Abstract:This paper was to reveal the key functional microbes and metabolic mechanisms in biological treatment of hypersaline production water with high levels of CODcr (chemical oxygen demand determined by Dichromate method) and ammonia. The illumina Miseq and real-time quantitative polymerase chain reaction(qPCR) were employed to detect the community and quantity of bacteria, AOB (ammonia oxidizing bacteria) and AOA (ammonia oxidizing archaea) within a full-scale A/O-MBR (anoxic / oxic pond - membrane bioreactor). Results showed that the average CODcr of the effluent was 211 mg/L, and the removal rate was 72.7%. Ammonia demonstrated a two-stage pattern of low/high removal rate (LS/HS), corresponding to 5.7% and 70.8%, respectively. Ammonia was completely transformed into nitrite. Nitrite was not further oxidized into nitrate, but partly reduced to nitrogen gas by denitrifying bacteria. Bacterial community revealed that the key bacteria involved in CODcr removal were Limnobacter, Thauera, Burkholderia, Aquamicrobium, Luteimonas and Arenimonas, and the key denitrifying bacteria were Stappia, Hyphomicrobium and Methylophaga. The dominant one in AOB was Nitrosomonas, accounting for more than 96.6%. AOA was primarily composed with Nitrososphaera, Nitrosopumilus and Nitrosocosmicus, all affiliated to Thaumarchaeota. The bacteria ranges were 2.83×10^9~4.01×10^9 copies/g sludge for both LS and HS samples, which resulted in the high efficiency of biological treatment. The AOB ranges were 7.27×10^7~8.47×10^7 copies/g sludge and 6.79×10^8~17.2×10^8 copies/g sludge, respectively for LS and HS samples, and this an order of magnitude difference led to LS/HS two-stage pattern. The AOA ranges were 4.06×10^4~8.88×10^4 copies/g sludge, and this less quantity revealed that those contributions to ammonia removal could be neglected. These results could be used for improving the efficiency and stability for biological treatment of hypersaline water from sour gas fields.