Abstract: Trickling filters are one of the biological treatment methods used to treat the wastewater before being discharged to the receiving water (rivers and drains) in order to keep water quality within specifications and hence can be used for other purposes. This study considers the performance evaluation of the sewage treatment plant in Al-Najaf city. Laboratory tests were carried out on the influent and effluent wastewater of the plant in 2014 and continued for 12 months (from Nov. 2014-2015). The determinants of environmental pollution of effluent wastewater that have been considered and evaluated comprise Biological Oxygen Domain (BOD), Chemical Oxygen Domain (COD), Total Suspended Solid (TSS), Phosphate ion (PO₄)^-3, Sulphate ion (SO₄)^-2, Nitrate (NO₃)^-3, Nitrite ion (No₂)^-3 , Hydrogen ion (pH), Chloride ion (CL)^-1, ammonia (NH₃), H₂S, grease and oils. Removal percentages have been employed as the quantitative measure for evaluating the performance efficiency of the sewage treatment plant. Based on the analysis results it was found that the removal percentages of (BOD), (COD) and (TSS) are 72.26, 63.26 and 43.7%, respectively. Similarly, the values of PO₄, SO₄, CL and NH₃ are 19.1, 9.64, 13.72 and 35.73%, respectively whereas the percentages regarding H₂S and grease and oils are 71.06, 28.05%. The results showed removal percentages with negative values for nitrate, nitrite and hydrogen ions that is the external concentrations were higher than internal ones due to the absence of advanced treatment in the plant. According to the results, the plant performance efficiency is generally acceptable, however, most of the concentrations of the pollutants of effluent wastewater exceeded the corresponding Iraqi standard specification due to the extra load resulted from internal and international visitors to Al-Najaf city in religious occasions.