After the bio-sludge was fully settled, the supernatant had to be removed (the removed volume of the supernatant was based on the operation program as mentioned in Table 3) within 0.5 hr (draw step: decant) and the system had to be kept under anoxic conditions (idle step) for 0.5 h. After that,fresh MIWW was filled into the reactor to the final volume of 20 l and the above operation program was repeated.