24    FATTY ACID DEGRADATION AS A TOOL TO MONITOR
ANAEROBIC SLUDGE ACTIVITY AND TOXICITY
Wei-Min Wu, Visiting Scholar
Robert F. Hickey, Senior Engineer
Lakshmi Bhatnagar, Senior Scientist
Mahendra K. Jain, Senior Scientist
J. Gregory Zeikus, Distinguished Senior Scientist and President
Michigan Biotechnology Institute
Lansing, MI 48909
INTRODUCTION
An essential step in evaluating the feasibility of anaerobic treatment for industrial wastewaters
which contain potential toxic or inhibitory materials is determination of the activity levels of anaerobic biomass in the presence of these compounds. Over the past ten years several methods have been
developed to assay the toxicity which were based on gas production or methane production.1"3 These
methods use acetate or an acetate/ propionate mixture as substrates. Methanogenesis from acetate
accounts for approximately 70% methane from acetate in anaerobic digesters4"° and most attention
has focused on determining the level of inhibition of the acetoclastic methanogens. The importance of
assessing the toxic effect produced on syntrophic acetogenesis has been mentioned.7 However, an
assay protocol to examine the level to which the different syntrophic acetogens are inhibited has not
been established.
A modified assay that uses methane production and fatty acid degradation rates to assay the level of
inhibition to these trophic groups has now been developed. This communication describes the application of this method for the evaluation of the toxicity of various chlorophenols and the feasibility of
anaerobically treating an industrial wastewater.
MICROBIOLOGICAL BACKGROUND
Complete mineralization of complex organic compounds to methane and C02 under anaerobic
conditions can be described as a multi-step process involving three different microbial trophic groups.
The hydrolytic-fermentative bacteria, syntrophic acetogenic bacteria, and methanogenic bacteria
(Figure 1) carry out the sequential degradative steps involved in this food chain. Hydrolytic-
fermentative bacteria convert complex compounds into volatile fatty acids (i.e., propionate, butyrate,
etc.), alcohols, as well as acetate, formate, and H2. Syntrophic acetogens then convert propionate,
butyrate, the higher molecular weight fatty acids and alcohols to acetate, hydrogen and formate.
Methanogens mediate the terminal reactions converting H2, formate and acetate into methane. The
maintenance of a low hydrogen partial pressure (or a low formate concentration) is essential for the
syntrophic acetogenic reactions to proceed.8"10
In anaerobic digester systems propionate and butyrate are thought to be the major intermediates of
the hydrolysis-fermentation stage." Butyrate degraders can utilize butyrate, valerate, and other
higher molecular weight fatty acids as substrates. Some of these acetogens can also utilize branched
volatile fatty acids (VFA) such as isobutyrate and 2-methylbutyrate. Known butyrate degraders such
as Syntrophomonas sp. and Clostridium sp.8, l2'" do not utilize propionate. Syntrophic propionate
degraders including Syntrophobacter wolini and other unspeciated strains9,15 use propionate as substrate but not butyrate or other VFAs. Other specific syntrophic acetogens such as isovalerate degraders,16 benzoate-degraders17 also exist in anaerobic ecosystems, but are much less important than
propionate and butyrate degraders in number and substrate turnover. It is reasonable, therefore, to
describe the syntrophic bacteria as two sub-trophic groups, propionate degraders and butyrate
degraders. The growth rates of these syntrophic acetogens is quite slow. Approximate doubling times
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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