75    THE CONTRIBUTION OF SULFATE REDUCING BACTERIA
TO ANAEROBIC DEGRADATION AND COD REMOVAL
BY ANAEROBIC GRANULAR SLUDGE
Wei-Min Wu, Associate Engineer
Robert F. Hickey, Senior Engineer
Michigan Biotechnology Institute
Lansing, Michigan 48909
INTRODUCTION
Sulfate reducing bacteria (SRB) are often observed in anaerobic ecosystems receiving input of
relatively high sulfate levels. Most research concerns SRBs has focused on investigating competitive
relationships between methane production and sulfate reduction for substrates such as hydrogen1-5
and acetate,6 or the inhibition of methane production caused by H2S produced by SRBs.78
Molybdate is a potent and relatively selective inhibitor of SRBs. In an attempt to selectively inhibit
SRBs in anaerobic reactors treating wastewater, high concentrations of molybdate (10-20 mM) were
added to the waste undergoing treatment. These attempts failed because addition of molybdate'also
inhibited methane production from acetate9 and the degradation of acetate and propionate.10
The contribution of SRBs to organic pollutant degradation in the presence of low sulfate concentrations has received little attention. In this work, the role of SRBs in the treatment of a volatile fatty
acid (VFA) mixture and a brewery wastewater containing low sulfate concentrations (10 and 58-125
mg S042"-S/L, respectively) by two granular sludges from UASB reactors was investigated.
MICROBIOLOGICAL BACKGROUND
The complete anaerobic digestion of complex organic compounds (polysaccharides, proteins and
lipids) requires at least three trophic groups of microorganisms, which form a microbial food
chain:1112
• Hydrolytic-fermentative microorganisms;
• Syntrophic acetogenic bacteria; and
• Methanogenic bacteria.
The first trophic group converts a variety of complex organic matter into simple organic compounds (formate, acetate, propionate, butyrate, ethanol, etc.), hydrogen and carbon dioxide. Eubac-
teria are prevalent hydrolytic-fermentative microorganisms, although fungi and protozoa also play a
role in some anaerobic ecosystems.
The second trophic group then converts the metabolic products from the first group, such as
ethanol, propionate, butyrate, valerate, isovalerate, benzoate etc., to direct methanogenic precursors,
principly acetate and hydrogen (or formate). These bacteria can only convert the above substrates in
the association with a hydrogen-utilizing partner, usually a methanogen (or sulfate reducer if sulfate is
available) to prevent the accumulation of intermediary products H2 or formate and, as a result, to
maintain thermodynamically favorable conditions.11,1314 The obligate syntrophic acetogenic species
isolated to date include, Syntrophobacter wolinii, a propionate degrader15 and butyrate degrading
Syntrophomans sp.16-18 and Sytrophospora bryantii.19-20
The third trophic group is comprised of methanogenic bacteria. Methanogens utilize the acetate,
Hj-COj and formate produced by fermentative organisms and syntrophic acetogens to produce
methane (and C02 in the case of acetate degradation). In anaerobic digestors, Methanothrix sp. is
major acetate utilizing methanogens. Methanobacterium, Methanospirillum and Methanobrevibacter
species are the most frequently observed H2-C02 and formate utilizing methanogens.
Sulfate reducing bacteria (SRB) are a diverse group of bacteria that exist in anaerobic ecosystems.
Using sulfate as an electron acceptor, SRBs can degrade almost all intermediates produced fermen-
46lh Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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