Aeration and sludge recirculation have been practiced
for many years at municipaJ mld industrial water treatment
plants. Aeration is used primarily for the physical
removal of gases or volatile compounds while sludge
recirculation can be beneficial for turbidity removal
and hardness reduction.
When the advantages of aeration and sludge recirculation
in water treatment were first recognized, energy
costs were so low that such considerations were seldom
of concern in treatment plant design and operation. With
the lO-fold increase in electricity cost that occurred in
some localities, however, it became necessm'y to review
the cost-effectiveness of all water treatment processes
that consume significant amounts of energy. This study