In this work we deal with the design of the robust feedback control of
wastewater treatment
system, namely the activated sludge process. This problem is formulated by a
nonlinear
ordinary differential system. On one hand, we develop a robust analysis when the
specific growth
function of the bacterium μ is not well known. On the other hand, when also
the substrate concentration
in the feed stream sin is unknown, we provide an observer of system and
propose a design
of robust feedback control in term of recycle rate, in order to keep the
pollutant concentration lower
than an allowed maximum level sd.