It is crucial to understand the response reproducibility of on-line
continuous-flow respirometers applied as biological toxicity monitors to
alert plant operators to contamination in wastewater influent. Used as
upset early warning systems, on-line respirometers could identify toxic
waste streams that should be diverted from activated sludge treatment
systems. This study used a continuous flow-through respirometer
(Bioscan™) to examine the reproducibility in the respiration
response of activated sludge biomass from a local wastewater treatment
plant and a commercially available seed (POLYTOX®), when each was
exposed to a toxic pH 3.0 buffer solution. Respiration response is
characterized in this study as a change in dissolved oxygen concentration
over time in the respirometer effluent. This response was segregated into
groups, including within an inoculated population of a given seed source
(activated sludge biomass or POLYTOX®), between different inoculations
of the same seed source, and by the two different seed sources.
Reproducibility in the response of the Bioscan™ respirometer
was as high as 22% within populations, 23% between populations, and 29%
for the same population over time. In addition, time dependency in the
respirometer response to the toxin was examined. It was demonstrated that
response to the same toxic pulse varies between and within populations and
that reduced sensitivity of the microorganisms can occur if repeatedly
exposed to an individual toxin. A genetic DGGE analysis for one population
suggests that the observed reduction in sensitivity may be caused by
changes in species composition.