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Oxygen Toxicity in Digesters
MEL FIELDS, Research Sanitary Engineer
FRANKLIN J. AGARDY, Vice President and Director
Environmental Systems Division
URS Research Company
San Mateo, California
INTRODUCTION
The anaerobic fermentation process, as employed in the wastewater field, is
subject to many environmental stresses ranging from shock loadings of toxic
materials to temperature drop-off due to heating system failures. In addition, it is
probable that some digestion failures could be attributed to air (oxygen) being
introduced to the system.
Extensive studies concerning activation, inhibition, and toxicity in anaerobic
systems have been carried out by McCarty (1,2,3), Speece (4), Kugelman (5), and
others (6,7,8,9), and the environmental conditions necessary to insure proper
digestion are well understood (10,11). However, no information exists concerning
the effect of free oxygen on the digestion process although the sensitivity of methane
fermenters to oxygen is discussed by Barker (12) and Burbank (13).
The purpose of this investigation was to determine the system parameter(s)
which signaled the onset of digestion failure when pure oxygen was introduced to the
system. Additionally, it was hoped that quantitative limits could be established for
concentrations of oxygen which could be tolerated by anaerobic systems as well as
concentrations which would cause irreversible effects to the system.
SYSTEM DESIGN
Three 10 liter digesters were set up in a temperature-controlled environment
(94 F ■* 1 F) and were initially seeded with digested sludge from the San Jose Water
Pollution Control Plant. The units were batch fed with 200 ml of raw sludge per day.
The sludge was allowed to reach room temperature (approximately 75 F) prior to
feeding to prevent thermal shock. The daily feeding and withdrawal regimen was
increased to 941 ml/day as the system acclimated. Mixing was accomplished by gas
recirculation. A comparison between the initial and stabilization conditions in the
systems is shown in Table I. In all, it took approximately two weeks to achieve a
steady state condition with gas production reaching 7.28 ml/day with a composition
of methane 39 percent and carbon dioxide 61 percent (percent by volume). The
volatile acids leveled off at 2,900 mg/l, HAc and the pH established at 6.8. While the
gas composition was not in the usual range of 3:1 to 4:1 for CH4/CO2 ratios, the
systems all exhibited uniform ratios indicating equilibrium conditions. Additionally,
the volatile acid composition consisted of propionic and buteric acids with no acetic
acid found. This was further indication that digestion was progressing satisfactorily
(14).
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