The treatment scheme consists of numerous steps, which
encompass preliminary, biological and tertiary treatments
(the total hydraulic retention time of the latter two is about
30 h) The preliminary treatments (grit removal,
screening, sand removal) are intended to remove coarse suspended
matters such as grit, sand and rags, pieces of fabric,
fibres, yarns. The biological compartment consists of a conventional
activated sludge plant composed of a predenitrification
stage, followed by biological oxidation and
secondary settling. Ozone is dosed on part of the recycle from
the secondary settler to the biological basin in order to reduce
sludge production and to solve some operational problems
due to filamentous bacteria growth and consequent biological
foam (sludge bulking problems and hindered sedimentation).
Then, tertiary treatments (coagulation-flocculation, clarification
and ozonation) are required to remove suspended solids,
recalcitrant compounds and colour.