Rotifers as a Bio-Index in Wastewater Treatment Plants(Based on Wisconsin DNR – Suspended Growth Processes Study Guide, Operator Certification...
Published on by Hossein Ataei Far, Deputy Manager of the Research, Technology Development, and Industry Relations Center at NWWEC
(Based on Wisconsin DNR – Suspended Growth Processes Study Guide, Operator Certification Program)
1. General Role
Rotifers are multicellular metazoa observed in activated sludge systems under conditions of proper stabilization and low organic loading. Their presence generally signifies that the facility is functioning in a mature, well-settled biological state, where easily degradable organic matter has been effectively consumed by bacteria and protozoa.
2. Operational Significance
Indicator of low F/M (food-to-microorganism ratio): Rotifers are most prevalent when the influent substrate is reduced, signifying a transition in the microbial community from active bacterial growth to a more stable population.
Sign of nitrification potential: The occurrence of rotifers often correlates with conditions conducive to nitrifying bacteria, as both prosper under extended aeration and minimal organic loading.
Polishing effect: Rotifers contribute to the “polishing” of effluent by grazing on suspended bacteria and fine particulates, thereby enhancing effluent clarity.
3. Population Dynamics and Control Insights
Elevated rotifer populations → signify extended sludge age (longer mean cell residence time, MCRT), well-settled biomass, and favorable effluent quality.
Low or absent rotifers → may denote elevated organic loading, immature sludge, or exposure to toxic shock.
Sudden population decline → can indicate the presence of toxic influent discharges (such as industrial chemicals or heavy metals) or abrupt operational fluctuations.
4. Practical Bio-Index Use
Expected occurrence of rotifers: Typically identified in activated sludge systems with prolonged aeration or low F/M ratios, following the establishment of protozoan populations (e.g., ciliates and stalked ciliates).
Microscopic monitoring: Operators can incorporate the observation and quantification of rotifers into a qualitative bio-index, alongside other protozoa such as free-swimming ciliates, stalked ciliates, and flagellates.
Complementary indicator: Rotifers should not be employed as a stand-alone control measure; rather, their presence should be assessed in combination with settling tests, effluent turbidity, and analysis of protozoan populations for optimal system evaluation.
5. Summary (Wisconsin DNR perspective)
Per the Wisconsin DNR training guide, rotifers are regarded as a positive indicator in activated sludge microscopy, signifying:
Stable operation
High effluent quality
Prolonged sludge age/extended aeration
By monitoring rotifers along with other protozoa and metazoa, operators can optimize sludge retention times, aeration settings, and solids management protocols.
📌 Reference:
Wisconsin Department of Natural Resources. Suspended Growth Processes Study Guide. Operator Certification Program, Wastewater.