A Technique for Estimating Active Nitrifying Mass
And Its Application In Designing Nitrifying Systems
E.G. SRINATH, Research Associate
T.B.S. PRAKASAM, Senior Research Associate
R.C. LOEHR, Professor
Department of Agricultural Engineering
Cornell University
Ithaca, New York 14850
INTRODUCTION
Investigations with aerobic treatment processes have shown that the extent of
stabilization is dependent on the concentration of active microorganisms in the sludge
and duration of treatment. Mixed liquor volatile suspended solids (MLVSS) is the most
common parameter used as an estimate of the concentration of the active microorganisms.
Other methods based on the biochemical characteristics of aerobic organisms, such as
deoxyribonucleic acid (DNA) content (1), dehydrogenase activity (2), and catalse activity
(3) have been suggested for estimating the active mass of the sludge.
In nitrifying systems, MLVSS is a very crude parameter of the concentration of
nitrifying organisms since the process of nitrification depends upon the metabolism of
autotrophic nitrifying organisms which represent a small fraction of the total microbial
population in the sludge. A reasonable estimate of the number of mass of the active
nitrifiers in an aerobic treatment process will provide an estimate of the nitrifying activity
in the treatment system and will permit a more meaningful approach for designing
nitrification systems. Although some methods are available for the estimation of the
population of nitrifiers, they are not suitable for measuring the nitrifying ability of a
system. In addition, the current procedures are tedious and require long periods of
incubation.
NITRIFYING ACTIVE MASS
To develop a meaningful parameter of the active mass of nitrifiers, a knowledge of
the physiology of these organisms is useful. There are seven genera of autotrophic nitrifiers listed in the Bergey's manual (4). Organisms belonging to the genera of Nitrosomonas
and Nitrobacter are generally encountered in soil and aquatic ecosystems and are, undoubtedly, the nitrifiers of importance. The other genera are rarely reported and the validity of some is debatable. Nitrosomonas derive their energy by the oxidation of NH-$ to
NO2 and Nitrobacter by the oxidation of NO; to NO5.
The generation time for nitrifying organisms is longer than that of the heterotrophs,
about 30 hours for ammonia oxidizers (5). The effect of factors such as pH, temperature,
dissolved oxygen, NHj; and NOj concentration on the kinetics of nitrification has been
examined. The pH optima for the growth of nitrifiers are not sharply defined, but have
been shown to be generally on the alkaline side of neutrality (6, 7). Laboratory studies on
activated sludge have indicated that the rate of nitrification increased with temperature
throughout the range of 5-35 C (8). Available information indicates that a DO concentration greater than 0.5 mg/1 has no inhibitory effect on nitrification.
In this paper the results available from the literature and the results from our research
on nitrification of highly nitrogenous wastes are used to develop a parameter for the
active mass of nitrifying organisms in an ecosystem such as a waste treatment facility.
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