42    DETERMINATION OF ACETOCLASTIC METHANOGENIC
ACTIVITY IN ANAEROBIC SYSTEMS
Chow F. Chiang, Research Assistant
Department of Civil and Construction Engineering
Iowa State University
Ames, Iowa 50011
Richard R. Dague, Professor and Chairman
Department of Civil and Construction Engineering
Iowa State University
Ames, Iowa 50011
INTRODUCTION
In recent years, a variety of high-rate anaerobic processes have become popular for applications in
industrial wastewater treatment.1 Among these are submerged media anaerobic reactors (SMARs)
making use of various feed and flow regimes, including upflow, downflow, and expanded bed, and
the upflow, anaerobic sludge-blanket (UASB) approach.
All of these anaerobic processes treat wastewaters at a high rate, as compared with traditional
anaerobic digesters treating organic sludges. These "high-rate" processes make use of short hydraulic
detention times (15-30 hours) and high organic loading rates (5-15 g COD/L/d). Efficient treatment
is achieved by maintaining a long solids retention time (SRT). The spatial distribution of biomass and
substrate within these processes also varies considerably.
The concept of biomass "activity" has been recognized for many years in aerobic treatment processes. Standard Methods1 provides a procedure (Method 213A) for determining activated sludge
activity based on specific oxygen consumption rate (SOCR, mg 02 consumed/gm VSS/d).
The activity of the biomass in anaerobic systems can be determined by an approach that is analogous to the SOCR in aerobic systems. The difference for anaerobic systems is that the activity is based
on the specific methane production rate (SMPR, mL CH4 at STP produced/gm VSS/d).
It is important to recognize that the active biomass in a reactor is the critical factor in achieving
efficient wastewater treatment. A long SRT, as reflected by traditional VSS measures, may not be the
most important indicator of potential system performance. From the standpoint of the design and
operation of anaerobic processes, biomass "activity" is of great importance.
The purpose of this chapter is to present a testing protocol for determining acetoclastic methano-
genic activity (AMA) using a serum bottle technique. The procedure that is presented is simple, yet
consistent, and can be used for routine monitoring of anaerobic systems. Biomass obtained from
various heights of three static-bed submerged media anaerobic reactors (SMARs) was used to evaluate
the test procedure. Interpretation of the AMA index for application in process design and operation
are also discussed.
THEORETICAL BACKGROUND
Anaerobic digestion of wastes generally follows three sequential stages: hydrolysis, acidogenesis/
acetogenesis, and methanogenesis, as shown in Figure 1. Nutritionally, methanogens can use only two
types of substrate as an energy source, proton-bearing (H2/C02 and HCOOH) and methyl-bearing
(CH3OH, CH3COOH, and methylamines). The acetate utilizers, which are of particular interest in
this work, are primarily Methanosarcina and Methanothr'tx? Methanothrix, an isolate from sewage
digesters, uses only acetate as the energy source. Methanosarcina has been characterized as being
mixotrophic (capable of using both proton- and methyl-bearing substrates as energy sources). However, under a high hydrogen partial pressure, acetate utilization can be inhibited significantly.4
The acetoclastic methanogens are defined as all the methanogens capable of using acetate as an
energy substrate. Previous studies have shown that, for domestic sludge, acetoclastic methanogenesis
43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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