In activated sludge systems, sludge age—defined by the number of times the flocculent sludge settles and returns to the aeration tank—is a fundamental parameter for process evaluation. As this age increases, the emergence and higher frequency of more complex organisms, such as rotifers, are observed.

Rotifer in an activated sludge sample. Phase-contrast microscopy at 400x magnification.

Rotifers are micro-metazoans widely found in activated sludge systems, wastewater, and surface waters, generally ranging from 200 to 500 µm in length. They are popularly known as "wheel animals" due to the presence of cilia in the cephalic region, whose coordinated movement creates the visual impression of two rotating wheels. These cilia play an essential role in capturing food.

Rotifers constitute an important component of zooplankton and feed on suspended material, which is why they are known as filter feeders. Additionally, they play a relevant role in lakes by removing small-sized phytoplankton (> 20 μm < 50 μm).

Rotifer feeding on suspended phytoplankton and bacteria. Phase-contrast microscopy at 400x magnification.

An interesting characteristic of rotifers is the excretion of mucous substances, which contribute to the coagulation of dispersed solids. In this way, these organisms interact positively with the process of floc formation and stabilization in activated sludge systems, favoring sludge sedimentation.

From a reproductive standpoint, rotifers exhibit various mechanisms, the most frequent being asexual parthenogenesis. In this process, females incubate unfertilized eggs in their posterior cavity, allowing for rapid multiplication under favorable environmental conditions.

Caption: Rotifer with an egg incubated in its cavity. Phase-contrast microscopy at 400x magnification.

In general, the presence of rotifers in activated sludge systems is considered a positive biological indicator, associated with high sludge ages, a high degree of oxidation, and good purification efficiency of the system.

Understanding and monitoring these microorganisms is another way to transform microbiology into a strategic ally in the operation and optimization of effluent treatment plants.

References:

01. SAAR, J.H. Microbiologia dos Lodos Ativados. 1st ed. Porto Alegre: Gênese, 2015.

02. CLASS, I.C. Lodos Ativados Princípios Teóricos Fundamentais, Operação e Controle. 1st ed. Porto Alegre: Evanfraf, 2007.

03. TUNDISI, J. G. Limnologia. São Paulo: Oficina de Textos, 2008.