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