Effect of oxygen on the stability and inducibility of the biodegradative capability of benzoate.

Anoxic zones in biological nitrogen removal systems are typically open to the atmosphere and receive oxygen from the atmosphere and the recirculation flow from the aerobic zone. This raises the question of how such oxygen input might influence the stability and inducibility of the enzyme systems involved in biodegradation of aromatic compounds. To investigate this, various amounts of oxygen were added to mixed culture denitrifying chemostats receiving benzoate at 667 mg/h as chemical oxygen demand (COD), and the stability and inducibility of the culture's benzoate biodegradative capability (BBC) were tested in aerobic and anoxic fed-batch reactors (FBRs). Cultures from chemostats receiving oxygen at 0, 33, 133, 266, and 466 mg O2/h lost almost all of their anoxic BBC within one hour after being transferred to an aerobic FBR and the first three cultures did not recover it upon being returned to anoxic conditions. The last two cultures recovered their anoxic BBC between 9 and 16 h during the 16 h aerobic exposure period that preceded their return to anoxic conditions and continued to increase their anoxic BBC as they were retained under anoxic conditions. In contrast, the culture from a chemostat receiving oxygen at 67 mg O2/h retained its anoxic BBC longer, recovered it within 3 h after its return to anoxic conditions, and increased it linearly thereafter. None of the cultures developed any aerobic BBC during the 16 h aerobic exposure period in FBRs. The results suggest that higher oxygen inputs into anoxic reactors helped the mixed microbial cultures recover and/or induced anoxic BBC more easily when they were exposed to alternating aerobic/anoxic environments. The exceptional behavior of the culture from the chemostat receiving oxygen at a rate of 67 mg O2/h may have been caused by the presence of a protective mechanism against the toxic forms of oxygen.

Microbial population dynamics in laboratory-scale activated sludge reactors.

As a first step in understanding nonlinear dynamics in activated sludge systems, two laboratory-scale sequencing batch reactors were operated under identical conditions and changes in their microbial communities were followed through microscopic examination, macroscopic observation, and denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene segments from the prokaryotic community. Two experiments were performed. The first used activated sludge from a local wastewater treatment plant to start the replicate reactors. The second used the biomass from the first experiment as a source by intermixing the two and equally redistributing the biomass into the two replicate reactors. For both experiments, the two reactors behaved fairly similarly and had similar microbial communities for a period of 60 days following start-up. Beyond that, the microbial communities in the two reactors in the first experiment diverged in composition, while those in the second experiment remained fairly similar. This suggests that the degree of change occurring in replicate reactors depends upon the severity of perturbation to which they are exposed. The DGGE data showed that the bacterial communities in both experiments were highly dynamic, even though the system performance of the replicate reactors were very similar, suggesting that dynamics within the prokaryotic community is not necessarily reflected in system performance. Moreover, a significant finding from this study is that replicate activated sludge systems are not identical, although they can be very similar if started appropriately.

Performance of a Suspended-Growth Bioscrubber for the Control of Methanol.

An experimental study is presented on the control of methanol emissions using a single-stage, laboratory-scale, suspended-growth bioscrubber. The inlet concentration was 50 or 100 ppmv. The bioscrubber was operated for over 80 days at a solids residence time of either one or two days. The overall removal efficiency of the scrubber with biomass ranged from 69.0 to 81.0%. The efficiency increased with the liquid flow rate. Model simulations were in good agreement with the data.