A Field Study of Inorganic
Nitrogen Oxidation Rates
C. T. WEZERNAK, Assistant Professor
J.J. GANNON, Professor
School of Public Health
The University of Michigan
Ann Arbor, Michigan
INTRODUCTION
It is generally recognized that nitrogenous compounds contained in waste
treatment plant effluents can have a dramatic impact on the oxygen resources of the
receiving stream. This fact assumes greatest importance when the effluent discharged
from waste treatment facilities forms a substantial part of the total flow of the receiving waters. Such is the case in the stream selected for this study.
The study described in this report was conducted on the Clinton River below
Pontiac, Michigan at the location shown on Figures 1 and 2. The Clinton River is a
small stream in this section with flows of less than 100 cfs and velocities ranging between one and two fps, under summer low flow conditions.
Various factors entered into the selection of this study area. The major
factor was a knowledge that dissolved oxygen levels were normally depressed in spite
of the fact that a high degree of treatment was provided by the two waste treatment
plants located immediately upstream from the study section.
TREATMENT PLANT EFFLUENT CHARACTERISTICS
The City of Pontiac operates two activated sludge plants which discharge their
effluent at the locations shown in Figure 2. Both plants are designed for an average
flow of 10 mgd each and normally provide 95 per cent BOD removal and 93 per cent
suspended solids removal. Typical values for the inorganic nitrogen components,
observed during the summer of 1967, are shown in Table I.
As a result of the inputs originating from the treatment facilities, ammonia
nitrogen concentrations increase abruptly downstream from the last plant, to values
ranging from 2.2 to 9.3 mg/1. Figure 3 illustrates the concentrations of inorganic
nitrogen found in the stream, immediately downstream from the last plant, during
the low-flow conditions of September 1967.
OBSERVED DISSOLVED OXYGEN PROFILES
Dissolved oxygen profiles determined for the study section showed a consistent
pattern. In all cases there was a sharp drop in dissolved oxygen at the beginning of
the study section followed by a long stretch of river in which no or very little increase in DO occurred; as shown in Figure 4. It should be noted that the sampling
period covered only daylight hours and hence does not indicate the lowest DO conditions. Also shown in Figure 4 is the DO profile of a slug of water sampled on the
basis of time of passage throughout the day. The difference between the two curves
is undoubtedly due to photosynthesis.
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