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Background and Aim: In the present study the influence of the different sulfate concentration on the anaerobic lagoon stabilization was investigated.

Materials and Methods: The present study is an experimental research carried out on anaerobic stabilization pond pilot for 7 months in Birjand wastewater treatment plant. After making sure of a steady state sulfate with different concentrations of 200, 300 and 400 mg/L were injected into the pilot. Then parameters including pH, organic nitrogen, ammonia nitrogen, BOD₅, COD and nitrate were measured. All of the experiments were carried out according to the methods presented in the book "Standard Method" for the examination of water and wastewater (2005).

Results: It was found that by increasing sulfate concentration from 200 to 300 mg/L all of parameters  except BOD5 (10% reduction) had no significant changes., but by increasing the sulfate concentration from 200 to 400 mg/L the removal efficiency of the parameters such as BOD5, COD, Organic nitrogen, total kjeldahl nitrogen, nitrate and sulfate reduced to 11, 8, 12, 26, 6 and 10 percent, respectively. PH in the first stage was alkaline and then changed to acidic.

Conclusion: Anaerobic stabilization ponds have different capacities for removal of organic compounds at different sulfate concentrations; so that; in sulfate concentration of 200 mg/L, the proper operation was seen and in concentration of 300 mg/L, sulfate-reducing bacteria get dominant and therefore odor is produced..  Alternatively, by increasing the concentration of sulphate to 400 mg/L, ammonia nitrogen increased 2.5 times (150%) in the effluent.