into 5 groups. The NH4 + –N concentrations were 0, 500, 1000, 2000,
3000 mg/L respectively. The methane production from the mediums
with 0 mg/L NH4 + –N was as control compared with that of
other groups. Methane production was measured every 2 h using
a glass gas displacement device filled with 3 mol/L of a NaOH solution.
The SMA was calculated from the linear range of the specific
methane production rate curve using linear regression. Specific
methanogenic activity ratio (SMAr) was used to determine the
degrees of inhibition of SMAs under different NH4 + –N concentrations,
SMAr was calculated by Eq. (1).