Abstract: In the process of biological oxidation pretreatment，a large amount of oxidation liquid will be produced. There are a large number of living bacteria，surplus ammonia nitrogen and toxic ions such as arsenic in the oxidation liquid. Living bacteria and surplus ammonia nitrogen are conducive to biological oxi⁃ dation reaction，while excessive toxic ions such as arsenic will inhibit biological oxidation. In order to solve the problem of how to effectively realize the recycling of oxidation liquid. Exploratory innovative experimen⁃ tal research on the recycling of biological oxidation liquid was carried out. The optimum concentration range of arsenic affecting the activity of oxidizing bacteria was determined and successfully applied to production practice. Industrial practice shows that when the concentration of toxic ion arsenic in the oxidation solution is controlled to below 8 542 mg/L，the partial recycling of the oxidation solution to the biological oxidation process not only improves the biological oxidation efficiency，reduces the cost of sewage treatment，but also greatly reduces the total amount of ammonia nitrogen emissions，which has significant economic and envi⁃ ronmental benefits.

Key words: biological oxidation solution, living bacteria, ammonia nitrogen, liquid arsenic, recycling