Period 2 (day 61–190): the NH4 + –N load increasing stage. The
NH4 + –N concentration in influent was increased stepwise, keeping
the COD concentration of 17,760 mg/L. When NH4 + –N concentration
in influent was reached 500 mg/L, the NH4 + –N concentration
in effluent was 750 mg/L. With pH between 8.0 and 8.2 in the reactor,
free ammonia concentration was 100 mg/L, which exceeded
80 mg/L, the minimum inhibitory level of the free ammonia
reported by Koster (1986). The COD removal efficiency and the
CH4 production rate dropped to 84.93% and 0.242 LSTP/h respectively.
After a period of operation, the COD removal efficiency
and the CH4 production rate recovered to about 90% and 0.260
LSTP/h respectively, since the methanogens have been acclimatized
to this NH4 + –N concentration (Sung and Liu, 2003). Further increasing
the NH4 + –N concentration in influent to 750 mg/L, the NH4 + –N
and free ammonia concentrations in effluent approached to
900 mg/L and 130 mg/L respectively. The COD removal efficiency