
Activated Sludge Process
RICHARD A. PODUSKA, Environmental Engineer
Tennessee Eastman Company
Kingsport, Tennessee 37662
JOHN F. ANDREWS, Professor
Civil and Environmental Engineering
University of Houston
Houston. Texas 77004
INTRODUCTION
Renewed interest in the problems created by the discharge of nitrogen compounds to
receiving waters has focused attention on the form and concentration of nitrogen in
wastewater treatment plant effluents. Most wastewater treatment plants are designed,
operated and evaluated based on the reduction of carbonaceous 5-day biochemical oxygen
demand (BOD) rather than on the total BOD of the effluent (carbonaceous and
nitrogenous). As a result, ammonium is the primary form of nitrogen in most wastewater
treatment plant effluents. Since ammonium may be biologically oxidized to nitrate in a
receiving water, the nitrogenous oxygen demand can represent a major portion of the total
oxygen demand for a treatment plant effluent.
Assuming a theoretical oxygen requirement for nitrification of 4.5 mg oxygen per mg
ammonium nitrogen oxidized to nitrate, an effluent with 20 mg NH+4-N per liter has a
nitrogenous oxygen demand (NOD) of 90 mg/1. This is considerably greater than that
contributed by the ultimate carbonaceous oxygen demand of normal domestic biological
treatment plant effluents (20-40 mg/1). The conversion of ammonium to nitrate within a
biological treatment plant would represent a significant improvement in effluent quality
through a reduced ultimate BOD. Additionally, the oxidation of ammonium to nitrate in a
treatment plant offers the advantage of reducing the amount of chlorine required to obtain
effective disinfection. This results from the decreased formation of chloramines which are
less effective disinfectants than free residual chlorine.
Nitrification can be accomplished in the activated sludge process if conditions suitable
for the retention and accumulation of the nitrifying bacteria are maintained. Since the
autotrophic nitrifiers have lower yields and growth rates, and are more sensitive to
environmental conditions than heterotrophic organisms, it is possible to obtain high
degrees of carbonaceous BOD removal and little, if any ammonium oxidation. Efficient
nitrification of industrial wastes is often difficult due to the presence of growth inhibiting
chemicals (1) and adverse pH conditions. The conditions necessary for nitrification in the
activated sludge process may be expressed in terms of sludge age (SA), pH, temperature and
dissolved oxygen concentration. The concentration of nitrifying bacteria will depend on
their specific growth rate and on the rate at which they are discharged from the system
through the effluent and waste sludge streams.
Several laboratory, pilot and full-scale investigations have been conducted with
nitrifying activated sludge systems and have demonstrated that high efficiencies of
nitrification are obtained at sludge ages of approximately 4 days or greater (2,3,4,5,6,7). It
is important to note, however, that at temperatures less than 20 C the sludge age required to
maintain a high nitrification efficiency increases substantially.
In the past, the design and modeling of biological wastewater treatment processes has
been based on steady state models even though the inputs to these processes are usually far
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