Age of Microbes

You can tell what type of microorganisms are found in sludge by looking at the sludge under a microscope.  For example, if you were to look at the organisms found in sludge of various ages under a microscope at 400 power, it might look like this:

Note the rotifers, large aerobic microorganisms found in old sludge.  Sludge of the right age has a few rotifers in it, but young sludge has none and old sludge has too many.  

Conditions other than age will also influence the composition of the sludge. As you can see in the picture below, low pH, high grease, low nitrogen, and low phosphorus conditions will result in foaming sludge with few rotifers.  

Anaerobic organisms are also present in sludge.  The picture below shows anaerobic organisms which will produce good settleability of sludge.  

All of the cells in the picture above are of the same cell type.  Sticky enzymes on the cells cause them to clump together, as you can see, which will make them settle out well in the clarifier.