Chemical Methods for
Nitrification Control
JAMES C. YOUNG, Assistant Professor
Civil Engineering Department
Iowa State University
Ames, Iowa
INTRODUCTION
Nitrification presents several major problems in the measurement and interpretation of BOD data. As a first example, nitrification normally proceeds much more
readily in effluents from biological treatment processes, which may contain a large
population of nitrifying organisms, than in raw wastes, which usually contain relatively few nitrifying organisms. Since nitrification proceeds simultaneously with the
carbonaceous oxygen demand, it is difficult to determine the contribution of nitrification to the measured BOD. Consequently, the efficiency of organic waste removal in a biological treatment process could be significantly in error.
Because of nitrification, it is difficult to compare the BOD of two different
wastes, or even the same wastewater stream sampled at two different times. This is
particularly true with effluents from trickling filters where nitrification takes place
rapidly, when compared to effluents from conventional high-rate activated sludge
systems where nitrification usually proceeds more slowly.
Reaction rates are difficult if not impossible to determine accurately when
nitrification occurs in a BOD test. In streams, where the oxygen uptake can be almost entirely nitrogenous demand, errors in determining reaction rates can be particularly misleading. Also the relative rates of nitrogenous and carbonaceous demand
change considerably with temperature so that in the winter nitrogenous demand may
not occur in the natural stream, yet could be a significant contribution in the 20 C
BOD test. Much more nitrification might then be indicated than would be possible in
the natural environment.
With nitrification, the indicated BOD may not represent the actual waste. This
error is particularly great with the standard dilution BOD test since ammonia nitrogen
is added with the nutrients and buffer.
Fortunately the microorganisms responsible for nitrification grow slowly and
do not grow well in the presence of soluble organic carbon. Thus nitrification usually
does not occur until five or more days after the start of a BOD test when using fresh
domestic sewage or industrial wastes. This apparently is one of the major reasons that
five days was set to be the standard incubation time in BOD tests. During long-term
tests of up to 20 days incubation, nitrification can seldom be eliminated, thereby
contributing significantly to the error in measurement of carbonaceous BOD. With
effluents from biological treatment units and with stream samples, nitrifying organisms
exist in sufficient numbers that nitrification proceeds rapidly in the standard BOD
test, possibly accounting for a major part of the measured BOD.
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