Qualitative and quantitative identification of microfauna

The identification of microfauna, especially protozoa and metazoa, is an excellent monitoring parameter. The predominance in the systems is used as bioindicators of the performance of the treatment systems.


Protozoa are excellent indicators of the quality of biological treatment processes, such as in activated sludge systems. They feed off bacterial cells free in the liquid or present on the edges of biological flakes and can also feed off solid particles or absorb soluble organic matter.

This “cleaning” activity contributes positively to the quality of the final effluent, by reducing the dispersed material, that is, reducing the turbidity of the final treated liquid (effluent clarification). In the absence of these microorganisms, a large number of particulate matter that does not settle can be washed away with the final effluent, reducing system efficiency and effluent clarification.

The vast majority of protozoa can be considered unicellular, and their size varies from 5 to 1000 μm in length. They can be subdivided into the following groups, according to their way of moving or capturing food. The predominance of certain protozoa can indicate a specific situation in the system or the phase in which it finds itself.

Amoebas (rhizopods or sarcodines): usually move through pseudopods, which are also used to capture food. They can be found with or without a carapace. As they are not extremely competitive in the search for food, in general, they predominate in systems with a large supply of food, a characteristic observed most often in systems that are starting up or in recovery. Thus, in general, its predominance is associated with systems with poor debugging, but some genera are indicators of good quality or of some specific characteristics.

Identificação da microfauna
Identificação da microfauna

Ciliates: are the most complex microorganisms compared to unicellular organisms. They move by means of cilia, which can be distributed throughout the entire body or grouped in some part of the body, and can be free or fixed by a peduncle, usually attached to the flakes. They are divided into free-swimming, reptants (flake predators), suctoria (carnivores) or fixed pedunculated. Because they feed on organic particles and other bacteria, they end up acting in the clarification or cleaning of the effluent in the treatment systems. Depending on their characterization and classification, they may indicate specific characteristics of the systems, but in general they are associated with more stabilized and good quality systems, when present in balance with the local microfauna.

Free swimming ciliates

Reptant free ciliates

Fixed pedunculated ciliates

Flagellates: these protozoa move using flagella, which they may have one or more depending on the species, performing undulatory movements in their locomotion. Often, the flagella are so small that they are difficult to visualize, but their movement is quite unique. Its predominance occurs in systems with a high organic load, feeding mainly on dissolved nutrients. Thus, as they are not so competitive, they are found in young systems, beginning of operation or in impact recovery, where there is low dissolved oxygen. They are more complex than sarcodines, and thus manage to withstand longer than them when food becomes scarcer.


Metazoans are also excellent quality indicators for the activated sludge process. Generally, they are more complex, multicellular, slower growing microorganisms, present in more stabilized systems and with older age. They are more sensitive to the occurrence of shocks, and most of them also have the characteristic of keeping the liquid clarified, feeding on the dispersed material and other bacteria, protozoa and metazoa.

This group can be subdivided into several groups, the most representative for activated sludge systems are the following:

Rotifers: quite common metazoans in activated sludge systems with good efficiency, as they are generally present in systems with high sludge age and high oxidation stages. They are easily identified, as they generally have elongated or rounded bodies, with claws on their tail and crowns of extraordinarily strong cilia, which are even capable of creating a stream of water and directing food to the oral region, in addition to helping in their locomotion process. Due to its complexity and sensitivity, it can be used as an excellent toxicity bioindicator, and its disappearance in systems may indicate the presence of toxic substances.

Nematodes: worms with an elongated and cylindrical body, without segmentation and with thinner ends. Its movements occur through rapid contractions, in the form of an S, feeding off particulate matter and other protozoa or metazoa. They are very resistant to extreme temperatures and low dissolved oxygen concentrations. They are capable of causing holes in biological flakes. Present in systems with higher sludge age and well stabilized systems.

Tardigrades: are metazoans rarely found in activated sludge systems, being popularly known as “water bear”. They move through four pairs of “legs”, feeding off cellular content of other protozoa and metazoa. They are sensitive to low concentrations of ammonia and can indicate systems with excellent nitrification. They have an extremely resistant carapace, being considered one of the most resistant animals on the planet, due to its ability to form cysts when in hostile environments.

Annelids: are the largest metazoans in activated sludge systems, with an elongated, cylindrical body, segmented into rings, usually with tufts of rigid bristles along its entire side, which helps in its displacement, in addition to the contractions used to move. They feed off particulate matter, algae and other microorganisms. They are found in stabilized systems and with high sludge age, but generally not predominant in the systems.

Análise microvida
Análise microvida