Evaluation of tracers for the determination of the mixing characteristics of activated sludge reactors

Four tracing agents widely used for tracing water movement, were assessed for their ability to indicate the mixing regime of an activated sludge reactor. These tracers were rhodamine WT, lithium chloride, spores of the bacterium Bacillus globigii, and bacteriophage to Serratia marcescens. The two biological tracers were inexpensive and their determination was simple. However, the bacteriophage suffered from a rapid die‐off (90% in 24h) whereas B. globigii spores demonstrated a propensity to bind to the floes. Lithium chloride was expensive with a time consuming assay and rhodamine WT proved the most suitable of the four tracing agents.