A stimulus-response and flow modelling technique was developed for the examination of internal mixing patterns in high rate anaerobic reactors. This method allowed the effects of biomass accumulation to be monitored in a non-intrusive manner which avoided interruption of the reactor operation. The analysis was applied to three pilot scale systems which utilized different reactor designs for anaerobic wastewater treatment.

The results of multiple tracer studies demonstrated that high rate anaerobic processes most closely resemble well mixed reactors. However, significant deviations from ideal behaviour were noted. A flow model incorporating bypass and dead volume was found to adequately describe the measured tracer results. Estimates of reactor actively mixed working volumes obtained by the modelling approach were highly correlated to void volumes measured by an intrusive technique. The stimulus-response methodology is suitable for routine application in full scale facilities where an assessment of biomass accumulation effects is required.

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