28    SULFIDE INHIBITION OF PROPIONATE UTILIZATION
IN ANAEROBIC TREATMENT OF LACTATE AND
ACETATE
D. M. McCartney, Research Assistant
T. Marstaller. Research Assistant
D. M. Heinrichs, Research Assistant
J. A. Oleszkiewicz, Associate Professor
Department of Civil Engineering
University of Manitoba
Winnipeg, Manitoba
CANADA R3T 2N2
INTRODUCTION
Inhibition of anaerobic biological processes either by toxic components of the wastewater or by
products of the microbial reactions has always been a concern to the process design engineer. A
typical example is hydrogen sulfide gas (H2S). It is a product of the reduction of sulfates (S04) by
sulfate reducing bacteria (SRB) and is a concern, particularly in the pulp and paper, fermentation,
and edible oil industry where sulfate concentration can reach several thousand milligrams per liter.
SRB may decrease the methanogenic rates of methane-producing bacteria (MPB) by either competing for available hydrogen sources (hydrogen and acetate) or producing sulfides which are known to
be inhibitory. The outcome of this competition and inhibition could have a significant influence on
yield of methane, effluent quality, and treatment efficiency.
Recent research1"4 suggests that sulfate-reducing bacteria (SRB): 1) generate sulfides that may
result in product inhibition or toxicity to methanogens; 2) increase the reactor pH; 3) accelerate
oxidation of organics (lactate or propionate) that are degraded at a lower rate by non-SRB incomplete
oxidizers; and 4) reduce rate by inhibition and quantity of methane generation by use of available
carbon.
Research conducted at the University of Manitoba3 has shown that SRB are more sensitive to
relatively high total sulfide (TS) concentrations than methane-producing bacteria (MPB), while both
are sensitive to relatively high H2S concentrations. Therefore, at relatively high TS and high pH the
MPB should be able to outcompete the SRB for substrate, by virtue of the lower inhibition TS
threshold evidenced by SRB. Yoda et al.4 concluded that at high acetate concentrations MPB would
outgrow SRB and would therefore be the dominant population. Other researchers5 concluded that
propionate breakdown was the rate-limiting step under H2S inhibition of methanogenesis. They
hypothesized that the drop in activity must be attributed to inhibition of either propionate-splitting
organisms (SRB or obligate H2 producers) or H2-oxidizing organisms (hydrogenotrophic SRB or
MPB).
Figure 1, shows the result of the most recent research conducted at the University of Manitoba.6,7
In batch methanogenic reactors, propionate utilization was inhibited three times as much as utilization of acetate and butyrate over the H2S range of 20 to 300 mg/L (0.6 to 9.6 mM) and pH range of
7.1 to 7.8. The observations were made in batch reactors identical to the ones described herein.
This chapter describes an experiment which addresses the question of inhibition of methanogenic
(MPB) and sulfate-reducing bacteria (SRB) by sulfide. The experiment consisted of 30 batch reactors
operating at various sulfide concentrations and at two pH levels treating two simple organics (lactate
and acetate). The two pH levels were chosen to cause various ratios of un-ionized sulfide (gaseous
H2S) to total sulfide (TS) according to Equation 1.
H2S = [1   +   1.02<pH-7)]-'    •   TS (1)
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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