On the surface of mature biofilms, there was a water
layer that remained relatively motionless under the
combined influences of capillary water tube inside the
biofilm and the adhering layer on the surface of biofilm.
For the biofilm, there was a cycling process composed of
the following four steps: First, the pollutants such as the
ammonia, phosphorus, and organic matter were adhered
to this water layer on the biofilm. And these pollutants
can be degraded to smaller molecular substances by the
extracellular enzymes from the microbes on the biofilm.
Second, these smaller molecular substances can adhere
and enter the inside of biofilm assisted by oxygen which
can enter the inside of the biofilm from the water. Third,
these substances can be metabolized to inorganic matter
such as carbon dioxide and water by the aerobes inside the
biofilm. Fourth, the produced inorganic materials can be
transferred from inside to outside the biofilm. The rates of
mass transportation on the biofilms were high and thus the
degradation rates of the pollutants were rapid.