51    POISONING OF SULFATE REDUCTION WITH
MOLYBDENUM IN ANAEROBIC REACTORS FED
GLUCOSE
Zablon Oonge, Graduate Research Assistant
Gene F. Parkin, Professor
Department of Civil and Environmental Engineering
The University of Iowa
Iowa City, Iowa 52242
INTRODUCTION
The presence of sulfate in wastewater can reduce the amount of methane that can be produced from
the degradation of wastewater organics. Sulfate reducing bacteria (SRB) use various electron donors
partially and completely oxidizing them while reducing S04" to H2S. It is well documented that both
sulfate reduction and methane production can occur in the same environment. It is also well documented that SRB may outcompete the consortium of bacteria responsible for methane production.
Several explanations for the reported predominance of SRB include energetic advantages, kinetic
advantages, and toxicity. There is considerable interest in preventing sulfate reduction to H2S from
occuring. One possibility to add molybdenum (Mo) since some literature reports indicate that Mo is
toxic to SRB at lower concentrations than those reported to be toxic to methane-producing
organisms.1"3
The microbial community involved in the anaerobic processing of complex organic matter such as
glucose has generally been divided into three groups. Fermentative bacteria break down complex
substrates into fatty acids, H2, and C02. Fatty acids larger than acetic acid, such as butyric and
propionic acids, are oxidized to acetate by a second group of bacteria known as acetogenic bacteria.
H2 and C02 are also formed by this group of bacteria. Finally, waste stabilization takes place when
the methane forming bacteria convert acetate and H2/C02 into CH4.
Sulfides produced from sulfate reduction are known to be toxic to methane producing bacteria.
Varying levels of dissolved sulfides (DS = H2S + HS" + S2"), from 100-800 mg/L, have reportedly
been associated with toxicity to methane production.4"16 It is widely believed that H2S is the toxic
sulfide form. As such, when levels of sulfides are known, pH data are also required to use in
determining H2S levels. Kroiss and Wabnegg17 have related methanogenesis to the level of free H2S in
solution. They found that a free H2S level of around 50 mg/L inhibits acetogens and methanogens by
about 50% while complete inhibition occurred at a free H2S level of around 200 mg/L.
It has also been reported that sulfides are toxic to SRB.1819 Recent work by Hilton and
Oleszkiewicz has shown that it is DS and not H2S that inhibits sulfate reduction. They propose that a
situation can be created where DS levels are high and H2S levels are low, such as in the case with high
pH, causing more of the substrate to be converted to methane by inhibiting the SRB.
The interaction between sulfides and heavy metals is an important phenomenon in anaerobic waste
treatment. Both sulfides and heavy metals are nutrients at low concentrations and toxic at high
concentrations. Even if both sulfide and heavy metals are added at sufficient concentrations, precipitation of the metal sulfide may make them both unavailable to the bacteria.
Molybdenum has been used to inhibit the activity of SRB in the studies of interaction of SRB with
methane producing consortia.''2t3'20 found that 20 mM of Mo04 completely inhibited sulfate reduction in intertidal sediments and inhibited acetate oxidation to C02 by 98.5%. They concluded that
unless Mo has unknown inhibitory effects on other respiratory processes, the degree to which it
inhibits acetate oxidation indicates the amount of acetate oxidized via sulfate reduction.
The purpose of the study described herein is to attempt to assess the feasibility of selectively
poisoning sulfate reduction by Mo. At the same time the effect of COD/S ratio on interaction
between SRB and methane producing consortia was studied for systems fed glucose.
45th Purdue Industrial Waste Conference Proceedings, © 1991 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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