Bacteria in a wastewater treatment plant

Bacteria in activated sludge from a wastewater treatment plant

It is a remarkable fact that we fully depend on microbes to treat or sewage water. In every wastewater treatment plant, from the simplest to the most modern ones, the essential activity is biological and is mainly carried out by bacteria. In modern plants, sewage water is directed to large aerated tanks in which the pollution is consumed by a mixture of microbes and organic matter known as  activated sludge.

The video below shows how an aerated tank looks like. You don’t want to take a swim in there… 

For a microbiologist, a wastewater treatment plant is a delight: I’ve never seen another environment with as much diversity in the size and shapes of cells. It is a true microbial jungle containing countless bacteria and many protozoans that feed on them.



Here's some of the bacterial morphologies that one can encounter in a wastewater treatment plant. Note that the scale is not the same in the different images.

These are from the aerated tank:

These filamentous bacteria come from a special digester for grease and oil:

Now it's interesting to realize that although the total number of bacterial species is tremendous, the number of different morphologies is far less impressive. To put it simply, most of the bacteria are either cocci (spherical), rods (straight or curved), corkscrew-shaped, appendaged, or filamentous. (Most of these are represented in the pictures above.)

For this reason, microbiologists do not generally rely on the cell shape and size to determine the identity of a bacterial species. A bacterium's metabolism can give a lot of clues (is it aerobic or anaerobic?, what carbon source can it use? , etc.), but it requires the isolation of the unknown species in a pure culture. Another tool of choice is to use the nucleic acids content, either through DNA sequencing or through the use of  fluorescent probes that target the ribosomal RNA (fluorescent in situ hybridization).

Here's an example with bacteria from the aerated tank. The first picture is a phase contrast image (normal light). The picture below shows the fluorescence emitted by a probe specific to the group betaproteobacteria. We can see that most cells (but not all!) are labelled with the probe, which gives us an indication about the composition of the bacterial community (in this case many betaproteos).

Here's another example with bacteria from the sludge digester, an anaerobic tank where the excess of activated sludge is pumped in order to be further 'digested'. This time we used a probe for the domain Archaea. Most likely these long rod cells are methanogens that help ferment the sludge into methane!

All micrographs were taken during my PhD at the University of Lausanne. At that time I was assisting in a practical course on bacterial community analysis.