Section Three
B. AEROBIC BIOLOGICAL PROCESSES
1. RESPIROMETRY
68    MODIFICATION OF AEROBIC RESPIROMETRY TO ASSESS
BIODEGRADABILITY OF COMPOUNDS IN
ANAEROBIC PROCESSES
Fernando Cadena, Associate Professor
Ivonne Santiago, Graduate Student
New Mexico State University
Las Cruces, New Mexico 88003
Martha I. Beach, President
N-CON Corporation, Inc.
Larchmont, New York, 10538
INTRODUCTION
Major advances have been made through the development of respirometric devices used to monitor
real-time oxygen uptake for determinations of biodegradability and toxicity under aerobic conditions.
However, little progress has been accomplished for similar applications in anaerobic conditions.
Inconsistent results are often obtained in conventional anaerobic systems due to the extended incubation times required to evaluate biodegradability and toxicity. Furthermore, commonly used methods
to measure biodegradability and toxicity under anaerobic conditions, such as the Biochemical Methane Potential (BMP) and Anaerobic Toxicity Assay (ATA), require intensive analytical work. As a
result, anaerobic systems are considered to be unreliable, when actually their lack of popularity seems
to stem from lack of understanding of their complexity.1 The novel instrument described in this
publication is based on a modification of a computerized respirometer2 as adapted for work with
anaerobic samples.
The main purpose in the development of the modified respirometer was to modify an existing
instrument to continuously monitor gas production in batch tests. The inherent nature of the instrument selected should overcome interferences or difficulties found in procedures used for measuring
toxicity and biodegradation under anaerobic conditions common in constant-volume manometry and
constant pressure manometry.
Interferences associated with constant-volume manometry include variations in barometric pressure2-3 and sensitivity to temperature variations.3'4 On the other hand constant-pressure manometry is
unaffected by barometric pressure fluctuations and less sensitive to temperature changes. However,
constant-pressure manometry requires complex and fragile glassware to operate.
The respirometer described by Cadena et al2 makes use of the advantages of constant pressure
manometry but overcomes the complexity of these systems. The same concepts applied in aerobic
respirometry with the pneumatic computerized BOD (PCBOD) instrument2 were applied for gas
production measurements in anaerobic conditions. Simple electrical and plumbing changes were made
to this aerobic respirometer to demonstrate that this instrument can be used for anaerobic studies.
Total gas production is used as an indicator of biodegradation in anaerobic studies instead of the
conventional oxygen uptake in aerobic tests.
DESCRIPTION OF AEROBIC OPERATION
The respirometer used in our studies is capable of continuously monitoring oxygen uptake without
being affected by several common interferences found in other respirometers such as, variable barometric pressure changes, sample evaporation losses, and oxygen depletion in the headspace.
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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