The influent of 2 m3·h−1 is pumped into
the anaerobic zone I, giving a hydraulic retention time
(HRT) of 1.53 and 14.4 h respectively in the anaerobic
zone (I+II) and the OD before it enters the secondary
clarifier. The RAS is pumped into the pre-concentration
tank for sludge pre-concentration. Then, the supernatant
fluid generated in the pre-concentration process
flows back into the OD bioreactor by gravity, and the
pre-concentrated RAS is pumped into the pre-anoxic
zone for eliminating NO3-N by endogenous denitrification.
Three mixers are set up for preventing sludge
settling in the pre-anoxic zone, anaerobic zone I and
aerobic zone II, and a disc aerator is employed for
supplying oxygen while a liquid propeller is equipped
for mixing in the OD bioreactor. Waste sludge is removed
from the system by discharging some RAS
twice a day in order to control the solid retention time
(SRT) at 18-20 d. Dissolved oxygen (DO), pH, oxidation-
reduction potential (ORP), and mixed liquor suspended
solid (MLSS) are measured on-line by probes.
Programmable logic controller (PLC) is adopted to
adjust the rotating speed of disc aerator according to
the DO value monitored by the DO probe in the aerobic
zone of the OD bioreactor. RAS pump and
pre-concentrated RAS pump are controlled by PLC
for different operational conditions.