2.1. Wastewater treatment process description
Themonitoring site chosen in this studywas theWWTP of the city of
Granollers, near Barcelona (Spain). This plant treats the domestic
wastewater of 112,000 population equivalents (P.E.). After primary
treatment and settling, the wastewater is treated biologically in two
parallel and identical plug–flow reactors where chemical oxygen demand
(COD) and nitrogen removal is performed. Then, the wastewater
flows into a secondary settler before being released into the environment.
Excess sludge is anaerobically digested in order to produce biogas
for electricity generation. A flow of reject wastewater (produced in the
anaerobic digester sludge dewatering process) is regularly released at
the inlet of the plant for its treatment. The configuration layout of
GranollersWWTP is shown in Fig. 1.