Hydraulic residence time (HRT) of the Orbal oxidation ditch
was about 18 h, the mixed liquid suspended solids (MLSS) were
2800–3600 mg/L, the returned sludge ratio was 100–120%, and
sludge residence time (SRT) was controlled at 20–30 days
through discharging the amount of surplus sludge from the
secondary settler in terms of the monitoring of MLSS concentrations
from the oxidation ditch and the wasted sludge