This is an experimental-cross-sectional study with applied nature. Ten carwashes were selected in cluster form at two sampling stages in March 2011 and July 2011. At the beginning of each sampling stage, samples were taken from every carwash and pH, COD, TDS, turbidity, electrical conductivity, oil and grease, and surfactant were measured. In the next stage, three carwashes with the highest, the lowest and the average COD concentrations were selected and they underwent re-sampling. The efficiency of the coagulation and clarification process was investigated through Jar test for treating the wastewater of these units using two coagulants of aluminum sulfate and iron sulfate. The results were analyzed by Excel; and statistical indices including mean value and standard deviation were calculated. The average value of water consumption for washing each car was estimated to be 97 liters in March 2016 and 100 liters in July 2017, respectively. On average, each carwash in Karaj generated 4557 liters wastewater per day in winter and 4900 liters per day in summer. The mean pH, TSS, TDS, EC, COD, oil and grease and surfactant and turbidity of wastewater generated from studied carwashes were obtained as 7.78±0.49,2948.7±25.3 mg/L,705±104.85mg/L, 1401.5±229microsiemes/cm, 989.89/ ±337.4mg/L, 157.87±47.42 mg/L, 14.71±5.92mg/L and 431± 286.6 NTU, respectively. In the coagulation and clarification process, the highest COD removal efficiency was obtained using aluminum sulfate coagulant with concentration of 450 mg /L, which its result with a homogeneous wastewater with initial COD was 1150 mg /L at pH equal to 8.5, equal to 82.03%. The highest removal efficiency of oil and grease for wastewater clarified with initial oil and grease was 248 mg /L at pH 7.31, using an iron sulfate coagulant with a concentration of 250 mg /L, 86.29%. The highest removal efficiency of turbidity in using iron sulfate coagulant with a concentration of 350 mg /L for homogeneous wastewater with an initial turbidity of 789 mg /L at pH equal to 9.5, equal to 99.64%. In using iron sulfate coagulant, the highest removal efficiency of surfactant in the concentration of iron sulfate at a concentration of 400 mg /L for homogeneous wastewater with initial surfactant was 26 mg /L at pH equal to 9.5, equal to 56.77%. The proposed method in this study is to treat the wastewater in an efficient way, but it does not reduce most of the pollutants to a standard level and requires another complementary method to meet the standards.
