Microscopic examination of activated sludge - why monitor its composition?

Microscopic examination of activated sludge - why monitor its composition?

Examination of activated sludge need not, and indeed should not, be limited to physico-chemical tests alone. Activated sludge under the microscope can provide a wealth of useful information to facilitate wastewater treatment.

Activated sludge is a flocculent suspension of microorganisms used in biological wastewater treatment systems. It consists mainly of heterotrophic bacteria and protozoa. However, slightly more complex organisms may also be present in activated sludge, such as: 

  • Rotifers,
  • Angiosperms,
  • Sporocerans,
  • Aborigines,

And also other microorganisms (e.g. fungi).

Rotifers (Rotaria)

Sedentary ciliate (Vorticella)


Activated sludge under the microscope will look different in every treatment plant with biological removal of pollutants - its composition depends on many factors, including:

  • the type and quantity of incoming wastewater,
  • the method of wastewater treatment,
  • variations in treatment parameters.

What is microscopic examination of activated sludge?

Microbiological testing of activated sludge involves in vivo observation (observation of a preparation with live micro-organisms) of the activated sludge under a microscope and its description. In addition, identification of filamentous bacteria in activated sludge can be carried out, where, in addition to in vivo observation, the bacteria must be stained.

Biological wastewater treatment methods can be troublesome - an imbalance in activated sludge can be very painful in its consequences. These range from a deterioration of the treatment parameters to sludge swelling or, in the worst case, the destruction of the entire activated sludge. Technologists at treatment plants are aware of these risks, but still not every treatment plant performs microscopic observations of its activated sludge.

Microscopic observations of activated sludge - why should they be carried out?

Systematic observations of activated sludge under a microscope bring a number of benefits. They enable us to:

  • find out the composition of a 'healthy' activated sludge, which will be our reference for working with a particular type of wastewater,
  • find out what changes have occurred in its composition and assess whether these are positive or negative and whether they will have an impact on the parameters of the treated wastewater,
  • react much more quickly to any negative changes in the activated sludge,
  • find out the exact type of filamentous bacterium that has grown, so that it can be combated.

Thus, examining activated sludge under a microscope provides us with valuable knowledge that is useful in many critical situations that sometimes occur in a wastewater treatment plant. SGS Polska performs professional and comprehensive activated sludge testing together with the microscopic observations and identification of filamentous bacteria discussed in the article.

Agnieszka Rybarska, SGS Poland expert in Industries & Environment

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