38    HEAVY METAL INHIBITION OF RESTING NITRIFYING
BACTERIA
Ahmed M. Ibrahim, Visiting Scholar
Environmental Engineering
Purdue University
W. Lafayette, Indiana 47907
INTRODUCTION
All heavy metals are potentially harmful to microorganisms at some level of exposure and absorption. Increased industrialization and domestic activities have accelerated the biogeochemical cycling
of many elements, including heavy metals, causing increased deposition of elevated amounts of these
metals into natural ecosystems, both aquatic and terrestrial.1 Unfortunately, it is known that nitrifying microorganisms are more susceptible to heavy metal inhibition than are the microorganisms
primarily responsible for the oxidation of carbonaceous material.2'3'4 However, little effort has been
made to characterize the effects of heavy metals on these nitrifying bacteria under substrate limiting
(i.e. resting cell) conditions.
This study was, therefore, developed to ascertain the relative susceptibility of nitrifying bacteria
under resting versus active metabolic conditions. All three of the inhibitory species selected for study
are relatively common metal contaminants found within raw wastewater streams, at levels in the sub-
and low-part per million range.
Based on existing documentation of metal inhibition effects, it is evident that the degree of negative
impact may vary widely in relation to individual bacterial species, pH, synergistic effects, and several
other factors. However, prior study of these inhibitory effects had not focused on the relative
metabolic state of the involved bacteria.
Hence, the purpose of this research was to investigate the effect of the three metals (lead, nickel and
chromium) upon the rate of respiration of resting and active nitrifying bacteria. Changes in respiration rate were used as the quantitative benchmark for inhibitory response, following one hour exposure periods.
MATERIALS AND METHODS
Enriched Nitrifier Culture
An enriched culture of nitrifying bacteria was developed and maintained within a continuous-flow
bench-scale reactor; specific details regarding this laboratory equipment and procedures have been
provided elsewhere5'6 Table I provides a brief synopsis of the critical reactor variables maintained
within this bench-scale system.
Table I.   Growth Conditions for Experimental Enriched Nitrifier Culture
Nitrogen loading rate 70 mg N/L/hr
Hydraulic retention time 24 hrs.
Solids retention time 55-75 days
MLSS -4000 mg/L
MLVSS -3300 mg/L
Effluent pH 8.1
Effluent ammonium-nitrogen 0.1-0.5 mg N/L
Maximal SOUR (Active cells) -0.25 mg 02/mg MLVSS/hr
Maximal SOUR (Resting cells) -0.005 mg 02/mg MLVSS/ hr
43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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