Which organism is commonly used as an indicator of healthy activated sludge when present?

• A. Vorticella
• B. Escherichia coli
• C. Paramecium
• D. Rotifer

Answer: (A)

In activated sludge, the make-up of tiny organisms on and in the flocs tells you a lot about how well the process is working. Vorticella, a sessile ciliate that attaches to sludge flocs, is a classic sign of healthy, stable biomass. It feeds on bacteria and helps maintain good floc structure, which supports strong settling and good effluent quality. Seeing Vorticella in the system indicates the biological community is balanced and operating under favorable conditions.

Escherichia coli is primarily a fecal contamination indicator and doesn’t reflect sludge health or settleability. Paramecium can be present in healthy sludge, but it’s not as strong or specific an indicator as Vorticella. Rotifers may appear in mature sludge, but they’re not as consistently associated with good settling as Vorticella.