26    HYDROGEN AND CARBON MONOXIDE AS EARLY
WARNING INDICATORS OF TOXIC UPSETS IN
ANAEROBIC DIGESTION
Eugenio Giraldo, Graduate Student
Kajsa Norgren, Graduate Student
Michael S. Switzenbaum, Associate Professor
Department of Civil Engineering
University of Massachusetts at Amherst
Amherst, Massachusetts 01003
Robert F. Mickey, Senior Engineer
Michigan Biotechnology Institute
Lansing, Michigan 48909
INTRODUCTION
The advantages of anaerobic digestion technology for the treatment of organic residues are well
known among environmental engineers.1 Significant advances in reactor configurations have been
achieved in recent years, broadening the spectrum of applications of the process to municipal wastewaters,2 and xenobiotic substances3 as well as the traditional applications to wastewater sludges and
industrial wastewaters. Nevertheless, the process control strategies available are still the same as those
which have long been used for sludge digesters.
One of the principal causes of process upsets in anaerobic digestion is toxic substances (e.g., heavy
metals and some organics).4 The possibility of using the trace gases carbon monoxide and hydrogen as
indicators of toxic upsets in the anaerobic digestion of a soluble substrate is explored in this chapter.
Results of batch inhibition studies on anaerobic digestion of a soluble substrate, sucrose, and a
particulate substrate, waste activated sludge (WAS) are compared. More detailed discussion of the
results of toxic studies on WAS is presented elsewhere.5,6 The heavy metals copper, zinc, nickel and
cadmium and the organics formaldehyde and bromoethanesulfonic acid, BES, were tested. Metals
and formaldehyde are found as common causes of operational difficulties in anaerobic digestion. BES
is a specific inhibitor of methanogenesis.7
BACKGROUND
The good performance of an anaerobic digester depends on the coordinated action of several
populations of microorganisms (Figure 1). Thus, one of the most important variables in anaerobic
digestion process control is the accurate characterization of the metabolic status of the microorganisms involved. Different parameters have been proposed for the characterization of anaerobic
digestion (e.g., volatile fatty acids-alkalinity ratio, methane and carbon dioxide composition, methane yield, redox potential, DNA content, ATP content, dehydrogenase activity, phosphatase activity,
specific cofactors of methanogens such as F-420, etc.); nevertheless, few of these parameters are
suitable for on-line real-time process monitoring. Additionally, it is not clear whether the information
that these parameters provide under stress situations of the process is a final result of an upset as
opposed to a warning of it.
Monitoring of trace gases such as hydrogen and carbon monoxide present several potential advantages as early warning upset indicators. They are slightly soluble gases and so they partition preferentially into the gaseous phase. Hydrogen is present in trace amounts and its theoretical turnover time
(e.g., concentration divided by the rate of consumption-production) is very short and should present a
quick response to environmental upsets. Monitoring of the gas phase is suitable to on-line real-time
data acquisition; the physical and chemical conditions of the gas phase of the digester are less severe
for the sensors than in the liquid phase; and the gas phase gives a better composite picture of the
conditions in the reactor.5,6
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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