Background: Wastewater produced by a dairy in Sanandaj is a major source of environmental pollution threatening the city. The dairy uses activated sludge treatment to remove organic pollution from the wastewater. The present study evaluated the performance of this process and its compliance with national requirements for chemical oxygen demand (COD), biochemical oxygen demand (BOD), and total suspended solids (TSS) remaining in the plant effluent. Methods: A total of 48 samples were obtained from the dairy inflow and outflow. The COD, BOD, and TSS were measured for each sample. The statistical sign test was used to assess the standards. Results: The results showed that the average BOD, COD and TSS in the input wastewater was 292.25, 422.92, and 198.33 mg/l, respectively. The ratio of BOD/COD was 0.69, which indicates the capacity of biological treatment was high. The BOD decreased to 64.22 mg/l (92% removal), COD to 33.74 mg/l (92% removal), and TSS to 43.11 mg/l (94% removal) in the effluent, indicating significant removal of water contaminants. The statistical sign test showed that TSS (P < 0.0001) and BOD (P = 0.031) were incompliance with national standards, but COD exceeded standard threshold (P = 0.076). Conclusion: Activated sludge treatment showed a good performance for TSS removal, but was not reliable for removal of BOD and COD pollutants.