Algeria water resources are facing quantity and quality issues. These resources are limited because of the climate (arid and semi-arid) and suffer a deterioration in their quality by the various pollutants releases. Pollution is a serious problem for the environment because of the emissions discharged into rivers; the untreated domestic wastewater is the main source of organic water pollution. To fill the water deficit, many farmers have had recourse to irrigation water discharges from waste water treatment plants. A follow-up 12 months hazard indicators (temperature, pH, electrical conductivity, BOD5, COD, SM, ammonium, nitrite, nitrate, orthophosphate) was conducted to determine the physical-chemical water quality of the station purification of polluted water at the entrance and exit to get an idea about the spatiotemporal evolution of the elements. The analytical monitoring showed a close correlation between the temperature and pH. The values of pollution parameters SM, BOD5, COD, NH4 + to NO3 - output uncross believes in the output show a treatment. The results of the Principal Components Analysis (PCA) revealed a close relationship between pollution parameters BOD5 at the outlet (S), SM (S), COD (S) and NH4 + (S), NO3 -(S). Regarding orthophosphtes, no change was observed and this shows the purification inefficiency. Monitoring of the spatiotemporal evolution of the organic pollution index "OPI" to reveal the nature of the pollutant water discharges from the STEP.