| In this paper, secondary floatation pool effluent of Shenghua oil refinery was studied, A1/A2/O1/O2 technics was discussed to remove organic substance and nitrogenous substance, reduction of residual sludge in hydrolyze acidification system and influence of residual sludge circumfluence on treatment effect were studied.Through the operation condition analysis of waste water treatment effect: A1/A2/O1/O2 technics had a strong capacity of shock load; the system can run stably within volume load (0.782~1.209 kgCOD·m-3·d-1); nitrogen removal effect would be enhanced with the increasing of COD/NH3-N, the shorten of HRT would affect nitrification; in order to achieve a better effect, HRT should remain 4.2h at least; the improvement of DO would enhance COD and ammonia nitrogen removal effect, and when DO gradient exists in the aerobic area, nitrogen removal effect would be enhanced; the improvement of sludge and mixed liquid return ratio would enhance the nitrogen removal effect.The residual sludge in secondary sedimentation tank returned to anaerobic area, through hydrolysis acid bacteria of short time and strong adaptability, could achieve the reduction effectively. Through the analysis of operation conditions which affect sludge reduction, 0.853kgCOD·m-3·d-1 was chosen as the normal volume load; HRT should be 4.2h in order to meet requirements of wastewater treatment and effective sludge reduction; sludge back to A1, N P release could effectively compensate C:N:P imbalance, thus cost of N,P addition to system could greatly reduce.Residual sludge back to A1 improved COD removal efficiency. COD removal efficiency increased from 18.97% to 22.62% in A1 when organic load is 0.853kgCOD·m-3·d-1 and HRT is 5.6h; there is little influence of sludge return on pH value; sludge return made NO3-N content increased, which was conductive to anti-nitrification, thus enhanced nitrogen removal; and TN removal efficiency increased with the growing of return ratio.
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